Sreenivasan and Venkataraman isolated and characterized 32 denitrifying bacteria from seawater, marine sand, and molluscs collected off the coast of South India. Twenty isolates were analyzed and classified into five groups, all belonging to the genus Pseudomonas. Two groups were proposed as new species: Pseudomonas marinodenitrifcans and Pseudomonas mytili. All isolates were capable of reducing nitrates to nitrous oxide and nitrogen gas, though at varying efficiencies. Denitrification proceeded more rapidly under anaerobic conditions. The bacteria required specific organic matter concentrations for growth and denitrification, and most showed inhibition by hydroxylamine, a proposed intermediate in nitrate reduction. The abundance of these denitrifiers in tropical marine environments contrasts with their scarcity in temperate waters like the Gulf of Maine, suggesting that plankton secretions and decomposing organic matter in the sea provide sufficient energy sources for active denitrification. This finding has implications for understanding seasonal nitrate fluctuations in tropical seawater.

Key findings

• Thirty-two denitrifying bacteria were isolated from South Indian seawater, marine sand, and molluscs; all isolates belonged to Pseudomonas species, with two proposed as new species (P. marinodenitrifcans and P. mytili)
• Denitrification occurred more rapidly under anaerobic conditions (48-72 hours) compared to aerobic conditions (7 days) for the same nitrate concentrations
• All isolates required a minimum threshold of organic matter (peptone) for both growth and denitrification, with 1500 ppm peptone enabling all cultures to denitrify
• Denitrifying bacteria were ubiquitous in South Indian marine environments, in contrast to their reported absence in temperate waters, suggesting tropical seas provide adequate organic substrates from plankton for denitrification
• The prevalence of denitrifiers may partially explain seasonal nitrate fluctuations observed in tropical seawater through nitrogen loss via denitrification