Diversity of denitrifying bacteria in the greenwater system of coastal aquaculture

Abstract

Greenwater technology is based on integrating finfishes inside pens for zero water exchange system of shrimp aquaculture. Nitrogen transformation could be improved by rearing euryhaline finfishes like grey mullet, milkfish which have a broad diet and tolerate poor water quality. The abundance of four denitrifying functional genes coding for <em>nitrate reductase</em> (<em>narG</em> and <em>napA</em>), <em>nitrite reductase</em> (<em>nirS</em>), <em>nitric</em><em>oxide reductase</em> (<em>qnorB</em>) and <em>nitrous</em><em>oxide reductase</em> (<em>nosZ</em>) has been examined in the greenwater system through a metagenomic approach. Phylogeny revealed homology of <em>narG</em> clones with uncultured environmental clones, whereas <em>napA</em> clone sequences were found to have homology with cultured (<em>Stappia aggregata</em>) and uncultured microorganisms. The <em>nirS</em> clones show uniqueness with <em>Marinobacter hydrocarbonoclasticus, Aromatoleum aromaticum,</em> and <em>Ruegeria pomeroyi</em>. The <em>qnorB</em> gene has been reported for the first time from culture systems along the Indian coast and clone exhibited 84–87 % identity with different uncultured bacteria. The <em>nosZ</em> clones are closely affiliated with <em>S. aggregate</em> and alpha bacterium. This study revealed denitrifying diversity from a greenwater system which could eventually be used in planning the future strategy for comprehending nitrogen fluxes, greenhouse gases and their mitigation in coastal aquaculture systems.