N2O emission by denitrifying bacteria isolated from freshwater environments by using a medium with low nitrate
1Tokyo University of Agriculture and Technology, 2Chiba University
Density of denitrifying bacteria in freshwater environments enumerated by MPN method using a medium (LC medium) containing 0.5 mM of nitrate and 80 mg C L-1 of organic substrates was 5-100 times higher than conventional rich medium, nutrient broth (NB) containing 5 mM of nitrate and more than 1 g C L-1 of organic substrates. Denitrifying bacteria isolated by using the LC medium belonged to genus Vogessella, Acidovorax, or family Oxalobacteraceae. Isolated denitrifying bacteria did not grow on agar plate nor indicate denitrifying activities when cultured in the rich medium. Although almost isolates possesses the N2O reductase gene (nosZ), they produced N2O as terminal gas of denitrification under anoxic condition (ORP, from -25 to -40 mV) in the LC medium. However, under anaerobic condition (ORP, -280 mV), 6 of 8 isolates sometimes produced N2 as terminal gas. These results suggested that some denitrifying bacteria in freshwater environments have been overlooked when the rich medium have been used. These denitrifying bacteria are considered to be highly sensitive to production of N2 as terminal gas.