Copepods promote bacterial community changes in surrounding seawater through farming and nutrient enrichment

Bacteria living in the oligotrophic open ocean have various ways to survive under the pressure of nutrient limitation. Copepods, an abundant portion of the mesozooplankton, release nutrients through excretion and sloppy feeding that can support growth of surrounding bacteria. We conducted incubation experiments in the North Atlantic Subtropical Gyre to investigate the response of bacterial communities in the presence of copepods. Bacterial community composition and abundance measurements indicate that copepods have the potential to influence the microbial communities surrounding and associating with them – their ‘zoosphere’, in two ways. First, copepods may attract and support the growth of copiotrophic bacteria including representatives of Vibrionaceae, Oceanospirillales and Rhodobacteraceae in waters surrounding them. Second, copepods appear to grow specific groups of bacteria in or on the copepod body, particularly Flavobacteriaceae and Pseudoalteromonadaceae, effectively ‘farming’ them and subsequently releasing them. These distinct mechanisms provide a new view into how copepods may shape microbial communities in the open ocean. Microbial processes in the copepod zoosphere may influence estimates of oceanic bacterial biomass and in part control bacterial community composition and distribution in seawater

Dissolved organic matter stimulates N₂ fixation and nifH gene expression in Trichodesmium

Mixotrophy, the combination of heterotrophic and autotrophic nutrition modes, is emerging as the rule rather than the exception in marine photosynthetic plankton. Trichodesmium, a prominent diazotroph ubiquitous in the (sub)tropical oceans, is generally considered to obtain energy via autotrophy. While the ability of Trichodesmium to use dissolved organic phosphorus when deprived of inorganic phosphorus sources is well known, the extent to which this important cyanobacterium may benefit from other dissolved organic matter (DOM) sources is unknown. Here we provide evidence of carbon-, nitrogen- and phosphorus-rich DOM molecules enhancing N₂ fixation rates and nifH gene expression in natural Trichodesmium colonies collected at two stations in the western tropical South Pacific. Sampling at a third station located in the oligotrophic South Pacific Gyre revealed no Trichodesmium but showed presence of UCYN-B, although no nifH expression was detected. Our results suggest that Trichodesmium may behave mixotrophically in response to certain environmental conditions, providing them with metabolic plasticity and adding up to the view that mixotrophy is widespread among marine microbes

Copepod-Associated Gammaproteobacteria Respire Nitrate in the Open Ocean Surface Layers

Microbial dissimilatory nitrate reduction to nitrite, or nitrate respiration, was detected in association with copepods in the oxygenated water column of the North Atlantic subtropical waters. These unexpected rates correspond to up to 0.09 nmol N copepod−1 d−1 and demonstrate a previously unaccounted nitrogen transformation in the oceanic pelagic surface layers. Genes and transcripts for both the periplasmic and membrane associated dissimilatory nitrate reduction pathways (Nap and Nar, respectively) were detected. The napA genes and transcripts were closely related with sequences from several clades of Vibrio sp., while the closest relatives of the narG sequences were Pseudoalteromonas spp. and Alteromonas spp., many of them representing clades only distantly related to previously described cultivated bacteria. The discovered activity demonstrates a novel Gammaproteobacterial respiratory role in copepod association, presumably providing energy for these facultatively anaerobic bacteria, while supporting a reductive path of nitrogen in the oxygenated water column of the open ocean

Dancing in the Dark - the never-resting ballet of animal life under the Arctic sea ice

While the Arctic Ocean is changing rapidly, we have barely begun to understand the dynamics of animal life under the permanent sea ice, as it exists today. During the MOSAiC expedition, RV Polarstern was moored to an ice floe and drifted more than 3,000 km across the central Arctic Ocean, enabling multidisciplinary observations of the inter-linked processes in atmosphere, sea ice, ocean and ecosystem. We studied year-round changes in diversity, abundance, vertical distribution, physiology, and ontogeny of Arctic ectotherms from the pack ice at the surface into the deep ocean. Imaging profilers show the fine-scale distribution of zooplankton at high resolution, how the vertical distribution and aggregation of different species change with season, and how zooplankton species prepare for reproduction already in the deepest winter. Systematic sampling with nets shows that the pelagic food web was active from the under-ice habitat down to bathypelagic depths throughout the winter, supporting a variety of predators, such as amphipods, polar cod (Boreogadus saida), and the understudied diversity of jellyfish. The first-ever hydroacoustic survey of the Transpolar Drift recorded the change in pelagic biomass in time and space, and highlights brief periods of diel vertical migration in spring and autumn. Video surveys with a Remotely Operated Vehicle (ROV) documented the use of the under-ice habitat by polar cod and various jellyfishes. A new under-ice net mounted on the ROV can provide new insights in the connection of the life cycles of sympagic amphipods with the seasonal change of sea-ice- and water column properties. We collected thousands of samples for the analysis of condition, physiological parameters, food preference, microplastic, and trophic biomarkers, and conducted numerous rate process measurements including respiration, feeding and reproduction, for key species with the goal of unravelling the sources and fate of carbon in the food web. The first results demonstrate how sampling techniques from the days of Nansen in combination with modern technology can unfold a comprehensive picture of the contribution of Arctic fauna to ecosystem functioning and biogeochemical cycles, as well as their resilience and potential changes in a future seasonally ice-covered Arctic Ocean. We will present and discuss first results and conclusions emerging from our data with regard to the scientific objectives of MOSAiC

Multiomics in the central Arctic Ocean for benchmarking biodiversity change

Multiomics approaches need to be applied in the central Arctic Ocean to benchmark biodiversity change and to identify novel species and their genes. As part of MOSAiC, EcoOmics will therefore be essential for conservation and sustainable bioprospecting in one of the least explored ecosystems on Earth

Dissolved organic matter stimulates N2 fixation and nifH gene expression in Trichodesmium

International audienc

Energy content measurements on Arctic zooplankton collected during MOSAiC (PS122): bomb-calorific measurements

This dataset contains energy content measurements performed on zooplankton collected in the Arctic Ocean during the MOSAiC expedition (PS122) from November 2019 untill September 2020. Energy content measurements were done on Apherusa glacialis, Themisto abyssorum, Chaetognatha, Thysanoessa longicaudata and Calanus hyperboreus. These species are all known prey of polar cod (Boreogadus saida), and their energy content was measured to be included in a bioenergetic model of the growth rate of this predator and to gain insight in the differences between prey species. The meaurements were performed on freeze-dried specimens using a 6725 semi-micro oxygen calorimeter (Parr, USA) connected to a 6772 calorimetric thermometer (Parr, USA)

Energy content measurements on Arctic zooplankton collected during MOSAiC (PS122)

This dataset contains energy content measurements performed on zooplankton collected in the Arctic Ocean during the MOSAiC expedition (PS122) from November 2019 untill September 2020. Energy content measurements were done on Apherusa glacialis, Themisto abyssorum, Chaetognatha, Thysanoessa longicaudata and Calanus hyperboreus. These species are all known prey of polar cod (Boreogadus saida), and their energy content was measured to be included in a bioenergetic model of the growth rate of this predator and to gain insight in the differences between prey species. The meaurements were performed on freeze-dried specimens using a 6725 semi-micro oxygen calorimeter (Parr, USA) connected to a 6772 calorimetric thermometer (Parr, USA)