Freshwater blooms of the microcystin-producing (toxigenic) cyanobacterium Microcystis may cause severe damage to human health and the environment. However, toxigenic and non-toxigenic strains co-exist within single blooms, and microcystin concentration in the water varies throughout the season. We hypothesized that strains that produce and do not produce microcystins represent different genotypes. To this end, we combined microcystin analysis with genomic analyses of individual strains isolated from a natural bloom of the morphospecies Microcystis botrys. Whole-genome assemblies revealed high genomic diversity among 19 strains, representing five different genospecies. Ten strains had intact microcystin cluster with all ten genes (mcyA-J) present and the nine remaining strains lacked all mcy genes. All non-microcystin producing strains were members of a single genospecies, while the microcystin producers were distributed across four different genospecies. One strain had all ten genes present but detectable levels of microcystin were not measured, possibly due to phenotypic plasticity in gene expression or genetic inactivation. Overall, these results demonstrate that even single-morphospecies blooms, from a single time point, consist of multiple populations of genospecies, with different microcystin genotypes and phenotypes. These findings are of great value to the field as the two common observations, presence of toxigenic and non-toxigenic strains within a single bloom and seasonal succession of toxigenicity, can now be attributed to species co-existence and species succession.

Nutrient limitation in the Baltic Proper exhibits temporal variations, with nitrogen limiting diatom and dinoflagellate-dominated spring blooms, while phosphorus constraints characterise the cyanobacterial summer blooms. Phosphorus is a key element for cellular functions and poses significant challenges for planktonic microbial communities under limited availability. Numerous studies have explored strategies phytoplankton and bacteria employ to cope with phosphorus scarcity. However, the temporal dynamics of phosphorus acquisition within natural communities remain poorly understood. Using metatranscriptomics, this study addresses this gap by examining how planktonic microbial communities acquire phosphorus over a year-long monitoring at an offshore station. Targeting genes related to phosphorus degradation, transport and membrane remodelling, we unveil diverse strategies employed by planktonic microbial communities to acquire phosphorus. Our findings highlight transporter and membrane remodelling-related genes are expressed at high levels across the year, suggesting their important role in coping with phosphorus acquisition. Our dataset reveals distinct strategies between phytoplankton and free-living bacteria under nutrient-limited conditions. While eukaryotic phytoplankton appear to rely more on recycling internal stores of phosphorus via membrane remodelling processes, free-living bacteria appear more prone to optimize extracellular scavenging mechanisms. These insights reveal the complex physiological adjustments of marine microbial communities to fluctuating phosphorus availability in the Baltic Sea.

Mixotrophic cultivation of microalgal polycultures can enhance biomass productivity and nutrient recovery from wastewaters compared to autotrophic systems. To better understand non-axenic polycultures under mixotrophic conditions, this study examined species composition and gene expression in a local freshwater green algal polyculture cultivated in outdoor raceway ponds in Southeast Sweden during spring and autumn. Cultures were cultivated under autotrophic conditions in leachate-KH₂PO₄ medium or under mixotrophic conditions with glucose or whey permeate supplementation. Community composition was analyzed using high-throughput sequencing of 18S and 16S rRNA genes, while transcriptomic responses were assessed by differential gene expression analysis. The eukaryotic community remained stable and dominated by green algae (Chlorophyta) under autotrophic conditions, whereas mixotrophic cultivation promoted increases in Ciliophora and Rotifera. Transcriptomic analysis revealed downregulation of photosynthesis-related genes alongside upregulation of nutrient transporters under mixotrophy, consistent with enhanced nutrient removal. Mixotrophic cultivation showed potential for enhanced carbohydrate accumulation and upregulated pathways for oleic acid and amino acid biosynthesis, including arginine, glutamine, glycine, proline, and asparagine. These results demonstrate that mixotrophic conditions drive both microbial community shifts and metabolic reprogramming, underscoring the importance of cultivation strategy and polyculture selection for targeted applications such as nutrient recovery and bioproduct generation.

In algal cultivation, nighttime biomass loss due to respiration and cell mortality can considerably reduce the amount of biomass produced during daylight. The adverse effect can be counteracted by mixotrophic cultivation, where an organic carbon (OC) source is used to supply the energy required for cell maintenance and division during darkness. The potential for mixotrophic cultivation to mitigate night biomass loss has yet to be tested under outdoor, large-scale conditions that use raw industrial waste streams, particularly during low-light seasons. We investigated night biomass loss in cultivation of the strain Monoraphidium minutum KAC90 in outdoor 1 m³ raceway ponds during the Nordic autumn. Flue gas condensate (nitrogen source) and cheese whey (phosphorus and OC source) were used for the mixotrophic treatment, while potassium monophosphate (phosphorus source) was used for the photoautotrophic control. Results indicate that under high OC availability, the mixotrophic treatment had a night biomass gain of 33% +/- 16%, whereas it experienced a night biomass loss of 10% +/- 9% under low OC. In contrast, the photoautotrophic control showed a night biomass loss of 5% +/- 15%. In the mixotrophic treatment, algal biomass had a higher carbohydrate content, but lower levels of lipids and proteins than the photoautotrophic cultures. The cultivation of algae using cheese whey may increase biomass accumulation in darkness, enhancing the overall production of algal biomass rich in carbohydrates.

In the marine environment, the prevailing paradigm is that larger organisms like diatoms are primary contributors to phytoplankton stoichiometry. Numerous studies investigated the stoichiometry of phytoplankton groups or total community but its dynamics among different size-groups are not resolved. In exploring the influence of phytoplankton community composition and succession on seasonal stoichiometry in the Baltic Sea, our study reveals that smaller size-groups, such as nano- and picoplankton, play a more significant role than traditionally thought. During seasonal transitions in nutrient availability—from nutrient-rich spring conditions favouring diatoms and dinoflagellates to nitrogen-limited summer conditions favourable for cyanobacteria—the Baltic Proper exhibits marked shifts in community structure and offers a unique system to investigate stoichiometric dynamics. Our yearly sampling at an offshore station using a size-fraction protocol unveils that the stoichiometry within larger size fractions (>20 µm) does not reflect the overall community's stoichiometry. Instead, nano- and picoplankton dominate nutrient cycling processes despite their smaller size. On any occasion, they represent between 55 and 90% of the biomass making them critical for nitrogen and phosphorus uptake and photosynthetic carbon fixation. These findings challenge the plankton stoichiometry paradigm and highlight the necessity to include these smaller phytoplankton groups into future climate change models to improve predictions regarding ecosystem responses to eutrophication and environmental changes.

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.

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.

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.

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.

In nature, the number of genome or chromosome copies within cells (ploidy) can vary between species and environmental conditions, potentially influencing how organisms adapt to changing environments. Although ploidy levels cannot be easily determined by standard genome sequencing, understanding ploidy is crucial for the quantitative interpretation of molecular data. Cyanobacteria are known to contain haploid, oligoploid, and polyploid species. The smallest cyanobacteria, picocyanobacteria (less than 2 μm in diameter), have a widespread distribution ranging from marine to freshwater environments, contributing significantly to global primary production. In this study, we determined the ploidy level of genetically and physiologically diverse brackish picocyanobacteria isolated from the Baltic Sea using a qPCR assay targeting the rbcL gene. The strains contained one to four genome copies per cell. The ploidy level was not linked with phylogeny based on the identity of the 16S rRNA gene. The variation of ploidy among the brackish strains was lower compared to what has been reported for freshwater strains and was more similar to what has been reported for marine strains. The potential ecological advantage of polyploidy among picocyanobacteria has yet to be described. Our study highlights the importance of considering ploidy to interpret the abundance and adaptation of brackish picocyanobacteria.