Linking mesozooplankton and SAR11 bacteria in Oxygen Deficient Zones and the open ocean

The gravitational biological pump is not large enough to account for microbial heterotrophic activity in the mesopelagic ocean. Migrating zooplankton may be a key source of organic matter transport to depth. Here we show signatures of zooplankton in the suspended organic matter at the zooplankton vertical migration depth in the Eastern Tropical North Pacific Oxygen Deficient Zone (ETNP ODZ). We examine the mesozooplankton community in metagenomic depth profiles using the mitochondrial cytochrome c oxidase (COI) gene as a marker in the ETNP and Eastern Tropical South Pacific (ETSP) ODZs and at the oxic Hawaii Ocean Timeseries (HOT). Additionally, eukaryotic transcripts (polyA-selected) were examined for zooplankton in the ETNP. While zooplankton eDNA increased in the ODZ, zooplankton eRNA decreased in the ODZ, similar to previous net-based data, implying that eDNA is better preserved under anoxia. At all stations, Cnidaria, often missed in net-based data, contributed greatly to the zooplankton eDNA/eRNA. SAR11 abundance, determined from the single-copy core gene ( rpoB ), significantly correlated with zooplankton eDNA, with R2 values >0.8 at all stations. Strong correlations between SAR11 and zooplankton have not been previously reported, but are logical as SAR11 bacteria consume and zooplankton excrete simple dissolved organic compounds. SAR11 bacteria possessed genes to utilize urea and taurine in the ODZ, both compounds known to be excreted by zooplankton. In ODZs, SAR11 bacteria preferentially used the taurine degradation pathway leading to C and N assimilation, not the pathway for organic S assimilation, probably due to additional sources of organic S in ODZs. ### Competing Interest Statement The authors have declared no competing interest.