Exploring microbial N2 O reduction: a continuous enrichment in nitrogen free medium.

N2 O is a potent greenhouse gas, but also a potent electron acceptor. In search of thermodynamically favourable - yet undescribed - metabolic pathways involving N2 O reduction, we set up a continuous microbial enrichment, inoculated with activated sludge, fed with N2 O as the sole electron acceptor and acetate as an electron donor. A nitrogen-free mineral medium was used with the intention of creating a selective pressure towards organisms that would use N2 O directly as source of nitrogen for cell synthesis. Instead, we obtained a culture dominated by microorganisms of the Rhodocyclaceae family growing by N2 O reduction to N2 coupled to N2 fixation. Biomass yields of this culture were 40% lower than those of a previously reported culture grown under comparable conditions but with an NH4+-amended medium, as expected from the extra energy expense of N2 fixation. Interestingly, we found no significant difference in yields whether N2 O or acetate was the growth-limiting substrate in the chemostat in contrast to the study with NH4+-amended medium, in which biomass yields were roughly 30% lower during acetate limiting conditions.