Genetic and physiological characterization of denitrifying bacteria from brackishwater shrimp culture ponds of India

• We isolated and characterized denitrifying bacteria from shrimp culture ponds.
• Most isolates harbored nirS gene and belonged to the order Alteromonadales.
• Few isolates indicated existence of denitrifying bacteria with genes yet to be characterized.
• Gene nirS expression is better under anoxic than under oxic conditions.
• Marinobacter isolates carried out denitrification under oxic and anoxic conditions.

Denitrifying bacteria in brackishwater shrimp culture pond sediments were isolated and characterized from a total of eight samples. Out of the 264 isolates obtained, only 108 (40.96%) were positive for nitrate reduction and only 14 (12.96%) were positive for both nitrate and nitrite reduction. These fourteen isolates have been characterized based on complete reduction of nitrate to gas, presence of denitrification genes (nirS, nirK and nosZ), reverse transcriptase PCR for nirS gene and quantification of nitrous oxide after blocking the nitrous oxide reductase with acetylene. Out of the 14 isolates, nine were identified as Marinobacter spp., and the others belonged to Shewanella sp., Aquamicrobium sp., Marinimicrobium sp., Microbulbifer sp. and Janibacter sp. None of the denitrification genes could be detected in two of the isolates (CDN1 and 12) that reduced both nitrate and nitrite. RT-PCR analysis of all the Marinobacter isolates showed that nirS gene expression is better under anoxic than under oxic conditions indicating their ability to adapt to varying dissolved oxygen concentration. Results revealed that all the Marinobacter isolates are capable of denitrification under oxic, oxygen tolerant and anoxic conditions and indicated that nirS gene containing bacteria of the order Alteromonadales are one of the predominant denitrifying bacteria in brackishwater shrimp culture ponds of India. The study highlights the significance of such brackishwater ecosystems as study sites for future investigations on distribution and diversity of denitrifying bacteria and their role in the nitrogen cycle.