Influence of nitrate on oxalate- and glyoxylate-dependent growth and acetogenesis by Moorella thermoacetica

Oxalate and glyoxylate supported growth and acetate synthesis by Moorella thermoacetica in the presence of nitrate under basal (without yeast extract) culture conditions. In oxalate cultures, acetate formation occurred concomitant with growth and nitrate was reduced in the stationary phase. Growth in the presence of [C-14]bicarbonate or [C-14]oxalate showed that CO2 reduction to acetate and biomass or oxalate oxidation to CO2 was not affected by nitrate. However, cells engaged in oxalate-dependent acetogenesis in the presence of nitrate lacked a membranous b-type cytochrome, which was present in cells grown in the absence of nitrate. In glyoxylate cultures, growth was coupled to nitrate reduction and acetate was formed in the stationary phase after nitrate was totally consumed. In the absence of nitrate, glyoxylate-grown cells incorporated less CO2 into biomass than oxalategrown cells. CO2 conversion to biomass by glyoxylategrown cells decreased when cells were grown in the presence of nitrate. These results suggest that: (1) oxalategrown cells prefer CO2 as an electron sink and bypass the nitrate block on the acetyl-CoA pathway at the level of reductant flow and (2) glyoxylate-grown cells prefer nitrate as an electron sink and bypass the nitrate block of the acetyl-CoA pathway by assimilating carbon via an unknown process that supplements or replaces the acetylCoA pathway. In this regard, enzymes of known pathways for the assimilation of two-carbon compounds were not detected in glyoxylate- or oxalate-grown cells.