When it rains after a dry, hot summer in Mediterranean climates, huge pulses of CO2 are released into the atmosphere. By monitoring the activity of soil microbes after soil re-wetting, Sarah Placella at the University of California, Berkeley, and colleagues have determined that the pulse is the result of successive resuscitation of specific phylogenetic groups of microorganisms, each re-activating at a different time following the initial onset of rain.
Placella and colleagues determined which taxa of microbes were most prevalent in soil samples near the end of the dry season, and quantified the ribosomal content—as a proxy for cell metabolism—of each taxon for up to 72 hours after wetting the samples. Researchers had thought that microbes in stasis reduce their number of ribosomes, but this study suggests that at least some taxa keep ribosomal concentration high to facilitate a quick response to favorable conditions.
Depending on their taxonomic grouping, some microbes responded more slowly to wetting, which was surprising, said microbiologist Tom Schmidt of Michigan State University, who was not involved in the study.
The puzzle that remains is where the microbes get all the carbon they release in those CO2 pulses. Available soil carbon is usually depleted in arid land. One popular idea was that the sudden stress of re-wetting caused microbes to burst, releasing their carbon. But recent research shows that while some kinds of microbes die, others multiply. How the microbes acquire so much carbon so quickly is still a mystery.