Acetogens were enumerated from root homogenates of the black needlerush Juncus roemerianus obtained from a nearly pristine salt marsh. An isolated colony, ST1, yielded acetogenic activity and was initially thought to be a pure culture; however, ST1 was subsequently found to be composed of an aerotolerant fermentative anaerobe (RC) and an acetogen (RST) (T indicates type strain). The two spore-forming mesophiles were separated by selective cultivation under conditions favouring the growth of either RC or RST. The 16S rRNA gene sequence of RC was 99 % similar to that of Clostridium intestinale, indicating that RC was a new isolate of this clostridial species. The rRNA gene sequence most similar to that of RST was only 96 % similar to that of RST and was from a species of the acetogenic genus Sporomusa, indicating that RST was a new sporomusal species; the name Sporomusa rhizae sp. nov. is proposed. RC grew at the expense of saccharides. H2-forming butyrate fermentation was the primary catabolism utilized by RC under anoxic conditions, while homolactate fermentation was the primary catabolism under oxic conditions. RC consumed O2 and tolerated 20 % O2 in the headspace of shaken broth cultures. In contrast, RST was acetogenic, utilized H2, lactate and formate, did not utilize saccharides, and could not tolerate high concentrations of O2. RST grew by trophic interaction with RC on saccharides via the uptake of H2, and, to a lesser extent, lactate and formate produced by RC. Co-cultures of the two organisms yielded high amounts of acetate. These results indicate that (i) previously uncharacterized species of Sporomusa are associated with Juncus roots and (ii) trophic links to O2-consuming aerotolerant anaerobes might contribute to the in situ activities and survival strategies of acetogens in salt marsh rhizospheres, a habitat subject to gradients of plant-derived O2.