In the heterotrophic growth mode they can ferment glucose and derive some ATP by substrate level phosphorylation. In so doing they generate carbon dioxide and hydrogen which can then be used to power the chemiosmotic mechanism which allows them to derive some ATP also by anaerobic respiration.
The overall stoichiometry of this growth mode of homoacetogens is shown below.
The first equation describes fermentation of glucose where ATP is generated by substrate level phosphorylation. The second equation shows the reduction of carbon dioxide where ATP is generated by anaerobic respiration. The third equation shows the sum of the two stoichiometries.
The acetate produced is an important substrate for intermediary metabolism in the host ruminant.
Surprisingly the presence of homoacetogens has only recently been reported in the reticulo-rumen as attested by the fact that the index to the most recent (second edition) of The Rumen Microbial Ecosystem by Hobson and Stewart has only one entry for the word acetogens; and there is only brief discussion of acetate production by two species of bacteria in this group. A much more thorough analysis is given in Fenchel and Finlay's monograph; this being due to the wider scope of their coverage which includes non-rumen anaerobic environments.
One interesting element in the analysis presented by Fenchel and Finlay relates to the competition in different growth environments between acetogens and methanogens for the hydrogen and carbon dioxide on which each can grow.
The reasons for one group predominating over the other in different
growth environments appears to be obscure. In the reticulo-rumen
it is clear that methanogens
generally predominate, but the activity of homoacetogens in the reticulo-rumen
contributes to explaining the importance of acetate
as an end product of the microbial metabolism of the host ruminant feedstock.