Acetogenic bacteria: what are the in situ consequences of their diverse metabolic versatilities?

Abstract

The four decades of the now classic studies by Harland G. Wood and Lars G. Ljungdahl lead to the resolution of the autotrophic acetyl-CoA 'Wood/Ljungdahl' pathway of acetogenesis. This pathway is the hallmark of acetogens, but is also used by other bacteria, including methanogens and sulfate-reducing bacteria, for both catabolic and anabolic purposes. Thus, the pathway is wide spread in nature and plays an important role in the global turnover of carbon. Because most historical studies with acetogens focused on the biochemistry of the acetyl-CoA pathway, the metabolic diversity and ecology of acetogens remained largely unexplored for many years. Although acetogens were initially conceived to be a somewhat obscure bacteriological group with limited metabolic capabilities, it is now clear that acctogens are arguably the most metabolically diverse group of obligate anaerobes characterized to date. Their anaerobic metabolic arsenal includes the capacity to oxidize diverse substrates, including aromatic, C1, C2, and halogenated compounds, and engage a large number of alternative energy-conserving, terminal electron-accepting processes, including classic fermentations and the dissimilation of inorganic nitrogen. In this regard, one might consider acetogens on a collective basis as the pseudomonads of obligate anaerobes. By virtue of their diverse metabolic talents, acetogens can be found in essentially all habitats. This review evaluates the metabolic versatilities of acetogens relative to both the engagement (regulation) of the acetyl-CoA pathway and the ecological roles likely played by this bacteriogical group.

Sporomusa silvacetica sp. nov., an Acetogenic Bacterium Isolated from Aggregated Forest Soil

“…Nitrate, rather than CO,, is the preferred terminal electron acceptor for Clostridium thermoaceticum and C. thermoautotrophicum and inhibits the ability of these acetogens to form acetate by repressing the electron transport system normally engaged in the acetyl coenzyme A (acetyl-CoA) pathway (21,26,57). When strain DG-lT was grown in the presence of nitrate, nitrate was not appreciably utilized and acetate production was not appreciably affected, indicating that nitrate was not used as an alternative electron acceptor by DG-lT. Sulfate was also not used as an alternative electron acceptor.…”

“…A number of acetogens can utilize multiple substrates simultaneously (7,38,41). In addition, most acetogens can utilize electron acceptors other than CO, for the conservation of energy (20,21,56). Strain DG-lT used H, and the methoxyl groups of vanillate as cosubstrates, a metabolic capacity that may enhance the in situ activities of certain acetogens (38 (18,41).…”

“…The broad substrate, pH, and temperature ranges of strain DG-lT may contribute to its competitiveness in a soil-based ecosystem subject to large environmental changes and limited substrate availability. It has been proposed that acetate forms a trophic link between anaerobic and aerobic processes in certain soils (21,35,63). This hypothesis is based on the observation that acetate is a stable end product under experimentally imposed anaerobic conditions and is converted to methane only after extensive lag periods (approximately 1 to 3 months) at in situ temperatures.…”

“…This hypothesis is based on the observation that acetate is a stable end product under experimentally imposed anaerobic conditions and is converted to methane only after extensive lag periods (approximately 1 to 3 months) at in situ temperatures. In the presence of oxygen, however, anaerobically produced acetate is readily oxidized to CO, (21,35,63). The anaerobic and acetogenic capacities of aggregated soils are relatively stable during periods of 0, enrichment and aerobic drying (63); oxic leaf litter also has acetogenic capacities (36).…”

Carbonic anhydrase in Acetobacterium woodii and other acetogenic bacteria

“…CO 2 is of particular importance to the acetyl coenzyme A pathway, and many acetogens cannot grow acetogenically under either heterotrophic or autotrophic conditions in the absence of exogenous CO 2 (3,16,40). Thus, one physiological function for CA in acetogens might be to increase intracellular CO 2 levels, as has been suggested for the cyanobacterium Synechococcus sp.…”