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

Abstract

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 [(14)C]bicarbonate or [(14)C]oxalate showed that CO(2) reduction to acetate and biomass or oxalate oxidation to CO(2) 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 CO(2) into biomass than oxalate-grown cells. CO(2) conversion to biomass by glyoxylate-grown cells decreased when cells were grown in the presence of nitrate. These results suggest that: (1) oxalate-grown cells prefer CO(2) 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 acetyl-CoA pathway. In this regard, enzymes of known pathways for the assimilation of two-carbon compounds were not detected in glyoxylate- or oxalate-grown cells.

Identification and Characterization of Oxalate Oxidoreductase, a Novel Thiamine Pyrophosphate-dependent 2-Oxoacid Oxidoreductase That Enables Anaerobic Growth on Oxalate

“…We have shown that M. thermoacetica metabolizes oxalate very simply by a novel CoA-independent OFOR that catalyzes the oxidative decarboxylation of oxalate to 2 mol of CO 2 , coupled to the reduction of ferredoxin (or other electron acceptors). The apparent reaction is C 2 O 4 2Ϫ ϩ Fd ox 7 2 CO 2 ϩ Fd red i , 3 although we have not yet shown whether CO 2 or bicarbonate is the product that is released from OOR. Our results are consistent with previous studies, which indicate that incorporation of oxalate into cell material requires conversion to CO 2 (3,14).…”

“…The apparent reaction is C 2 O 4 2Ϫ ϩ Fd ox 7 2 CO 2 ϩ Fd red i , 3 although we have not yet shown whether CO 2 or bicarbonate is the product that is released from OOR. Our results are consistent with previous studies, which indicate that incorporation of oxalate into cell material requires conversion to CO 2 (3,14). For example, M. thermoacetica can grow on oxalate even in CO 2 -free medium; however, when cells are grown on oxalate and CO 2 , very little radioactivity from 14 C-oxalate is incorporated into biomass (14).…”

“…Many bacteria that use oxalate as an energy source also use oxalate as a carbon source by reduction to glyoxylate, which is incorporated into the central metabolite 3-phosphoglycerate (39,40), but enzymes involved in glyoxylate incorporation have not been detected in oxalategrown cultures of M. thermoacetica (3,13). We have shown that M. thermoacetica metabolizes oxalate very simply by a novel CoA-independent OFOR that catalyzes the oxidative decarboxylation of oxalate to 2 mol of CO 2 , coupled to the reduction of ferredoxin (or other electron acceptors).…”

“…For example, M. thermoacetica can grow on oxalate even in CO 2 -free medium; however, when cells are grown on oxalate and CO 2 , very little radioactivity from 14 C-oxalate is incorporated into biomass (14). Thus, OOR enables growth on oxalate by linking the production of CO 2 and reducing equivalents to the Wood-Ljungdahl pathway of autotrophic anaerobic acetyl-CoA formation (3,14).…”

“…However, oxalate is unique in its properties as an electron donor by M. thermoacetica for acetogenic growth. When both nitrate and CO 2 are provided to cells growing on oxalate, CO 2 is reduced to acetate during exponential growth phase, and nitrate is only used as the electron acceptor during stationary phase (3). Oxalate is the only substrate with which M. thermoacetica has been shown to reduce CO 2 instead of nitrate when both are present.…”