Source:  Eoin Brodie and Dan Hawkes

Hopland_dry_wetThe contrasting seasons in a Mediterranean grassland.  Image: Sarah Placella

Microorganisms feature a variety of states that permit survival through harsh conditions, such as those they experience in the soil environment during a hot, dry Mediterranean-climate summer. Many microbes produce small cyst-like cells restructured from vegetative cells; others produce spores. Each response provides protection from certain kinds of environmental stress, (e.g., high heat, lack of water). Although these mechanisms may be important for surviving an extended drought, organisms that have entered a dormant state must return to an active state once the rains return. This resuscitation process can be responsible for extremely large increases in respiration (and thus CO2 release to the atmosphere). A high degree of gene coordination may be necessary for surviving the sudden deluge, as well as for entry into and resuscitation from dormant states.

F3bigWithin a Mediterranean-type ecosystem, which in California includes the area west of the Sierra Nevada down to coastal Baja California, the sudden increase in carbon dioxide released to the atmosphere from the first rainfall after a dry summer is both substantial and well documented, with the carbon released comparable to the annual carbon release of many other terrestrial systems in the world. In a recent paper, a group of Berkeley scientists, including ESD’s Eoin Brodie and Mary Firestone, show how this carbon “pulse” is produced by a three-step resuscitation of microbes (Placella et al., 2012).

Specific phylogenetic groups of microorganisms activate and contribute to the CO2 pulse at different times after a simulation of the first rainfall following a severe summer drought. Most bacteria could be classified as rapid responders (within 1 hour of rainfall), intermediate responders (between 3 and 24 hours after rainfall), or delayed responders (24–72 hours after rainfall), based on their changes in ribosomal RNA (rRNA) abundances. Notably, the rRNA quantities of rapid responders were as high in the pre-rain dry soils as at any other time, suggesting that specific groups of organisms may be poised to respond to rain, in that they preserve their capacity to synthesize proteins rapidly. Microbial response patterns displayed phylogenetic clustering, in that similar organisms display similar response strategies. This suggests that resuscitation strategies after rain following drought may be a phylogenetically conserved ecological trait.

Rainfall patterns in many parts of the world, including California, are predicted to change over the coming decades, with periods of drought and extreme rainfall events becoming more frequent. The identification in soil microbes of a few strategies for desiccation tolerance and resuscitation from desiccation suggests that understanding the controls of a few common pathways could be used to improve estimates of CO2 flux during these significant pulsed events.


Placella, S.A., E.L. Brodie, and M.K. Firestone (2012), Rainfall-induced carbon dioxide pulses result from sequential resuscitation of phylogenetically clustered microbial groups. PNAS, DOI: