Biogeochemical transformations in the oceanic environment are carried out by diverse groups of micro-organisms. These biogenic transformations are in turn responsible for the removal of fixed nitrogen through gaseous end products.   Such gaseous transformations intensify at low oxygen concentrations.  Apart from the sedimentary environments, such low oxygen concentrations exist in the water column (oxygen minimum zones, OMZ) in three regions of the world oceans, namely, Eastern Tropical North Pacific (ETNP), Eastern Tropical South Pacific (ETSP) and the Arabian Sea.  Although, these three regions together comprise only 0.1 - 0.2% of the total volume of the World Ocean, they contribute disproportionately to the atmospheric nitrous oxide budget, a greenhouse gas.  They also are hotspots that account for ~30% of the oceanic fixed nitrogen removal. It is not known, however, which organisms are responsible for the removal of fixed nitrogen from these environments, through the production of molecular nitrogen and nitrous oxide. For many years it was believed that denitrifying organisms were mainly responsible of the production of nitrous oxide and entirely for the removal of fixed nitrogen.  Recent studies, however, show that anammox bacteria could be equally, if not more important, in the nitrogen removal process, and nitrifying bacteria are implicated in the production of nitrous oxide.

We have used the signature genes that encode the enzyme nitrite reductase (nirS for the iron type enzyme and nirK for the copper type enzyme) to investigate the diversity of microbes responsible for denitrification in the Arabian Sea.  The denitrifiers in the OMZ of the Arabian Sea are not as diverse as in the coastal regions or sedimentary environments. These denitrifiers in the Arabian Sea OMZ are unique and their  genes exhibit no close similarities to the genes from organisms in culture or to denitrfiers in other denitrifying environments. Our results also indicate that the communities of these denitrifying organisms are highly dynamic in structure (number as well as diversity):  They are low in number but highly diverse during the initial stages of denitrification. Then an increase in intensity of denitrification is accompanied by a bloom in one or a few geno-types. The type that dominates the bloom varies in space and time and such blooms are responsible for the most of the removal of the fixed nitrogen. Our study in the Arabian Sea also shows that anammox organisms do not undergo such bloom dynamics and are much less abundant than the denitrifiers. Research is underway currently in the Ward lab to investigate the relative contribution of anmmox and denitrifying bacteria to the nitrogen removal process.

Ward Lab Participants: Amal Jayakumar