Picophytoplankton (<2 µm diameter), a diverse group of picocyanobacteria and photosynthetic picoeukaryotes, are significant contributors to primary production. Predatory mortality controls picophytoplankton biomass and thereby energy transfer in the marine food web. The 2 major pathways of picophytoplankton mortality are grazing and viral lysis. Grazing passes carbon directly to higher trophic levels, while lysis products are passed into the viral loop. Picophytoplankton are abundant in the Baltic Sea but little is known about their predatory mortality. Using a modification of the dilution approach, we calculated growth and mortality rates of picophytoplankton and studied the effect of predation on community structure during late August and September. The experiments were conducted coinciding with the peak in picophytoplankton abundance (similar to 10(5) cells ml(-1)) at the Linnaeus Microbial Observatory in the Baltic Sea Proper. The results showed that grazing is an important controller of picocyanobacteria and photosynthetic picoeukaryote populations, while no significant viral lysis effect was detected. Grazing on picocyanobacteria was proportional to growth rates, while grazing on photosynthetic picoeukaryotes exceeded growth. Selective grazing of phylogenetically distinct picocyanobacterial clades had a significant effect on community structure, suggesting that grazing has an impact on the seasonal dynamics of co-occurring clades. Picocyanobacteria had a higher carbon transfer contribution to higher trophic levels than photosynthetic picoeukaryotes at the time of the experiments. The study shows that picophytoplankton are important contributors to carbon cycling in the Baltic Sea microbial food web and should be considered for future ecological models.

Aphanizomenon, together with Dolichospermum and Nodularia, constitute the major genera of bloom forming filamentous nitrogen fixing cyanobacteria in the Baltic Sea. Like the other genera, Aphanizomenon displays summer blooms that are highly variable in magnitude and duration but unlike the others it is considered a holoplanktonic species. Still, the molecular mechanisms enabling Aphanizomenon year-round presence in surface waters are currently unknown. Here we combine analysis of Aphanizomenon population dynamics at the Linnaeus Microbial Observatory (LMO) station in the Baltic Proper over nine years (2011-2019) with associated gene expression patterns during 2016-2017 to identify annual abundance, and metabolic and life cycle strategies. Aphanizomenon biomass showed large annual variability and a consistent biovolume peak in summer, with bloom intensity ranging from 78-1334 mm3 m-3. 16S rRNA gene amplicon sequence data showed that one Aphanizomenon amplicon sequence variant (ASV) dominated, and its relative abundance correlated with biovolume measurements. Metatranscriptomic reads that mapped to an Aphanizomenon metagenome- assembled genome (MAG) revealed annually repeating gene expression patterns, resulting in distinct gene expression profiles during different meteorological seasons. Genes encoding proteins involved in several important functional classes, e.g. carbon fixation, photosynthesis, and associated photopigments showed seasonal variation, but were detected year round. Other genes, particularly those involved in nitrogen fixation, were highly expressed in summer, while absent in winter. Vitamin metabolism and phosphorus scavenging genes were preferentially expressed during the colder periods of the annual cycle. Together, these data show that Aphanizomenon regulates the molecular machinery on the seasonal scale, providing context to the observed dynamics of Aphanizomenon in the Baltic Proper and a foundation for understanding the ecology of these cyanobacteria. 

Climate change is likely to have large effects on the Baltic Sea ecosystem. Simulations indicate 2-4 degrees C warming and 50-80 % decrease in ice cover by 2100. Precipitation may increase similar to 30 % in the north, causing increased land runoff of allochthonous organic matter (AOM) and organic pollutants and decreased salinity. Coupled physical-biogeochemical models indicate that, in the south, bottom-water anoxia may spread, reducing cod recruitment and increasing sediment phosphorus release, thus promoting cyanobacterial blooms. In the north, heterotrophic bacteria will be favored by AOM, while phytoplankton production may be reduced. Extra trophic levels in the food web may increase energy losses and consequently reduce fish production. Future management of the Baltic Sea must consider the effects of climate change on the ecosystem dynamics and functions, as well as the effects of anthropogenic nutrient and pollutant load. Monitoring should have a holistic approach, encompassing both autotrophic (phytoplankton) and heterotrophic (e.g., bacterial) processes.

The production and release of chemicals from human activities are on the rise. Understanding how the aquatic environment is affected by the presence of an unknown number of chemicals is lacking. We employed the chemical activity concept to assess the combined effects of hydrophobic organic contaminants on the phyto-plankton species Rodomonas salina. Chemical activity is additive, and refers to the relative saturation of a chemical in the studied matrix. The growth of R. salina was affected by chemical activity, following a chemical activity-response curve, resulting in an Ea50 value of 0.078, which falls within the baseline toxicity range observed in earlier studies. The chlorophyll a content exhibited both increases and decreases with rising chemical activity, with the increase possibly linked to an antioxidant mechanism. Yet, growth inhibition provided more sensitive and robust responses compared to photosynthesis-related endpoints; all measured endpoints correlated with increased chemical activity. Growth inhibition is an ecologically relevant endpoint and integrates ther-modynamic principles such as membrane disruption. Our study utilized passive dosing, enabling us to control exposure and determine activities in both the medium and the algae. The concept of chemical activity and our results can be extended to other neutral chemical groups as effects of chemical activity remain independent of the mixture composition.

In this study we have investigated whether the carbon isotopic signature differs between different groups and species of marine phytoplankton depending on growth phase, nutrient conditions and salinity. The 15 investigated algal species, representing the Bacillariophyceae, Chlorophyceae, Cryptophyceae, Cyanophyceae, Dinophyceae and Haptophyceae classes were grown in batch monocultures and analysed for delta(13)C in both exponential and stationary phase. For all the cultured species, delta(13)C signatures ranged from -23.5 parts per thousand (Imantonia sp.) to - 12.3 parts per thousand (Nodulania spumigena) in the exponential phase and from - 18.8 parts per thousand (Amphidinium carterae) to - 8.0 parts per thousand (Anabaena lemmermannii) in the stationary phase. Three species (Dunaliella tertiolecta, Rhodomonas sp.. Heterocapsa triquetra) were also grown under nutrient sufficient and nitrogen or phosphorus deficient conditions. Nitrogen limitation resulted in a more negative delta(13)C signature, whereas no effect could be observed during phosphorus limitation compared to nutrient sufficient conditions. Growth of Prymnesium parvum in two different salinities resulted in a more negative delta(13)C signature in the 26 parts per thousand-media compared to growth in 7 parts per thousand-media. Our results show that the carbon isotopic signature of phytoplankton may be affected by salinity, differ among different phytoplankton species, between exponential and stationary phase, as well as between nutrient treatments.

Marine bacterioplankton are essential in global nutrient cycling and organic matter turnover. Time-series analyses, often at monthly sampling frequencies, have established the paramount role of abiotic and biotic variables in structuring bacterioplankton communities and productivities. However, fine-scale seasonal microbial activities, and underlying biological principles, are not fully understood. We report results from four consecutive years of high-frequency time-series sampling in the Baltic Proper. Pronounced temporal dynamics in most investigated microbial variables were observed, including bacterial heterotrophic production, plankton biomass, extracellular enzyme activities, substrate uptake rate constants of glucose, pyruvate, acetate, amino acids, and leucine, as well as nutrient limitation bioassays. Spring blooms consisting of diatoms and dinoflagellates were followed by elevated bacterial heterotrophic production and abundances. During summer, bacterial productivity estimates increased even further, coinciding with an initial cyanobacterial bloom in early July. However, bacterial abundances only increased following a second cyanobacterial bloom, peaking in August. Uptake rate constants for the different measured carbon compounds varied seasonally and inter-annually and were highly correlated to bacterial productivity estimates, temperature, and cyanobacterial abundances. Further, we detected nutrient limitation in response to environmental conditions in a multitude of microbial variables, such as elevated productivities in nutrient bioassays, changes in enzymatic activities, or substrate preferences. Variations among biotic variables often occurred on time scales of days to a few weeks, yet often spanning several sampling occasions. Such dynamics might not have been captured by sampling at monthly intervals, as compared to more predictable transitions in abiotic variables such as temperature or nutrient concentrations. Our study indicates that high resolution analyses of microbial biomass and productivity parameters can help out in the development of biogeochemical and food web models disentangling the microbial black box.

Bioassay experiments were performed two times (with 2 years in between) in order to investigate if nitrogen(N, ammonium), phosphorus (P, phosphate) and carbon (C, glucose) additions would stimulate the growth ofbacteria and phytoplankton differently in three different tropical aquatic environments. The water and theirindigenous microbial communities were taken from a freshwater coastal lake (Cabiunas), a coastal (Anjos),and an offshore marine station (Sonar) in the Atlantic outside Cabo Frio, Rio de Janeiro State, Brazil. Ammonium,phosphate and glucose were added alone or in combination to triplicate bottles. In the lake, P seemedto be the primary limiting factor during the first experiment, since both bacterial production and phytoplanktongrowth was stimulated by the P addition. Two years later, however, addition of P inhibited phytoplanktongrowth. During both years, C was closely co-limiting for bacteria since CP additions increased the responseconsiderably. For both the coastal and offshore seawater stations, phytoplankton growth was clearly stimulatedby N addition in both years and the bacteria responded either to the P, N or C additions (alone or incombination). To conclude, the results from these tropical aquatic systems show that it is possible that phytoplanktonand bacteria may compete for a common resource (P) in lakes, but can be limited by different inorganicnutrients in marine waters as well as lakes, suggesting that phytoplankton and bacteria do notnecessarily compete for the same growth limiting nutrient in these environments.

Hydrophobic organic contaminants (HOCs) affect phytoplankton at cellular to population levels, ultimately impacting communities and ecosystems. Baseline toxicants, such as some HOCs, predominantly partition to biological membranes and storage lipids. Predicting their toxic effects on phytoplankton populations therefore requires consideration beyond cell uptake and diffusion. Functional traits like lipid content and profile can offer insights into the diverse responses of phytoplankton populations exposed to HOCs. Our study investigated the vulnerability of five phytoplankton species populations to varying chemical activities of a mixture of polycyclic aromatic hydrocarbons (PAHs). Population vulnerability was assessed based on intrinsic sensitivities (toxicokinetic and toxicodynamic), and demography. Despite similar chemical activities in biota within the exposed algae, effects varied significantly. According to the chemical activity causing 50% of the growth inhibition (Ea50), we found that the diatom Phaeodactylum tricornutum (Ea50 = 0.203) was the least affected by the chemical exposure and was also a species with low lipid content. In contrast, Prymnesium parvum (Ea50 = 0.072) and Rhodomonas salina (Ea50 = 0.08), both with high lipid content and high diversity of fatty acids in non-exposed samples, were more vulnerable to the chemical mixture. Moreover, the species P. parvum, P. tricornutum, and Nannochloris sp., displayed increased lipid production, evidenced as 5-10% increase in lipid fluorescence, after exposure to the chemical mixture. This lipid increase has the potential to alter the intrinsic sensitivity of the populations because storage lipids facilitate membrane repair, reconstitution and may, in the short-term, dilute contaminants within cells. Our study integrated principles of thermodynamics through the assessment of membrane saturation (i.e. chemical activity), and a lipid trait-based assessment to elucidate the differences in population vulnerability among phytoplankton species exposed to HOC mixtures. A chemical mixture caused diverse responses across five phytoplankton species. Analysis of lipid profiles and changes in neutral lipid content enhanced our understanding of the vulnerability of phytoplankton populations to chemical pollution.

Thiamin (vitamin B1) is mainly produced by bacteria and phytoplankton and then transferred to zooplankton and higher trophic levels but knowledge on the dynamics of these processes in aquatic ecosystems is lacking. Hence, the seasonal variation in thiamin content was assessed in field samples of copepods and in pico-, nano- and micro-plankton of two size classes (0.7–3 µm and > 3 µm) collected monthly in the Baltic Sea during 3 years and in the Skagerrak during 1 year. Copepods exhibited species-specific concentrations of thiamin and Acartia sp. had the highest carbon-specific thiamin content, at both locations. Even members of the same genus, but from different systems contained different levels of thiamin, with higher thiamin content per specimen in copepods from the Skagerrak compared to congeners from the Baltic Sea. Furthermore, our results show that the small plankton (0.7–3 µm) had a higher carbon-specific thiamin content compared to the large (> 3 µm). Additionally, there was a large seasonal variation and thiamin content was highly correlated comparing the two size fractions. Finally, there was an overall positive correlation between thiamin content in copepods and plankton. However, for periods of high thiamin content in the two size fractions, this correlation was negative. This suggests a decoupling between thiamin availability in pico-, nano- and micro-plankton and zooplankton in the Baltic Sea. Knowledge about concentrations of this essential micronutrient in the aquatic food web is limited and this study constitutes a foundation for further understanding the dynamics of thiamin in aquatic environments.

The planktonic realm from bacteria to zooplankton provides the baseline for pelagic aquatic food webs. However, multiple trophic levels are seldomly included in time series studies, hampering a holistic understanding of the influence of seasonal dynamics and species interactions on food web structure and biogeochemical cycles. Here, we investigated plankton community composition, focusing on bacterio-, phyto- and large mesozooplankton, and how biotic and abiotic factors correlate at the Linnaeus Microbial Observatory (LMO) station in the Baltic Sea from 2011 to 2018. Plankton communities structures showed pronounced dynamic shifts with recurring patterns. Summarizing the parts of the planktonic microbial food web studied here to total carbon, a picture emerges with phytoplankton consistently contributing > 39% while bacterio- and large mesozooplankton contributed ~ 30% and ~ 7%, respectively, during summer. Cyanophyceae, Actinobacteria, Bacteroidetes, and Proteobacteria were important groups among the prokaryotes. Importantly, Dinophyceae, and not Bacillariophyceae, dominated the autotrophic spring bloom whereas Litostomatea (ciliates) and Appendicularia contributed significantly to the consumer entities together with the more traditionally observed mesozooplankton, Copepoda and Cladocera. Our findings of seasonality in both plankton composition and carbon stocks emphasize the importance of time series analyses of food web structure for characterizing the regulation of biogeochemical cycles and appropriately constraining ecosystem models. 

Thiamin (vitamin B1) is primarily produced by bacteria and phytoplankton in aquatic food webs and transferred by ingestion to higher trophic levels. However, much remains unknown regarding production, content and transfer of this water-soluble, essential micronutrient. Hence, the thiamin content of six phytoplankton species from different taxa was investigated, along with the effect of thiamin amendment on thiamin content. Furthermore, thiamin transfer to copepods was estimated in feeding experiments. Prey type, not phytoplankton thiamin content per se, was the most important factor for the transfer of thiamin, as it was lowest from filamentous Cyanophyceae and highest from more easily ingested prey like Dunaliella tertiolecta and Rhodomonas salina. Cyanophyceae had the highest thiamin content of the investigated species, eightfold higher than the lowest. Phytoplankton varied in thiamin content related to the supply of thiamin, where thiamin addition enabled higher thiamin content in some species, while copepod thiamin content was less variable. In all, thiamin transfer is not only dependent on the prey thiamin content, but also the edibility and/or digestibility is of importance. Thiamin is essential for all organisms, and this study constitutes an important building block to understanding the dynamics and transfer of thiamin in the aquatic food web.

Top predators in several aquatic food webs regularly display elevated reproductive failure, caused by thiamin(vitamin B1)deficiency. The reasons for these low-thiamin levels are not understood and information about the transfer of thiamin from the producers (bacteria and phytoplankton) to higher trophic levels is limited. One main concern is whether cyanobacterial blooms could negatively affect thiamin transfer in aquatic systems. Laboratory experiments with Baltic Sea plankton communities and single phytoplankton species were used to study the effect of filamentous cyanobacteria on the transfer of thiamin from phytoplankton to zooplankton. Experiments showed that the thiamin content in copepods was reduced when exposed to elevated levels of cyanobacteria, although filamentous cyanobacteria had higher levels of thiamin than any other analyzed phytoplankton species. Filamentous cyanobacteria also had a negative effect on copepod egg production despite high concentrations of non-cyanobacterial food. Phytoplankton species composition affected overall thiamin concentration with relatively more thiamin available for transfer when the relative abundance of Dinophyceae was higher. Finally, phytoplankton thiamin levels were lower when copepods were abundant, indicating that grazers affect thiamin levels in phytoplankton community, likely by selective feeding. Overall, high levels of thiamin in phytoplankton communities are not reflected in the copepod community. We conclude that presence of filamentous cyanobacteria during summer potentially reduces the transfer of thiamin to higher trophic levels by negatively affecting phytoplankton and copepod thiamin content as well as copepod reproduction, thereby lowering the absolute capacity of the food web to transfer thiamin through copepods to higher trophic levels.

Cyanobacterial blooms covering almost the entire Baltic Sea is a yearly feature during July-August. For the tourism industry at Öland island, SE Sweden, the economical losses during the summer 2005 amounted to 17-23 million euros. Remote sensing satellite images show that all the Öland beaches are covered with decomposing algae. In reality, these blooms rarely reach the western side of the island. To more accurately inform the public on the quality of the water for swimming, with the help of volunteers, a daily real-time surveillance of the algal densities on the beaches was performed. The volunteers (from 15 years old to pensioners) were trained at the Linnaeus University, from simple laboratory techniques, to more complicated ones such as identification and enumeration of the toxic cyanobacteria species. By latest 9.00 a.m., the public had access to information on the algal situation on 17 beaches. We could show that: 1) although remote sensing images showed Öland being surrounded by the blooms, our surveillance showed no algal accumulations on the beaches 2) that the real-time warning system boosted public confidence in the local water quality and during the first “Miss Algae”-summer 2006, the economical losses by the tourism industry turned in profits, the gain amounting to 17 million euros, 3) this kind of real-time surveillance is economical feasible due to low-costs involved, but also, the project has a great social value for the volunteers who mostly were pensioners. The volunteers who participated in “Miss Algae” had a good knowledge about the area they monitored (as their houses are located nearby) and could disseminate knowledge to the public in these areas. This kind of project also render a lot of interest regional, national and international, and can be used in advertising campaigns to increase tourism in the areas affected by algal blooms.

Reproduction in planktonic animals depends on numerous biotic and abiotic factors. One of them is predation pressure, which can have both directconsumptive effects on population density and sex ratio, and non-consumptive effects, for example on mating and migration behaviour. In copepods, predatorvulnerability depends on their sex, motility pattern and mating behaviour. Therefore, copepods can be affected at multiple stages during the mating process. We investigated the reproductive dynamics of the estuarine copepod Eurytemora affinis in the presence and absence of its predator the mysid Neomysis integer in a mesocosm experiment. We found that the proportion of ovigerous females decreased in the presence of predators. This shift was not caused by differential predation as the absolute number of females was unaffected by mysid presence. Presence of predators reduced the ratio of males to non-ovigerous females, but not by predation of males. Our combined results suggest that the shift from ovigerous to non-ovigerous females under the presence of predators was caused by either actively delayed egg production or by shedding of egg sacs. Nauplii production was initially suppressed in the predation treatment, but increased towards the end of the experiment. The proportion of fertilized females was similar in both treatments, but constantly fell behind model predictions using a random mating model. Our results highlight the importance of non-consumptive effects of predators on copepod reproduction and hence on population dynamics.

Micronutrients such as vitamins are transferred from lower to higher trophic levels, but no general ecological concept describes the factors regulating this process. Here, we investigated thiamin (thiamine, vitamin B1), which is an example of a metabolically important water-soluble micronutrient. Thiamin is produced by organisms such as bacteria and phytoplankton, and all consumers, such as zooplankton and fish, rely on a continuous intake of thiamin through their diet and possibly from de novo-synthesized thiamin by gut microbiota. A deficiency in thiamin negatively affects reproduction in fish and bird populations worldwide. The aim of this study was to quantify thiamin transfer in a planktonic food web in response to thiamin and/or nutrient addition, using an outdoor mesocosm system (an approximately 1.9 m3 bag submerged in sea water). These estimates were then compared with literature data on thiamin concentrations at different trophic levels. The results showed that thiamin was rapidly taken up by phytoplankton in both the ambient and nutrient-amended treatments. However, large differences in thiamin concentrations in phytoplankton did not lead to any significant changes in community composition or abundance. Nitrogen addition led to changes in the abundance and community composition of picoplankton and phytoplankton but there were no additional major effects of thiamin addition. Differences in thiamin concentrations in phytoplankton were not detected at the next trophic level in zooplankton. Although the concentrations did not change, a greater abundance of some zooplankton taxa were developed in the thiamin treatments. Comparing the mesocosm results with literature data demonstrated a gradual reduction in thiamin concentrations along the food chain, with six percent of the concentration in producers occurring in top consumers (i.e., piscivorous fish). Overall, these observations illustrate the concept of trophic dilution of micronutrients where concentrations decrease along the food web from phytoplankton via zooplankton and planktivorous fish to piscivorous fish.

Environmental conditions regulate the germination of phytoplankton resting stages. While some factors lead to synchronous germination, others stimulate germination of only a small fraction of the resting stages. This suggests that habitat filters may act on the germination level and thus affect selection of blooming strains. Benthic “seed banks” of the toxic dinoflagellate Alexandrium ostenfeldii from the Baltic Sea are genetically and phenotypically diverse, indicating a high potential for adaptation by selection on standing genetic variation. Here, we experimentally tested the role of climate-related salinity and temperature as selection filters during germination and subsequent establishment of A. ostenfeldii strains. A representative resting cyst population was isolated from sediment samples, and germination and reciprocal transplantation experiments were carried out, including four treatments: Average present day germination conditions and three potential future conditions: high temperature, low salinity, and high temperature in combination with low salinity. We found that the final germination success of A. ostenfeldii resting cysts was unaffected by temperature and salinity in the range tested. A high germination success of more than 80% in all treatments indicates that strains are not selected by temperature and salinity during germination, but selection becomes more important shortly after germination, in the vegetative stage of the life cycle. Moreover, strains were not adapted to germination conditions. Instead, highly plastic responses occurred after transplantation and significantly higher growth rates were observed at higher temperature. High variability of strain-specific responses has probably masked the overall effect of the treatments, highlighting the importance of testing the effect of environmental factors on many strains. It is likely that A. ostenfeldii populations can persist in the future, because suitable strains, which are able to germinate and grow well at potential future climate conditions, are part of the highly diverse cyst population. OPEN RESEARCH BADGES: This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.c8c83nr. © 2019 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.

Picocyanobacteria are widespread and globally significant primary producers. In brackish waters, picocyanobacterial populations are composed of diverse species with both single-cell and colony-forming lifestyles. Compared to their marine counterparts, brackish picocyanobacteria are less well characterized and the focus of research has been weighted toward single-cell picocyanobacteria. Here, we investigate the uptake dynamics of single and colony-forming picocyanobacteria using incubations with dual carbon-13 and inorganic (ammonium and nitrate) or organic (urea and amino acids) nitrogen-15 sources during August and September 2020 in the central Baltic Sea. Phytoplankton community and group-specific uptake rates were obtained using an elemental analyzer isotope ratio mass spectrometer (EA-IRMS) and nano secondary-ion mass spectrometry (NanoSIMS). Picocyanobacteria contributed greater than one third of the ammonium, urea, amino acids, and inorganic carbon community uptake/fixation in September but < 10% in August when phytoplankton biomass was higher. Overall, single-cell ammonium and urea uptake rates were significantly higher for single-celled compared to colonial picocyanobacteria. In a 6-yr offshore central Baltic Sea time series (2015-2020), summer abundances of colonial picocyanobacteria reached up to 10(5) cells mL(-1) and represented > 5% of the average phytoplankton biomass, suggesting that they are periodically important for the ecosystem. Colonial strain identification was not distinguishable using 16S rRNA gene amplicon data, highlighting a need for refined tools for identification of colonial forms. This study shows the significance of single-celled brackish picocyanobacteria to nutrient cycling and the importance of considering uptake and lifestyle strategies when assessing the role of picocyanobacteria in aquatic ecosystems.

The microbial part of the pelagic food web is seldom characterized in models despite its major contribution to biogeochemical cycles. In the Baltic Sea, spatial and temporal high frequency sampling over three years revealed changes in heterotrophic bacteria and phytoplankton coupling (biomass and production) related to hydrographic properties of the ecosystem. Phyto- and bacterioplankton were bottom-up driven in both coastal and offshore areas. Cold winter temperature was essential for phytoplankton to conform to the successional sequence in temperate waters. In terms of annual carbon production, the loss of the spring bloom (diatoms and dinoflagellates) after mild winters tended not to be compensated for by other taxa, not even summer cyanobacteria. These results improve our ability to project Baltic Sea ecosystem response to short- and long-term environmental changes.

The uptake rates of different nitrogen (N) forms (NO3-, urea, and the amino acids glycine and glutamicacid) by N-deficient, laboratory-grown cells of the mixotrophic haptophyte, Prymnesium parvum, weremeasured and the preference by the cells for the different forms determined. Cellular N uptake rates(rcell, fmol N cell-1 h-1) were measured using 15N-labeled N substrates. P. parvum showed highpreference for the tested amino acids, in particular glutamic acid, over urea and NO3 under the culturenutrient conditions. However, extrapolating these rates to Baltic Seawater summer conditions, P. parvumwould be expected to show higher uptake rates of NO3- and the amino acids relative to urea because ofthe difference in average concentrations of these substrates. A high uptake rate of glutamic acid at lowsubstrate concentrations suggests that this substrate is likely used through extracellular enzymes.Nitrate, urea and glycine, on the other hand, showed a non-saturating uptake over the tested substrateconcentration (1–40 mM-N for NO3- and urea, 0.5–10 mM-N for glycine), indicating slower membranetransportrates for these substrates

We investigated the ability of the ichthyotoxic haptophyte Prymnesium parvum to use sewage-originated nutrients applying stable carbon (C) and nitrogen (N) isotope techniques. P. parvum was cultured under N and phosphorus (P) sufficient and deficient conditions in either sewage effluent-based medium or in a nitrate- and phosphate-based control. Cell densities and toxicities were monitored and stable carbon N isotopes signatures (delta(13)C and delta(15)N) of P. parvum and the sewage effluent analysed. Nitrogen and P sufficient cultures achieved the highest biomass followed by P and N deficient cultures, regardless of sewage effluent additions. The P deficient cultures with sewage effluent had higher toxicity, estimated as haemolytic activity (9.4 +/- 0 x 10(-5) mg Saponin equiv. cell(-1)) compared to the P deficient control and to all N deficient and NP sufficient cultures. Nutrient deficient conditions had no effect on the cell delta(15)N, but a decreasing effect on delta(13)C in the inorganic N deficient treatment. Growth in sewage-based media was followed by a substantial increase in the cell delta(15)N (10.4-16.1 parts per thousand) compared to the control treatments (2.4-4.9 parts per thousand), showing that P. parvum is capable of direct use of sewage-originated N, inorganic as well as organic. Uptake of terrestrial derived C in the sewage treatments was confirmed by a decrease in cell delta(13)C, implying that P. parvum is able to utilize organic nutrients in sewage effluent.

The study evaluated removal of industrial CO2 from cement flue gas using algal cultivation. Local polycultures were grown in an up-scaled outdoor photobioreactor over 5 years in northern Europe. Algal biomass was harvested 2–3 times per week and the closed panel system was re-filled with seawater amended with nutrients. Flue gas was fed to the photobioreactor circulatory system in one direction or re-circulated. Removal efficiency of CO2 averaged 9 % in non-recirculation and 17 % in re-circulation modes and reached 20–60 % under best cultivation conditions. Recovery of carbon into algal biomass reached up to 10 g m2d−1 in non-recirculation mode. Biomass performance was explained by circulation mode and shift of polyculture traits. Stability of biomass quality was shown over seasons, with higher relative content of protein in autumn. Toxic elements in biomass were below legal thresholds for upcycling. The study shows feasibility of algal solutions for conversion of waste, applied in temperate climate.

Vitamin B1 (thiamin) deficiency is an issue periodically affecting a wide range of taxa worldwide. In aquatic pelagic systems, thiamin is mainly produced by bacteria and phytoplankton and is transferred to fish and birds via zooplankton, but there is no general consensus on when or why this transfer is disrupted. We focus on the occurrence in salmon (Salmo salar) of a thiamin deficiency syndrome (M74), the incidence of which is highly correlated among populations derived from different spawning rivers. Here, we show that M74 in salmon is associated with certain large-scale abiotic changes in the main common feeding area of salmon in the southern Baltic Sea. Years with high M74 incidence were characterized by stagnant periods with relatively low salinity and phosphate and silicate concentrations but high total nitrogen. Consequently, there were major changes in phytoplankton and zooplankton, with, e.g., increased abundances of Cryptophyceae, Dinophyceae, Diatomophyceae and Euglenophyceae and Acartia spp. during high M74 incidence years. The prey fish communities also had increased stocks of both herring and sprat in these years. Overall, this suggests important changes in the entire food web structure and nutritional pathways in the common feeding period during high M74 incidence years. Previous research has emphasized the importance of the abundance of planktivorous fish for the occurrence of M74. By using this 27-year time series, we expand this analysis to the entire ecosystem and discuss potential mechanisms inducing thiamin deficiency in salmon.

Microalgal solutions to clean waste streams and produce biomass were evaluated in Nordic conditions during winter, spring, and autumn in Southeast Sweden. The study investigated nitrogen (N) removal, biomass quality, and safety by treating industrial leachate water with a polyculture of local microalgae and bacteria in open raceway ponds, supplied with industrial CO2 effluent. Total N (TN) removal was higher in spring (1.5 g-2d-1), due to beneficial light conditions compared to winter and autumn (0.1 and 0.09 g-2d-1). Light, TN, and N species influenced the microalgal community (dominated by Chlorophyta), while the bacterial community remained stable throughout seasons with a large proportion of cyanobacteria. Winter conditions promoted biomass protein (19.6-26.7%) whereas lipids and carbohydrates were highest during spring (11.4-18.4 and 15.4-19.8%). Biomass toxin and metal content were below safety levels for fodder, but due to the potential presence of toxic strains, biofuels or fertilizer could be suitable applications for the algal biomass.Practitioner points Microalgal removal of nitrogen from leachate water was evaluated in Nordic conditions during winter, spring, and autumn. Total nitrogen removal was highest in spring (1.5 g-2d-1), due to beneficial light conditions for autotrophic growth. Use of local polyculture made the cultivation more stable on a seasonal (light) and short-term (N-species changes) scale. Toxic elements in produced algal biomass were below legal thresholds for upcycling. The study investigated nitrogen removal, biomass quality, and safety by treating industrial leachate water with a polyculture of local microalgae and bacteria in open raceway ponds, supplied with industrial CO2 effluent. Nitrogen removal by the polyculture was highest in spring and the biomass biochemical composition changed with season. image

Algal lipids have been observed to increase during autumn conditions (low light, low mean temperature, and diurnal shift in temperature), in large‐scale outdoor photobioreactors. In this paper, we tested the effect of diurnal shifts in temperature (DS) and nitrogen (N) limitation on algal BODIPY lipid fluorescence cell⁻¹ (BPF). We show that DS increased BPF in algal biomass up to 28% more compared with N limitation, the standard stressor to boost neutral lipids (NL) in commercial production. Biomass yield was constant, regardless the DS range (6°C‐12°C). A combination of both stressors had an additive effect on algal BPF. A polyculture from an outdoor photobioreactor was cultivated under controlled conditions at different regimes of light, temperature, and N limitation. DSs were mimicking autumn conditions with a difference of 6°C, 10°C, and 12°C between day and night. Biomass and BPF were monitored over one to two weeks, and NLs were stained with a fluorescent marker (BODIPY) and detected with flow cytometry. Results indicate that, during autumn conditions, daily heating and cooling processes in contrast to N limitation do not challenge the trade‐off between biomass production and BPF. During seasons when day temperature is still relatively high, DSs are rapid BPF boosting stressors, while N limitation could be applied to boost BPF further during other seasons.

Functionally uniform monocultures have remained the paradigm in microalgal cultivation despite the apparent challenges to avoid invasions by other microorganisms. A mixed microbial consortium approach has the potential to optimize and maintain biomass production despite of seasonal changes and to be more resilient toward contaminations. Here we present a 3-year outdoor production of mixed consortia of locally adapted microalgae and bacteria in cold temperate latitude. Microalgal consortia were cultivated in flat panel photobioreactors using brackish Baltic Sea water and CO2 from a cement factory (Degerhamn, Cementa AB, Heidelberg Cement Group) as a sustainable CO2 source. To evaluate the ability of the microbial consortia to maintain stable biomass production while exposed to seasonal changes in both light and temperature, we tracked changes in the microbial community using molecular methods (16S and 18S rDNA amplicon sequencing) and monitored the biomass production and quality (lipid, protein, and carbohydrate content) over 3 years. Despite changes in environmental conditions, the mixed consortia maintained stable biomass production by alternating between two different predominant green microalgae (Monoraphidium and Mychonastes) with complementary tolerance to temperature. The bacterial population was few taxa co-occured over time and the composition did not have any connection to the shifts in microalgal taxa. We propose that a locally adapted and mixed microalgal consortia, with complementary traits, can be useful for optimizing yield of commercial scale microalgal cultivation.

Mixotrophic microalgal solutions are efficient nutrient recovery methods, with potential to prolong the cultivation seasons in temperate climates. To improve operation sustainability, the study used landfill leachate for nitrogen source and whey permeate for phosphorus and organic carbon. A non-axenic polyculture, dominated by green algae, was cultivated in mixotrophic mode on glucose or whey permeate compared to a photoautotrophic control in outdoor pilot-scaled raceway ponds during Nordic spring and autumn. The whey permeate treatment had the highest algal growth rate and productivity (0.48 d(-1), 183.8 mg L-1 d(-1)), nutrient removal (total nitrogen: 21.71 mg L-1 d(-1), total phosphorus: 3.05 mg L-1 d(-1)) and recovery rate (carbon: 85.19 mg L-1 d(-1), nitrogen: 17.01 mg L-1 d(-1), phosphorus: 2.58 mg L-1 d(-1)). When grown in whey permeate, algal cultures demonstrated consistent productivity and biochemical composition in high (spring) and low light conditions (autumn), suggesting the feasibility of year-round production in Nordic conditions.

Microalgal cultivation for biodiesel and feed requires recycled nutrient resources for a sustainable long-term operation. Whey permeate (WP) from dairy processing contains high organic load (lactose, oils, and proteins) and nitrogen (resources tested for microalgal cultivation) and organic phosphorus (P) that has not yet been tested as a P source for microalgal cultivation. We explored the potential of green algae strains (brackish) and polyculture (freshwater) in exploiting P from WP added to a medium based on either seawater (7 psu) or landfill leachate. Both strains showed a capacity of using organic P in WP with equal growth rates (0.94-1.12 d(-1)) compared with chemical phosphate treatments (0.88-1.07 d(-1)). The polyculture had comparable growth rate (0.25-0.57 d(-1)) and biomass yield (152.1-357.5 mg L-1) and similar or higher nutrient removal rate in the leachate-WP medium (1.3-6.4 mg L-1 day(-1) nitrogen, 0.2-1.1 mg L-1 day(-1) P) compared with the leachate-chemical phosphate medium (1.2-4.7 mg L-1 day(-1) nitrogen, 0.3-1.4 mg L-1 day(-1) P). This study showed that WP is a suitable P source for microalgal cultivation over a range of salinities. To date, this is the first study demonstrating that raw WP can replace mineral P fertilizer for algal cultivation. Practitioners PointsWhey permeate is a comparable phosphorus source to standard fertilizers used in algal cultivation.Green algae removed phosphorus effectively from whey permeate.Microalgal cultivation is a good approach for treatment of whey permeate in combination with a nitrogen-rich wastewater.

We show high feasibility of using cement industrial flue gas as CO2 source for microalgal cultivation. The toxicity of cement flue gas (12-15% CO2) on algal biomass production and composition (lipids, proteins, carbohydrates) was tested using monocultures (Tetraselmis sp., green algae, Skeletonema marinoi, diatom) and natural brackish communities. The performance of a natural microalgal community dominated by spring diatoms was compared to a highly productive diatom monoculture S. marinoi fed with flue gas or air-CO2 mixture. Flue gas was not toxic to any of the microalgae tested. Instead we show high quality of microalgal biomass (lipids 20-30% DW, proteins 20-28% DW, carbohydrates 15-30% DW) and high production when cultivated with flue gas addition compared to CO2-air. Brackish Baltic Sea microalgal communities performed equally or better in terms of biomass quality and production than documented monocultures of diatom and green algae, often used in algal research and development. Hence, we conclude that microalgae should be included in biological solutions to transform waste into renewable resources in coastal waters. (C) 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

The prospect of using constructed communities of microalgae in algal cultivation wasconfirmed in this study. Three constructed communities of diatoms (Diatom), greenalgae (Green) and cyanobacteria (Cyano), were each mixed with a natural communityof microalgae as baseline. The communities were cultivated in batch and semicontinuousmode and fed CO2 or cement flue gas (12-15 % CO2). Diatom had thehighest growth rate but Green had the highest yield. Dynamic changes in thecommunity composition occurred from start through batch to semi-steady state. Greenalgae were the most competitive group during the experiment. Euglenoids wererecruited from scarce species in the natural community and became a large part of thebiomass in semi-steady state in all communities. High temporal and yield stabilitywas demonstrated in all communities during semi-steady state. Biochemicalcomposition (lipids, proteins and carbohydrates) was similar for the threecommunities with lipids ranging 14-26 % of dry weight (DW), proteins (15-28 %DW) and carbohydrates (9-23 % DW). Filamentous cyanobacteria were outcompetedearly in the experiment. However, their minute presence in Cyano associated withhigher lipid and lower carbohydrates compared to Diatom and Green, suggesting theimportance of chemical interactions among microorganisms. Our results indicate thatculture functions (stability, biomass quality) were maintained while dynamic changesoccurred in community composition. We propose that a multi-species communityapproach can aid sustainability in microalgal cultivation, through complementary useof resources and higher culture stability. Local environmental conditions,complementary microalgal traits, and interactions among functional groups (algae,bacteria) should be considered in community design where natural succession andcrop rotation will likely provide stability for commercial-scale algal cultivation.

Several marine and freshwater diatoms produce polyunsaturated aldehydes (PUA) in wound-activated processes. These metabolites are also released by intact diatom cells during algal blooms. Due to their activity in laboratory experiments, PUA are considered as potential mediators of diatom-bacteria interactions. Here, we tested the hypothesis that PUA mediate such processes in a close-to-field mesocosm experiment. Natural plankton communities enriched with Skeletonema marinoi strains that differ in their PUA production, a plankton control, and a plankton control supplemented with PUA at natural and elevated concentrations were observed. We monitored bacterial and viral abundance as well as bacterial community composition and did not observe any influence of PUA on these parameters even at elevated concentrations. We rather detected an alternation of the bacterial diversity over time and differences between the two S. marinoi strains, indicating unique dynamic bacterial communities in these algal blooms. These results suggest that factors other than PUA are of significance for interactions between diatoms and bacteria. 

Many marine dinofiagellates, including several species of the genus Alexandrium, Gymnodinium catenatum, and Pyrodinium bahamense are known for their capability to produce paralytic shellfish toxins (PST), which can cause severe, most often food-related poisoning. The recent discovery of the first PST biosynthesis genes has laid the foundation for the development of molecular detection methods for monitoring and study of PST-producing dinofiagellates. In this study, a probe-based qPCR method for the detection and quantification of the sxtA4 gene present in Alexandrium spp. and Gymnodinium catenatum was designed. The focus was on Alexandrium ostenfeldii, a species which recurrently forms dense toxic blooms in areas within the Baltic Sea. A consistent, positive correlation between the presence of sxtA4 and PST biosynthesis was observed, and the species was found to maintain PST production with an average of 6 genomic copies of sxtA4. In August 2014, A. ostenfeldii populations were studied for cell densities, PST production, as well as sxtA4 and species-specific LSU copy numbers in Foglo, Aland, Finland, where an exceptionally dense bloom, consisting of 6.3 x 10(6) cells L-1, was observed. Cell concentrations, and copy numbers of both of the target genes were positively correlated with total STX, GTX2, and GTX3 concentrations in the environment, the cell density predicting toxin concentrations with the best accuracy (Spearman's p = 0.93, p < 0.01). The results indicated that all A. ostenfeldii cells in the blooms harbored the genetic capability of PST production, making the detection of sxtA4 a good indicator of toxicity. (C) 2015 Elsevier B.V. All rights reserved.

Studies of predator-prey systems in both aquatic and terrestrial environments have shown that grazers structure the intraspecific diversity of prey species, given that the prey populations are phenotypically variable. Populations of phytoplankton have traditionally considered comprising only low intraspecific variation, hence selective grazing as a potentially structuring factor of both genetic and phenotypic diversity has not been comprehensively studied. In this study, we compared strain specific growth rates, production of polyunsaturated aldehydes, and chain length of the marine diatom Skeletonema marinoi in both grazer and non-grazer conditions by conducting monoclonal experiments. Additionally, a mesocosm experiment was performed with multiclonal experimental S. marinoi populations exposed to grazers at different levels of copepod concentration to test effects of grazer presence on diatom diversity in close to natural conditions. Our results show that distinct genotypes of a geographically restricted population exhibit variable phenotypic traits relevant to grazing interactions such as chain length and growth rates. Grazer presence affected clonal richness and evenness of multiclonal Skeletonema populations in the mesocosms, likely in conjunction with intrinsic interactions among the diatom strains. Only the production of polyunsaturated aldehydes was not affected by grazer presence. Our findings suggest that grazing can be an important factor structuring diatom population diversity in the sea and emphasize the importance of considering clonal differences when characterizing species and their role in nature.