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Cryptobiotic Crusts and CO2: Good News for Deserts
Brostoff, W.N., Sharifi, M.R. and Rundel, P.W.  2002.  Photosynthesis of cryptobiotic crusts in a seasonally inundated system of pans and dunes at Edwards Air Force Base, western Mojave Desert, California: laboratory studies.  Flora 197: 143-151.

What was done
Net photosynthesis rates of cryptobiotic desert-surface crusts composed of cyanobacterial algae living on dunes and playas of the Mojave Desert were measured under a variety of carefully controlled conditions to determine their responses to variations in light intensity, crust moisture content, antecedent crust moisture content, and atmospheric CO2 concentration.  In the case of atmospheric CO2, the responses of the crusts were determined when other parameters were maintained within ranges conducive to maximizing net photosynthesis.

What was learned
The photosynthetic rates of the desert-surface crusts increased in linear fashion with increasing atmospheric CO2 concentration to at least 1000 ppm, which was the upper limit of reliability of the instrumentation used in the study.  Compared to natural atmospheric CO2 values, the authors report that photosynthetic rates at 1000 ppm CO2 were doubled for the playa crusts and tripled for the dune crusts.

What it means
Although cryptobiotic crusts have often been overlooked in studies of desert ecosystems, the authors say that the substantial photosynthetic rates they observed "reiterate the ecosystem-wide importance of their carbon fixation."  Specifically, they note that "the ability of the cryptobiotic crusts to take up CO2 at much higher than normal levels calls attention to their potentially important role in global warming studies."  Indeed, because the crusts tend to reduce wind and water erosion, stabilize soil moisture, and increase the nitrogen available to other plants by boosting algal nitrogen fixation rates, it can be appreciated that the CO2-induced increases in the crusts' photosynthetic rates may significantly enhance the abilities of deserts to sequester ever greater amounts of carbon as the air's CO2 content rises ever higher.  Hence, we have yet another example of a biologically-modulated negative feedback process that tends to temper any potential for CO2-induced changes in earth's near-surface air temperature.

Reviewed 31 July 2002