Trophic interaction of the aerotolerant anaerobe Clostridium intestinale and the acetogen Sporomusa rhizae sp. nov. isolated from roots of the black needlerush Juncus roemerianus

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 (R[S.sup.T]) (T indicates type strain). The two spore-forming mesophiles were separated by selective cultivation under conditions favouring the growth of either RC or R[S.sup.T]. 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 R[S.sup.T] was only 96% similar to that of R[S.sup.T] and was from a species of the acetogenic genus Sporomusa, indicating that R[S.sup.T] was a new sporomusal species; the name Sporomusa rhizae sp. nov. is proposed. RC grew at the expense of saccharides. [H.sub.2]-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 [O.sub.2] and tolerated 20% [O.sub.2] in the headspace of shaken broth cultures. In contrast, R[S.sup.T] was acetogenic, utilized [H.sub.2], lactate and formate, did not utilize saccharides, and could not tolerate high concentrations of [O.sub.2]. R[S.sup.T] grew by trophic interaction with RC on saccharides via the uptake of [H.sub.2], 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 [O.sub.2]-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 [O.sub.2].