Plants, Fungi, and Microbes: Symbiosis in Carbon and Nitrogen Cycling

Arbuscular mycorrhizal fungi perform an important ecosystem service.

The Science

Arbuscular mycorrhizal (AM) fungi form intimate affiliations with the roots of many plant types. This classic example of symbiosis is commonly understood to involve AM fungi helping the plants take up soil nutrients. In exchange, the fungi receive some of the sugars generated by the plants from photosynthesis. Although AM fungi play a large role in carbon and nitrogen cycling in terrestrial environments, details of how they actually function remain poorly understood. In particular, the impact of AM fungi on soil microbe communities has not been examined in detail due to the difficulty of tracking nanoscale processes in complex soil habitats.

U.S. Department of Energy researchers at the University of California–Berkeley and Lawrence Livermore National Laboratory used a combination of “omics” tools and nanoscale tracking of isotopically labeled compounds to dissect interactions of AM fungi and soil microbial communities in carefully constructed soil microcosms. Plant-affiliated AM fungi were allowed to colonize small chambers containing soil samples and radiolabelled dead plant material (“litter”). The team found that the AM fungi have a significant impact on surrounding microbial community composition, increasing the abundance of microbes involved in plant litter degradation. During degradation of litter in soil, microbes play an important role in liberating nitrogen compounds bound in dead plant matter. The team observed significant uptake of microbially released nitrogen (but not carbon) by the AM fungi.

The Impact

By microarray analysis, researchers demonstrated that the AMF G. hoi significantly altered the relative abundance of approximately 10% of the soil bacterial taxa inhabiting decomposing litter. Researchers found that taxa within the phylum Firmicutes (Clostridia and Bacilli) increased in relative abundance in the presence of AMF, while taxa within the phyla Actinobacteria and Bacteroidetes decreased in the presence of AMF. These findings reveal another layer of complexity in this symbiotic system and yield another important puzzle piece towards understanding the complex routes by which carbon and nitrogen flow through ecosystems.

Principal Investigator

Mary K. Firestone
University of California–Berkeley
[email protected]

BER Program Manager

Dawn Adin

U.S. Department of Energy, Biological and Environmental Research (SC-33)
Biological Systems Science Division
[email protected]

References

Nuccio, E. E., A. Hodge, J. Pett-Ridge, D. J. Herman, P. K. Weber, and M. K. Firestone. 2013. “An Arbuscular Mycorrhizal Fungus Significantly Modifies the Soil Bacterial Community and Nitrogen Cycling During Litter Decomposition,” Environmental Microbiology, DOI: 10.1111/1462-2920.12081.