The Diversity and Co-occurrence Patterns of N2-Fixing Communities in a CO2-Enriched Grassland Ecosystem
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Diazotrophs are the major organismal group responsible for atmospheric nitrogen (N2) fixation in natural ecosystems. The extensive diversity and structure of N2-fixing communities in grassland ecosystems and their responses to increasing atmospheric CO2 remain to be further explored. Through pyrosequencing of nifH gene amplicons and extraction of nifH genes from shotgun metagenomes, coupled with co-occurrence ecological network analysis approaches, we comprehensively analyzed the diazotrophic community in a grassland ecosystem exposed to elevated CO2 (eCO2) for 12 years. Long-term eCO2 increased the abundance of nifH genes but did not change the overall nifH diversity and diazotrophic community structure. Taxonomic and phylogenetic analysis of amplified nifH sequences suggested a high diversity of nifH genes in the soil ecosystem, the majority belonging to nifH clusters I and II. Co-occurrence ecological network analysis identified different co-occurrence patterns for different groups of diazotrophs, such as Azospirillum/Actinobacteria, Mesorhizobium/Conexibacter, and Bradyrhizobium/Acidobacteria. This indicated a potential attraction of non-N2-fixers by diazotrophs in the soil ecosystem. Interestingly, more complex co-occurrence patterns were found for free-living diazotrophs than commonly known symbiotic diazotrophs, which is consistent with the physical isolation nature of symbiotic diazotrophs from the environment by root nodules. The study provides novel insights into our understanding of the microbial ecology of soil diazotrophs in natural ecosystems.
KeywordsnifH Soil diazotrophs Community structure Co-occurrence patterns Elevated CO2
We thank James W. Voordeckers for editing this paper. This work is supported by the U.S. Department of Agriculture (project 2007-35319-18305) through the NSF-USDA Microbial Observatories Program, by the Department of Energy under contract DE-SC0004601 through Genomics: GTL Foundational Science, Office of Biological and Environmental Research, and by the National Science Foundation under grants DEB-0716587 and DEB-0620652 as well as grants DEB-0322057, DEB-0080382 (the Cedar Creek Long Term Ecological Research project), DEB-0218039, DEB-0219104, DEB-0217631, and DEB-0716587 (BioComplexity, LTER and LTREB projects), the DOE Program for Ecosystem Research, and the Minnesota Environment and Natural Resources Trust Fund.
Conflict of Interest
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