Marine phytoplankton are central to global seascapes, acting as key conduits in element cycling and oceanic food webs. Phytoplankton cell size spans several orders of magnitude (0.2 to >200 mu m) and is an important trait that governs metabolism. However, the vast taxonomic diversity within phytoplankton size classes makes it challenging to link specific taxa to bulk community changes in productivity and elemental stoichiometry. To explore phytoplankton biogeography and biogeochemical roles in field populations, we analyzed 3 years of 16S and 18S rRNA gene amplicon sequencing variant (ASV) data alongside biochemical measurements across the dynamic latitudinal gradient of the North Pacific Transition Zone. We identified picophytoplankton community members associated with patterns in net community production (NCP), particulate organic carbon (POC), and particulate organic nitrogen (PON) and uncovered co-occurring species that may influence their growth and abundance. Multivariate linear mixed modeling revealed that the occurrence of chlorophytes explained 22.6% of NCP values, followed by stramenopiles and cyanobacteria. In contrast, POC and PON spatial patterns were best explained by chlorophyte and dinoflagellate spatial patterns. Weighted co-expression network analysis further showed NCP, POC, and PON correlations with a subset of similar to 40 ASVs belonging to chlorophytes, cyanobacteria, stramenopiles, haptophytes, and dinoflagellates that range in trophic strategy. Association network inference recapitulated these findings and revealed additional co-occurring phytoplankton, grazers, and heterotrophic bacteria. Together, our integrated computational analyses identified key picophytoplankton and co-occurring mixotrophs as major contributors to shaping regional biogeochemical dynamics in the North Pacific Ocean.

Nitrogen fixation, the microbial reduction of dinitrogen to ammonia, is increasingly recognized to occur in the Arctic Ocean. However, knowledge about the composition, biogeography, abundance, and ecology of nitrogen-fixing organisms (diazotrophs) is poor. This ultimately hinders the prediction of ecosystem productivity fueled by nitrogen fixation in this rapidly changing and predominantly nitrogen-limited ocean. We assessed the composition and abundance of total and nifH-expressing diazotrophs in subsurface water (8 m; amplicon sequencing and quantification of the marker gene nifH) over similar to 3400 km from the mouth of the brackish Baltic Sea to the sea ice edge in the Arctic Ocean. Upon entering nutrient-rich waters in the Atlantic gateway to the Arctic, we discovered an abrupt transition from autotrophic to heterotrophic diazotrophy (nifH expression). Our findings therefore suggest that diazotrophy is functionally distinct in the Arctic Ocean compared to adjacent temperate-boreal waters-a difference likely driven by inorganic nutrients, salinity, and temperature. We identify three key non-cyanobacterial diazotroph groups in the Arctic Ocean with Arctic-specific (Rhodocyclales and Oceanospirillales) or more widespread (unknown Gammaproteobacterium) distribution patterns and report their nifH gene transcription levels (up to 10³ nifH transcripts L^-1). In contrast, nifH expression in the warmer and more nutrient-poor Norwegian Sea with coastal-influenced water was dominated by sublineages of Candidatus Atelocyanobacterium thalassa (UCYN-A1, UCYN-A2, UCYN-A4; up to 10⁴ nifH transcripts L^-1). With ongoing atlantification of the Arctic pushing oceanic provinces and biogeographical ranges poleward, we predict a future displacement of the transition from autotrophic to heterotrophic diazotrophy with likely significant changes in nitrogen fixation.

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.

Due to climate change, sea ice more commonly retreats over the shelf breaks in the Arctic Ocean, impacting sea ice-pelagic-benthic coupling in the deeper basins. Nitrogen fixation (the reduction of dinitrogen gas to bioavailable ammonia by microorganisms called diazotrophs) is reported from Arctic shelf sediments but is unknown from the Arctic deep sea. We sampled five locations of deep-sea (900-1500 m) surface sediments in the central ice-covered Arctic Ocean to measure potential nitrogen fixation through long-term (> 280 days) stable-isotope (N¹⁵₂) incubations and to study diazotroph community composition through amplicon sequencing of the functional marker gene nifH. We measured low but detectable nitrogen fixation rates at the Lomonosov Ridge (0.6 pmol N g^-1 day^-1) and the Morris Jessup Rise (0.4 pmol N g^-1 day^-1). Nitrogen fixation was observed in sediments with the lowest organic matter content and bacterial abundance, and where sulphate-reducers like Desulfuromonadia and Desulfosporosinus sp. were prominent. Most nifH genes were distantly related to known diazotrophs. In this study, we show a potential for nitrogen fixation in Arctic bathypelagic sediments, considerably extending the known biome of marine nitrogen fixation. It raises the question of the significance of low but potentially widespread nitrogen fixation in deep-sea sediments.

With climate change-induced sea ice decline in the Arctic Ocean, nitrogen is expected to become an increasingly important determinant of primary productivity. Nitrogen fixation is the conversion of molecular nitrogen to bioavailable ammonium by microorganisms called diazotrophs. Here, we report nitrogen fixation rates, diazotroph composition, and expression under different stages of declining sea ice in the Central Arctic Ocean (multiyear ice, five stations) and the Eurasian Arctic (marginal ice zone, seven stations). Nitrogen fixation in the Central Arctic Ocean was positively correlated with primary production, ranging from 0.4 +/- 0.1 to 2.5 +/- 0.87 nmol N L^-1 d^-1. Along two transects across the marginal ice zone, nitrogen fixation varied between days and ice regime from below detection up to 5.3 +/- 3.65 nmol N L^-1 d^-1 associated with an ice-edge phytoplankton bloom. We show nitrogen fixation in sea ice-covered waters of the Arctic Ocean and provide insight into present and active non-cyanobacterial diazotrophs in the region.

Diazotrophs are a diverse group of microorganisms that can fertilize the ocean through biological nitrogen fixation (BNF). Due to the high energetic cost of this process, diazotrophy in nitrogen-replete regions remains enigmatic. We use multidisciplinary observations to propose a novel framework for the ecological niche construction of nitrogen fixers in the upwelling region off NW Iberia-one of the most productive coastal regions in Europe-characterized by weak and intermittent wind-driven upwelling and the presence of bays. The main diazotroph detected (UCYN-A2) was more abundant and active during summer and early autumn, coinciding with relatively high temperatures (_16 degrees C), low nitrogen:phosphorus ratios (N:P _ 7.2), and a large contribution of ammonium (_75%) to the total dissolved inorganic nitrogen available. Furthermore, nutrient amendment experiments showed that BNF is detectable when phytoplankton biomass and productivity are nitrogen limited. Seasonally recurrent biogeochemical processes driven by hydrography create an ecological niche for nitrogen fixers in this system. During the spring-summer upwelling, nondiazotroph autotrophs consume nitrate and produce organic matter inside the bays. Thereafter, the combined effect of intense remineralization on the shelf and sustained positive circulation within the bays in late summer-early autumn, conveys enhanced ammonium content and excess phosphate into the warm surface layer. The low N:P ratio confers a competitive advantage to diazotrophs since they are not restricted by nitrogen supply. The new nitrogen supply mediated by BNF could extend the productivity period, and may be a key reason why upwelling bays are more productive than upwelled offshore waters.

In recent years, new contributors to the marine silica cycle have emerged, including pico-sized phytoplankton (<2-3 μm in size) such as Synechococcus and picoeukaryotes. Their contribution and relevance to silica cycling are still under investigation. Field studies reporting the biogenic silica (bSi) standing stock in the pico-sized fraction are limited to silica-poor oligotrophic environments, and the mechanism of bSi accumulation in picoplankton remains unknown. We investigated the variability of bSi standing stocks in two size fractions (picoplankton, 0.22-3 mu m and microplankton, >3 μm) in the dissolved silica-replete Baltic Sea via biweekly time series samplings spanning 2 years. Time series data showed that the large changes in bSi standing stock in the Baltic Proper were primarily related to microplankton biomass and community composition. Meanwhile, picoplankton were, at times, surprisingly high contributors to total bSi year-round (up to 21.6%). Simultaneously, we performed microcosm incubation experiments with natural phytoplankton communities in each season to examine how nutrient additions affected bSi concentrations. In these experiments, increases in microplankton bSi were directly correlated to increases in diatom biomass, highlighting their influential role in the Baltic Sea silica cycle. Meanwhile, phosphorus additions triggered an increase in picoplankton bSi accumulation in all experiments. This uncovers a potential control of bSi accumulation in picoplankton, which can help identify the cellular mechanisms behind this process and uncover their role in silica cycling. The results link phytoplankton community composition and silica cycling, which is important for understanding the consequences of organism shifts due to climate change.

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.

Planktonic foraminifera are calcifying protists that represent a minor but important part of the pelagic microzooplankton. They are found in all of Earth's ocean basins and are widely studied in sediment records to reconstruct climatic and environmental changes throughout geological time. The Arctic Ocean is currently being transformed in response to modern climate change; however, the effect on planktonic foraminiferal populations is virtually unknown. Here, we provide the first systematic sampling of planktonic foraminifera communities in the "high" Arctic Ocean - defined in this work as areas north of 80 degrees N - specifically in the broad region located between northern Greenland (the Lincoln Sea with its adjoining fjords and the Morris Jesup Rise), the Yermak Plateau, and the North Pole. Stratified depth tows down to 1000 m using a multinet were performed to reveal the species composition and spatial variability in these communities below the summer sea ice. The average abundance in the top 200 m ranged between 15 and 65 individuals m(-3) in the central Arctic Ocean and was _0.3 individuals m(-3) in the shelf area of the Lincoln Sea. At all stations, except one site at the Yermak Plateau, assemblages consisted solely of the polar specialist Neogloboquadrina pachyderma. It predominated in the top 100 m, where it was likely feeding on phytoplankton below the ice. Near the Yermak Plateau, at the outer edge of the pack ice, rare specimens of Turborotalita quinqueloba occurred that appeared to be associated with the inflowing Atlantic Water layer. Our results would suggest that the anticipated turnover from polar to subpolar planktonic species in the perennially ice-covered part of the central Arctic Ocean has not yet occurred, in agreement with a recent meta-analysis from the Fram Strait which suggested that the increased export of sea ice is blocking the influx of Atlantic-sourced species. The presented data set will be a valuable reference for continued monitoring of the abundance and composition of planktonic foraminifera communities as they respond to the ongoing sea-ice decline and the "Atlantification" of the Arctic Ocean basin. Additionally, the results can be used to assist paleoceanographic interpretations, based on sedimented foraminifera assemblages.

Through biosilicification, organisms incorporate dissolved silica (dSi) and deposit it as biogenic silica (bSi), driving the silicon (Si) cycle in aquatic systems. While Si accumulation in marine picocyanobacteria has been recently observed, its mechanisms and ecological implications remain unclear. This study investigates biosilicification in marine and brackish picocyanobacteria of the Synechococcus clade and two model freshwater coccoid cyanobacteria. Brackish strains showed significantly higher Si quotas when supplemented with external dSi (100 mu M) compared to controls (up to 60.0 +/- 7.3 amol Si.cell-1 versus 9.2 to 16.3 +/- 2.9 amol Si.cell-1). Conversely, freshwater strains displayed no significant differences in Si quotas between dSi-enriched treatments and controls, emphasizing that not all phytoplanktons without an obligate Si requirement accumulate this element. The Si-accumulating marine and brackish picocyanobacteria clustered within the Synechococcus clade, whereas their freshwater counterparts formed a distinct sister group, suggesting a link between phylogeny and silicification. Rapid culture growth caused increased pH and led to dSi precipitation, influencing apparent dSi uptake; this was mitigated by pH control through bubbling. This phenomenon has significant implications for natural systems affected by phytoplankton blooms. In such environments, pH-induced silicon precipitation may reduce dSi availability impacting Si-dependent populations like diatoms. Our findings suggest brackish picocyanobacteria could significantly influence the Si cycle through at least two mechanisms: cellular Si accumulation and biologically induced changes in dSi concentrations.IMPORTANCEThis work provides the first evidence of biogenic silica accumulation in brackish picocyanobacteria and uncovers a link between phylogeny and biosilicification patterns. Our findings demonstrate that picocyanobacterial growth induces pH-dependent silica precipitation, which could lead to overestimations of cellular Si quotas by up to 85%. This process may drive substantial silica precipitation in highly productive freshwater and coastal marine systems, with potential effects on silica cycling and the population dynamics of Si-dependent phytoplankton. The extent of biosilicification in modern picocyanobacteria offers insights into the rock record, shedding light on the evolutionary and ecological dynamics that influence sedimentary processes and the preservation of biosilicification signatures in geological formations. Overall, this research adds to the significant impact that microorganisms lacking an obligate silica requirement may have on silica dynamics.