If two similar species are present in an environment and occupy the same niche, competition will eventually produce a winner and a loser, leading to elimination or exclusion of one species. But niche differentiation by similar species can allow communities of similar bacteria to coexist. A report published in Environmental Microbiology reveals a new mechanism of ecological coexistence in a community — redox niche differentiation — by sympatric Achromatium species in sediments.

Fluorescence in situ hybridization (FISH) and microautoradiography are techniques that allow ecologists to correlate functional and phylogenetic information in mixed communities. Niche specialization can occur through a variety of mechanisms; spatial and temporal separation, or cross-feeding between species. Similar Achromatium species were thought to coexist owing to differences in carbon metabolism or redox chemistry. Previously, FISH studies of a natural sediment population showed that carbon metabolism was unlikely to be important, since the species studied were all mixotrophs.

Using microcosms, Gray et al. now show that altering redox conditions by using aerated or anoxic water overlaid on the sediment changed the abundance of different Achromatium species. Nitrate addition, either in one batch or by sustained additions, can mimic a wide range of redox conditions and altered the Achromatium community structure.

Redox conditions in sediments are likely to vary with sediment depth owing to diffusion of chemicals such as nitrates, so that different Achromatium strains could specialize owing to redox sensitivity and avoid competing with each other. Clearly the genetic diversity observed correlates to functional diversity, and provides a new mechanism for structuring microbial communities.