Description: Phytoplankton forms the base of aquatic food webs and element cycling in diverse aquatic systems. The fate of phytoplankton-derived organic matter, however, often remains unresolved as it is controlled by complex, interlinked remineralization and sedimentation processes. We here investigate a rarely considered control mechanism on sinking organic matter fluxes: fungal parasites infecting phytoplankton. We demonstrate that bacterial colonization is promoted 3.5-fold on fungal-infected phytoplankton cells in comparison to non-infected cells in a cultured model pathosystem (diatom Synedra, fungal microparasite Zygophlyctis, and co-growing bacteria), and even ≥17-fold in field-sampled populations (Planktothrix, Synedra, and Fragilaria). Additional data obtained using the Synedra–Zygophlyctis model system reveals that fungal infections reduce the formation of aggregates. Moreover, carbon respiration is 2-fold higher and settling velocities are 11–48% lower for similar-sized fungal-infected vs. non-infected aggregates. Our data imply that parasites can effectively control the fate of phytoplankton-derived organic matter on a single-cell to single-aggregate scale, potentially enhancing remineralization and reducing sedimentation in freshwater and coastal systems.

Description: In the Arctic Ocean, climate change effects such as warming and ocean acidification (OA) are manifesting faster than in other regions. Yet, we are lacking a mechanistic understanding of the interactive effects of these drivers on Arctic primary producers. In the current study, one of the most abundant species of the Arctic Ocean, the prasinophyte Micromonas pusilla, was exposed to a range of different pCO2 levels at two temperatures representing realistic current and future scenarios for nutrient-replete conditions. We observed that warming and OA synergistically increased growth rates at intermediate to high pCO2 levels. Furthermore, elevated temperatures shifted the pCO2 optimum of biomass production to higher levels. Based on changes in cellular composition and photophysiology, we hypothesise that the observed synergies can be explained by beneficial effects of warming on carbon fixation in combination with facilitated carbon acquisition under OA. Our findings help to understand the higher abundances of picoeukaryotes such as M. pusilla under OA, as has been observed in many mesocosm studies.

Description: Marine snow aggregates are microhabitats for diverse microbial communities with various active metabolic pathways. Rapid recycling and symbiotic transfer of nutrients within aggregates poses a significant challenge for accurately assessing aggregate‐associated turnover rates. Although single‐cell uptake measurements are well‐established for free‐living microorganisms, suitable methods for cells embedded in marine snow are currently lacking. Comparable cell‐specific measurements within sinking pelagic aggregates would have the potential to address core questions regarding aggregate‐associated fluxes. However, the capacity to perform microscale studies is limited by the difficulty of sampling and preserving the fragile aggregate structure. Furthermore, the application of nano‐scale secondary ion mass spectrometry (NanoSIMS) to aggregates is complicated by technical requirements related to vacuum and ablation resistance. Here, we present a NanoSIMS‐optimized method for fixation, embedding, and sectioning of marine snow. Stable isotope labeling of laboratory‐generated aggregates enabled visualization of label incorporation into prokaryotic and eukaryotic cells embedded in the aggregate structure. The current method is also amenable to various staining procedures, including transparent exopolymer particles, Coomassie stainable particles, nucleic acids, and eukaryotic cytoplasm. We demonstrate the potential for using structural stains to generate three‐dimensional (3D) models of marine snow and present a simplified calculation of porosity and fractal dimension. This multipurpose method enables combined investigations of 3D aggregate structure, spatial microbial distribution, and single‐cell activity within individual aggregates and provides new possibilities for future studies on microbial interactions and elemental uptake within marine snow.

Description: The biological carbon pump is largely driven by the formation and sinking of marine snow. Because of their high organic matter content, marine snow aggregates are hotspots for microbial activity, and microbial organic matter degradation plays an important role in the attenuation of carbon fluxes to the deep sea. Our inability to examine and characterize microscale distributions of compounds making up the aggregate matrix, and of possible niches inside marine snow, has hindered our understanding of the basic processes governing marine carbon export and sequestration. To address this issue, we have adapted soft‐embedding and sectioning to study the spatial structure and components of marine aggregates at high resolution. Soft‐embedding enables rapid quantitative sampling of undisturbed marine aggregates from the water column and from sediment traps, followed by spatially resolved staining and characterization of substrates of the aggregate matrix and the microorganisms attached to it. Particular strengths of the method include in situ embedding in sediment traps and successful fluorescence in situ hybridization (FISH)‐probe labeling, supporting studies of microbial diversity and ecology. The high spatial resolution achieved by thin‐sectioning of soft‐embedded aggregates offers the possibility for improved understanding of the composition and structure of marine snow, which directly influence settling velocity, microbial colonization and diversity, degradation rates, and carbon content. Our method will help to elucidate the small‐scale processes underlying large‐scale carbon cycling in the marine environment, which is especially relevant in the context of rising anthropogenic CO2 emissions and global change.

Description: Over the past decades, two key grazers in the Southern Ocean (SO), krill and salps, have experienced drastic changes in their distribution and abundance, leading to increasing overlap of their habitats. Both species occupy different ecological niches and long-term shifts in their distributions are expected to have cascading effects on the SO ecosystem. However, studies directly comparing krill and salps are lacking. Here, we provide a direct comparison of the diet and fecal pellet composition of krill and salps using 18S metabarcoding and fatty acid markers. Neither species’ diet reflected the composition of the plankton community, suggesting that in contrast to the accepted paradigm, not only krill but also salps are selective feeders. Moreover, we found that krill and salps had broadly similar diets, potentially enhancing the competition between both species. This could be augmented by salps’ ability to rapidly reproduce in favorable conditions, posing further risks to krill populations.

Description: Climatic changes in the Southern Ocean have strong implications for the global marine carbon cycle, for example through changes in phytoplankton community composition. These shifts, in turn, can affect the strength and efficiency of the biological carbon pump, i.e. the process by which carbon is exported from the surface ocean to the deep sea via the aggregation and sinking of phytoplankton and other organic matter. At depth, carbon can be sequestered over long periods of time, effectively “buffering” increasing atmospheric CO2 concentrations. For the Southern Ocean, specifically the Weddell Sea, we only have limited data on carbon export due to the difficulties of accessing these remote and often ice-covered regions. Based on various phytoplankton bottle incubation experiments which simulated future climatic changes a possible shift in phytoplankton community composition from large diatoms to small flagellates such as Phaeocystis sp. is indicated, with unknown consequences for nutrient cycling and carbon export. To address these unknowns, we conducted in situ measurements and roller tank experiments with contrasting diatom-to-Phaeocystis ratios during a Polarstern cruise to the Southern Weddell Sea in spring 2021 to characterize aggregate formation degradation and sinking of marine snow. The same set of parameters were also assessed in controlled laboratory experiments with well-defined diatom-to-Phaeocystis ratios. Based on our results from field and laboratory, we hypothesised that a climate-mediated shift towards Phaeocystis in the future would reduce the efficiency of the biological carbon pump due to decreased silica-ballasting and increased concentrations of positively buoyant exopolymeric substances associated with Phaeocystis colonies. To our surprise, preliminary results reveal that higher Phaeocystis cell numbers relative to diatoms do not lead to a statistically significant reduction in aggregate mass density and size-specific sinking velocity. At the same time, there was a trend towards larger particles when Phaeocystis was abundant. Our results from field and laboratory observation together reveal that Phaeocystis-dominated communities do not impede carbon export in the Weddell Sea, but may actually enhance it.

Description: Due to their high organic matter content, marine snow particles are hotspots for microbial activity. The heterogeneous composition of marine snow makes microbial dynamics and microbe-substrate interactions hard to examine using standard filtration and microscopy. As spatial information is crucial to better understand these interactions, we have developed cryosectioning of frozen embedded marine snow as new tool for high-resolution 3D visualization of individual aggregates. We used this method on in situ collected marine snow to conduct a series of incubations where we compared the colonization potential of a) motile Marinobacter adhaerens and their aflagellate mutants and b) bacteria extracted from two different water depths. Surprisingly, we observed attachment and penetration for M. adhaerens with and without flagella, suggesting that bacterial motility is not the only controlling factor for aggregate colonization. Our method and findings shed new light on the role of special adaptations of aggregate-associated microorganisms and pave the way for future research on specialized microbe-substrate interactions and sequential degradation of organic compounds.