Anaerobic nitrate reduction to ammonium in two strains isolated from coastal marine sediment: A dissimilatory pathway

A total of 28 nitrate-reducing bacteria were isolated from marine sediment (Mediterranean coast of France) in which dissimilatory reduction of nitrate to ammonium (DRNA) was estimated as 80% of the overall nitrate consumption. Thirteen isolates were considered as denitrifiers and ten as dissimilatory ammonium producers. 15N ammonium production from 15N nitrate by an Enterobacter sp. and a Vibrio sp., the predominant bacteria involved in nitrate ammonification in marine sediment, was characterized in pure culture studies. For both strains studied, nitrate-limited culture (1 mM) produced ammonium as the main product of nitrate reduction (> 90%) while in the presence of 10 mM nitrate, nitrite was accumulated in the spent media and ammonia production was less efficient. Concomitantly with the dissimilation of nitrate to nitrite and ammonium the molar yield of growth on glucose increased. Metabolic products of glucose were investigated under different growth conditions. Under anaerobic conditions without nitrate, ethanol was formed as the main product; in the presence of nitrate, ethanol disappeared and acetate increased concomitantly with an increased amount of ammonium. These results indicate that nitrite reduction to ammonium allows NAD regeneration and ATP synthesis through acetate formation, instead of ethanol formation which was favoured in the absence of nitrate.