Role of free-living (or attached to non-sinking suspended particles) versus attached to fast sinking particles heterotrophic prokaryotes in the biological carbon pump at the PAP site (NE Atlantic Ocean)

The ocean’s biological carbon pump is dominated by sinking of organic marine snow from the surface ocean to its interior where carbon sequestration occurs. Most studies have shown that particle flux does not supply sufficient amount of carbon to sustain the observed biological carbon demand of organisms living in the mesopelagic. This highlights our limited understanding of the functioning of the ocean’s interior. Commonly, prokaryotes (major actors of remineralization in the mesopelagic) are sampled using Niskin bottles. However, heterotrophic prokaryotic activity of particle associated communities have yet to be constrain. Here we present field measurements of prokaryotic activities and diversity in the North Atlantic. Samples were collected using both Niskin bottles and Marine Snow Catcher (segregating particles into suspended (Susp), Fast sinking (FS) and slow sinking (SS)) pools). Carbon remineralisation rates associated to each particles pool were compared to results computed using a mesopelagic food web model (Anderson & Tang, 2010). Using 16S rDNA and 16S rRNA MiSeq sequencing, we characterized the prokaryotic community diversity associated to each pools of marine particles. Both activity and diversity show clear differences between fast-sinking particles and conventional Niskin samples. We estimate that free-living prokaryotes or attached to suspended particles prokaryotes are responsible from 82% (model) to 400% (measured) of mesopelagic remineralization of organic carbon in the study area (28-500 m). We discuss the implications of such result on discrepancies between sources and consumption of carbon in mesopelagic waters.