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Diversity of nitrite oxidizers in greenwater system of coastal aquaculture by metagenomic approach targeting NxrB genes

V.Kathiravan, K.K.Krishnani, I.S.Azad, M.Kailasam, A.G.Ponniah

Greenwater culture system is based on the integrated culture of economically important fish in pens inside shrimp culture ponds. However, the mechanisms on how the system works to bioremediate chemical and microbial pollutants are still unclear. Nitrite oxidizing bacteria (NOB) are an essential functional group for oxidation of nitrite into less toxic nitrate. The gene NxrB is unique to NOB, which encodes nitrite oxidoreductase enzyme that helps in oxidation of nitrite into nitrate. In the present study, the diversity of NOB was analyzed using the nxrB gene as a functional marker for screening constructed metagenomic library. The clones obtained had less similarity with NxrB of Nitrobacter sp. PJN1 and N. hamburgensis. However, the phylogenetic analysis showed that the clones obtained exhibit similarity with the sequences of nitrate reductase (nar) from Nocardioides sp. and Mycobacterium sp. of Acitinobacteria group. These results indicate that NOB population in a greenwater system of coastal aquaculture is much more diverse, and thus could be useful for planning a bioremediation strategy for nitrogenous metabolites and also for understanding of nitrogen fluxes generated through nitrite oxidation in coastal shrimp aquaculture.