Assessing the link between zooplankton, microbes and the global biogeochemical cycles

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Crustacean zooplankton are one of most abundant macro organisms in the ocean. They are important constituents of the marine trophic food webs, channeling the newly synthesized organic matter from phytoplankton towards higher trophic levels. However, they also interact with heterotrophic microbes, providing microenvironments rich in organic matter and nutrients, low oxygen concentration and pH. Consequently, they facilitate the proliferation of specific microbial groups and promote metabolic processes that are generally underrepresented in open water of marine pelagic ecosystems, such as fermentation and metanogenesis. Therefore, zooplankton and the zooplankton-associated microbiome play an important role in the organic matter and nutrients re-cycling. However, little is known about rates and fluxes of the biogeochemical processes mediated by zooplankton and their microbiome in marine pelagic waters, where zooplankton usually reside. The proposal aims to link two aspects of the marine food web usually independently studied, zooplankton ecology and microbial oceanography. The proposed study will investigate the influence of zooplankton activity on the composition and activity of prokaryotic communities in coastal waters. We propose to use a multidisciplinary approach that combines analytical chemical and microbiological techniques to advance our understanding on the interaction between zooplankton and the main drivers of the biogeochemical cycles, the prokaryotes. Therefore, the expected results from this study should also contribute to better understand the overall impact of zooplankton and their associated microbiome on the global biogeochemical cycles. The dominant copepod groups from the southern North Sea will be collected and the community composition of the copepod-associated microbial community will be determined. Special focus will be devoted in three of the main copepod species from the coastal North Sea, i.e., Acartia tonsa, Pseudocalanus and Calanus helgolandicus. The response of the free living microbial community composition and activity to zooplankton grazing on phytoplankton will be assessed during incubation experiments, and compared to the microbial community from the ambient waters. Single cell activity measurements, metagenomics and metatranscriptomics will be used to assess the microbial groups and metabolic pathways stimulated by zooplankton grazing in comparison to ambient water microbes not exposed to zooplankton grazing by-products. Additionally, the incubation experiments will be used to assess the potential role of zooplankton grazing in nutrient re-cycling, e.g., iron, and methane release in oxygenated waters.