Lock-and-Key Model Incompatible With Tropical Panamanian Legumes

Abstract

Different species of legume are known to fix nitrogen (N) at vastly different rates, regardless of nutrient concentration. This N fixation provides an important ecosystem service in the world’s diverse tropical biome, but the only factor found associated with the relative fixation rates of legumes is the species itself – high fixing species will fix N regardless of nutrient levels.1,2 The rhizobia responsible for the actual fixation may dictate this fixation, however, studies on the structure and organization of the microbial communities that are responsible for this N fixation are rare. Those that exist are outdated because they use culturing and Sanger sequencing methods to identify rhizobia, which miss the potential of identifying low abundance strains of N-fixing rhizobia, as well as neglecting other members of the microbial community that are present within the nodule 3,4. To these limitations, this study used a culture-independent method and high-throughput sequencing of the 16S rRNA and nifH genes to examine the structure of the microbial community at the level of both the ecosystem and individual nodules. Our results showed that unique host-rhizobia relationships are rare, and that the overall microbial community is dominated by a few strains of Bradyrhizobia. Furthermore, Actinobacteria Amycolatopsis were found to be the second most prevalent microbes in a nodule, after Bradyrhizobia, which has been found to be the dominant N-fixing bacteria in tropical legumes. These results reveal the lack of unique, evolutionarily stable symbiotic relationships with evolutionarily old and young legume species, indicating that tropical forests of Panama favor a dynamic, competitive model of symbiosis. This information on the underlying structure of the legume-rhizobia symbiosis is imperative to understanding the functions of the associated bacteria.