| Speaker: Dr. Clara Fuchsman (Assistant Professor, UMCES-HPL) Abstract: Up to half of marine N losses from N2 production occur in marine oxygen deficient zones (ODZs). Organic matter flux from surface waters is thought to be a primary control on N2 production rates. While the measured offshore primary production and export rates were nearly oligotrophic in the offshore Eastern Tropical North Pacific, published rate measurements for denitrification and anammox have demonstrated ample microbial activity in the offshore portion of this ODZ. A secondary chlorophyll a maximum located within the ODZ contained an actively growing cyanobacterial population, as determined by metaproteomic data, and this population produced additional organic matter within the ODZ, some of which was transferred to sinking particles. Flow cytometry revealed Prochlorococcus was the numerically dominant photoautotroph at the secondary chlorophyll maximum. A very high cyanophage to cyanobacteria ratio on particle-metagenomes implies a role for viruses in this in situ organic matter cycling. In samples from January 2017, the extent of virally infected Prochlorococcus was measured using a molecular method that detects viral DNA inside individual cells. Virally infected Prochlorococcus had a maximum in the ODZ, corresponding to the maximum in Prochlorococcus cells, with 2% of the community infected. Using elemental cell quotas for the ODZ Prochlorococcus, we estimate infected cells at the secondary chlorophyll maximum contained 20 nM C. If the infected portion of the Prochlorococcus population lysed every three days, the organic matter produced would be enough to sustain 5 nM/d of N2 production from denitrification. N2 production rates in the presence of sinking organic matter varied between 2-5 nM/d at that time. We propose that offshore N2 production is partly supported by autochthonous production of organic matter within the ODZ liberated by viral lysis. Host: Dr. Helen Dooley |
