Temperature Drives the Continental-Scale Distribution of Key Microbes in Topsoil Communities

by Elizabeth Medford

While it has become fairly well known that global warming will cause plant and animal species to migrate toward cooler areas or cause range loses, until now it has been unclear whether this will also be true for microorganisms. Microorganisms play a key role in soil fertility and erodibility making this study relevant both for future agricultural endeavors as well as future efforts relating to ecological protection. Garcia-Pitchel et al. (2013) conducted continental-scale compositional surveys of soil crust microbial communities in the arid regions of North America. The results from these surveys imply that temperature caused latitudinal replacement between two key topsoil cyanobacteria. The cyanobacteria Microcoleus vaginatus behaved more psychrotolerant and less thermotolerant while M. steenstrupii behaved more thermotolerant. These results imply that the later may replace the former as temperature increases globally. Further studies are required to fully understand the impact of this microbial replacement.

Soil crusts, also known as biocrusts, are largely microbial communities that play crucial roles in soil management for plant interspaces in arid lands. Some of the roles of these photosynthetic assemblages include helping to stabilize the soil against erosion, modifying the hydrological properties of soil, and exporting biologically fixed carbon and nitrogen. These contributions to the desert ecosystem are important globally and locally because of the extent of arid lands. Cyanobacteria were chosen for this study because of the abundant prior descriptive work available on these bacteria that make up the biocrust’s dominant phylum. The two cyanobacteria, M. vaginatus and M. steenstrupii, dominated the phototroph community, which despite having similar generic names are not closely related phylogenetically. Despite these differences, both species are rope-formers meaning they help to stabilize soil on contact and they act as biocrust pioneers. Garcia-Pichel et al. also choose these species because of their role as keystone species in desert communities.

To evaluate the ranges of the two chosen cyanobacteria the authors performed a continental-scale survey of bacterial diversity using 16S rRNA gene diversity in community DNA. They additionally evaluated each site on soil type, geochemistry, texture, geography, and climate. Simple correlation and multiple regression analyses applied to the distribution of individual taxa helped to determine which cyanobacteria abundances correlated best with mean annual temperature. To test the temperature segregation hypothesis, the authors used two cultivation avenues, which included observing enrichment cultures at different incubated temperatures. The results from these incubations indicated that there were differential responses to high temperature between the two species. None of the M. vaginatus culture survived incubation at 40ºC while M. steenstrupii strains performed well at 35ºC. Conversely, at low temperatures (10ºC) as well as in the psychrophylic range, M. vaginatus strains grew more than the M. steenstrupii strains. This experimental data as well as the geographic distribution imply that M. vaginatus represents a more psychrotolerant taxon while M. steenstrupii is a more thermotolerant family. With this information Garcia-Pichel et al. can logically predict that a few degrees of temperature increase caused by anthropogenic climate change can cause a replacement in the dominance of M. vaginatus by M. steenstrupii especially on the cooler side of the current boundary. According to climate models, the arid southwestern parts of the United States are among the most likely to see temperature increases in the order of one degree per decade. Therefore, in fifty years all of the M. vaginatus will be replaced with mostly unknown consequences for soil management and ecological restoration efforts in arid landscapes.

Garcia-Pichel, F., Loza, V., Marusenko, Y., 2013. Temperature Drives the Continental-Scale Distribution of Key Microbes in Topsoil Communities. Science 340, pp. 1574–1577. Full paper at: http://bit.ly/1uLtfWW

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