Denitrification prevails over anammox in tropical mangrove sediments (Goa, India)

Denitrification, anammox (Anx) and di-nitrogen fixation were examined in two mangrove ecosystems- the anthropogenically influenced Divar and the relatively pristine Tuvem. Stratified sampling at 2 cm increments from 0-10 cm depth revealed denitrification as the main process of N2 production in mangrove sediments. At Divar, denitrification was ∼3 times higher than at Tuvem with maximum activity of 224.51±6.63 nmol N2 g -1 h -1 at 0-2 cm. Denitrifying genes (nosZ) numbered up to 2x10 7 copies g −1 sediment and belonged to uncultured microorganisms clustering within Proteobacteria. Anammox was more prominent at deeper depths (8-10 cm) mainly in Divar with highest activity of 101.15±87.73 nmol N2 g -1 h -1 which was 5 times higher than at Tuvem. Di-nitrogen fixation was detected only at Tuvem with a maximum of 12.47±8.36 nmol N2 g -1 h -1 . Thus, in these estuarine habitats prone to high nutrient input, N2-fixation is minimal and denitrification rather than Anx serves as an important mechanism for counteracting N loading.