Alternative Nitrogen Fixation and Its Role In Nutrient Cycling

The fixation of nitrogen by diazotrophic bacteria is a crucial process in global nutrient cycling. It plays an especially important role in the development of microbial communities that are involved in pedogenesis. Despite its significance, the particular pathways of nitrogen fixation employed by bacteria in nutrient-poor environments (e.g., rock outcrops) are not well understood. The most common and efficient pathway is dependent upon molybdenum, while an alternative pathway uses iron in its place. Iron is generally much more available than molybdenum in nutrient-poor and/or acidic environments, suggesting that the alternative iron-dependent pathway may be crucial to nitrogen fixation in such environments. This study provides evidence for the presence of both the molybdenum-dependent and iron-dependent nitrogen fixation systems in bacteria associated with lichens, some of the first and most crucial organisms in primary biological succession. These findings not only contribute greatly to our understanding of lichen ecology and physiology, but also provide evidence that alternative nitrogen fixation may play a larger role in global nutrient cycling than previously demonstrated.