Insights into Prokaryotic Community and Its Potential Functions in Nitrogen Metabolism in the Bay of Bengal, a Pronounced Oxygen Minimum Zone

Abstract

Ocean oxygen minimum zones (OMZs) around the global ocean are expanding both horizontally and vertically. Multiple studies have identified the significant influence of anoxic conditions (≤1 μM O₂) on marine prokaryotic communities and biogeochemical cycling of elements. However, little attention has been paid to the expanding low-oxygen zones where the oxygen level is still above the anoxic level. Here, we studied the abundance and taxonomic and functional profiles of prokaryotic communities in the Bay of Bengal (BoB), where the oxygen concentration is barely above suboxic level (5 μM O₂). We found the sinking of <i>Trichodesmium</i> into deep water was far more efficient than that of <i>Prochlorococcus</i>, suggesting <i>Trichodesmium</i> blooms might be an essential carbon and nitrogen source for the maintenance of the BoB OMZ. In addition to the shift in the prokaryotic community composition, the abundance of some functional genes also changed with the change of oxygen concentration. Compared to oxic (>60 μM O₂) Tara Ocean and high-hypoxic (>20 to ≤60 μM O₂) BoB samples, we found more SAR11-<i>nar</i> sequences (responsible for reducing nitrate to nitrite) in low-hypoxic (>5 to ≤20 μM O₂) BoB waters. This suggested SAR11-<i>nar</i> genes would be more widespread due to the expansion of OMZs. It seems that the nitrite-N was not further reduced to nitrogen through denitrification but likely oxidized to nitrate by <i>Nitrospinae</i> in the BoB OMZ and then accumulated in the form of nitrate-N. However, the lack of N₂ production in the BoB would change if the BoB OMZ became anoxic. Together, these results suggested that reduction of oxygen concentration and OMZ expansion may increase the use of nitrate by SAR11 and N₂ production in the BoB. <b>IMPORTANCE</b> Recognizing the prokaryotic community and its functions in hypoxic (>5 to ≤60 μM O₂) environments before further expansion of OMZs is critical. We demonstrate the prokaryotic community and its potential functions in nitrogen metabolism in the Bay of Bengal (BoB), where oxygen concentration is barely above suboxic level. This study highlighted that <i>Trichodesmium</i> might be an essential carbon and nitrogen source in the maintenance of the BoB OMZ. Additionally, we suggest that the lack of N₂ production in the BoB would change if the BoB OMZ became anoxic, and the expansion of OMZs in the global ocean may potentially increase the use of nitrate by SAR11.