Omics Informed Ecology of Prochlorococcus in the Eastern Tropical North Pacific Oxygen Deficient Zone

Marine Oxygen deficient zones (ODZs) are home to a community of heterotrophic and autotrophic microbes that together reduce the amount of bio-available nitrogen in the marine system through the production of N2O and N2 gas. In all three major marine ODZs, there are regions where the top of the ODZ shoals to within the photic zone, often resulting in a secondary chlorophyll maximum entirely within the anoxic zone. This secondary chlorophyll maximum is dominated by cyanobacteria, largely Prochlorococcus,the numerically dominant phototroph in the oligotrophic oceans. In this regime Prochlorococcus is photosynthesizing under very low light levels, fixing carbon and producing oxygen, with implications for the surrounding microbial community. Prochlorococcusprovide an additional fixed carbon source for heterotrophic denitrifiers in the ODZ, but they may also create microscale suboxic zonesallowing nitrifying archaea and bacteria to oxidize ammonia and nitrite. Here we use metagenomics to examine the role of Prochlorococcus in the ODZ microbial community. Nitrogen assimilation genes suggest a complex marketplace and potential role for small organic nitrogen sources as ODZ Prochlorococcus possess genes to assimilate nitrate, nitrite and urea while nitrite oxidizers (Nitrospina) possessed genes to assimilate nitrite, urea and cyanate; and ammonia-oxidizing Thaumarchaeota possessed genes to utilize urea. ODZ Prochlorococcusalso possessed a hydroxylamine reductase that may also play a role in nitrate reduction. We also identified an oxygen-independent pigment synthesis gene not found in other Prochlorococcus. Finally we examined host-derived genes carried on cyanophage in the secondary chlorophyll maxima and found they have numerous genes from the purine biosynthesis pathway rather than light or phosporus related host-derived genes as is common in cyanophage in surface waters.