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Lowly Lichens and Algae Help Arid Lands Sequester More Carbon as the Air's CO2 Content Rises
The surfaces of many arid and semi-arid lands are covered with thin crusts of living and dead organic matter composed of lichens and/or algae plus their remains.  These cryptobiotic soil crusts, as they are called, perform a number of important functions: they reduce wind and water erosion (Evans and Johansen, 1999), help preserve soil moisture (Yair, 1990), and provide much-needed nitrogen for larger vascular plants by means of their nitrogen-fixing activities (Evans and Belnap, 1999).  Hence, these inconspicuous little plants help to stabilize shifting desert sands and provide an opportunity for higher plants to gain a foothold in regions that without them would sometimes be virtually uninhabitable.

So how do the lichens and algae of cryptobiotic soil crusts respond to increases in the air's CO2 content?  In a recent study that addressed this very question, Brostoff et al. (2002) collected pieces of algal-dominated soil crusts from dune tops and playa bottoms in the western Mojave Desert of California that were located within the confines of Edwards Air Force Base.  Bringing them back to the laboratory, they maintained the crusts in controlled environment chambers, where they determined their photosynthetic responses to variations in light intensity, crust moisture content and atmospheric CO2 concentration.

The CO2 responses were determined when all other environmental parameters were maintained at values that promoted optimal rates of net photosynthesis.  Under these conditions, Brostoff et al. observed the net photosynthetic rates of the soil crusts from both the dunes and the playas to rise in linear fashion as the air's CO2 concentration rose from 150 to 1000 ppm (which was the highest concentration they were able to test with their particular instrumentation).  In the case of the playa crusts, the net photosynthetic rate of the algae rose by a factor of two in going from the ambient CO2 concentration characteristic of their normal environment (385 ppm) to the maximum value the scientists investigated (1000 ppm), while in the case of the dune crusts, the net photosynthetic rate tripled.

The substantial photosynthetic rates of the soil crusts led the scientists to comment on "the ecosystem-wide importance of their carbon fixation."  Furthermore, "the ability of the cryptobiotic crusts to take up CO2 at much higher than normal levels," they said, "calls attention to their potentially important role in global warming studies."  Indeed, their observations clearly indicate that as the air's CO2 content rises and the perceived need to remove CO2 from the atmosphere intensifies, the cryptobiotic soil crusts of the Mojave Desert rise to the challenge and significantly increase the rate at which they do just that.

But what happens when environmental conditions are not optimal?  Lange et al. (1999) studied this situation with lichens, finding that when water contents were supra-optimal - and net photosynthesis rates were depressed below their normal maximum values because of too much water - atmospheric CO2 enrichment almost always alleviated the photosynthetic depression.  In many instances, in fact, the CO2-enriched air actually boosted the lichens' rate of CO2 uptake to 20 to 30% above the maximum values observed under optimal moisture conditions.

At the other end of the spectrum, Tuba et al. (1998) studied the effects of sub-optimal water contents, when lack of water depressed the photosynthetic rates of lichens.  In this case, a doubling of the air's CO2 content allowed photosynthetic carbon gains during experimental dry-downs to be maintained 14% longer than what was typically observed in normal air.  Furthermore, the total assimilation of carbon during the dry-downs was determined to be 50% greater in the CO2-enriched air than in the ambient air.

In light of these findings, Tuba et al. conclude that "desiccation-tolerant plants will be among the main beneficiaries of a high CO2 future."  What is doubly exciting about this observation is the fact that the high-CO2-induced benefits reaped by desiccation-tolerant cryptobiotic soil crusts will be passed on to other plants in the world's arid and semi-arid lands, due to the fact that the crusts provide (1) added protection from the erosive effects of wind and rain, (2) enhanced soil moisture retention, and (3) an augmented supply of nitrogen.  Hence, not only will the crusts themselves add to the store of carbon in these vast regions of the earth, they will encourage the growth of many other plants that would not be able to survive there without the help they provide; and these plants will remove far greater amounts of CO2 from the atmosphere than the soil crusts could ever do on their own.

Once again, we thus have another example of a multi-step biologically-modulated negative feedback phenomenon that tends to temper the tendency for CO2-induced global warming: the higher the air's CO2 content rises, the more the world's cryptobiotic soil crusts work to take carbon out of the air and make the planet a more habitable place for still other plants that can do an even better job of withdrawing CO2 from the atmosphere and sequestering its carbon in their tissues and the soils in which they grow.

How true it is that out of small things proceedeth that which is great!

Dr. Sherwood B. Idso Dr. Craig D. Idso

References
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.

Evans, R.D. and Belnap, J.  1999.  Long-term consequences of disturbance on nitrogen dynamics in an arid ecosystem.  Ecology 80: 150-160.

Evans, R.D. and Johansen, J.R.  1999.  Microbiotic crusts and ecosystem processes.  Critical Reviews in Plant Sciences 18: 183-225.

Lange, O.L., Green, T.G.A. and Reichenberger, H.  1999.  The response of lichen photosynthesis to external CO2 concentration and its interaction with thallus water-status.  Journal of Plant Physiology 154: 157-166.

Tuba, Z., Csintalan, Z., Szente, K., Nagy, Z. and Grace, J.  1998.  Carbon gains by desiccation-tolerant plants at elevated CO2Functional Ecology 12: 39-44.

Yair, A.  1990.  Runoff generation in a sandy area of the Nizzana Sands, western Negev, Israel.  Earth Surface Processes and Landforms 15: 597-609.