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Lowly Lichens Are An Important Part of Earth's "Bio-Thermostat"
Reference
Kuhn, U. and Kesselmeier, J. 2000. Environmental variables controlling the uptake of carbonyl sulfide by lichens. Journal of Geophysical Research 105: 26,783-26,792.

Background
Carbonyl sulfide or COS - the most stable and abundant reduced sulfur gas in the atmosphere - is a major player in determining earth's radiation budget. After making its way into the stratosphere, it can be photodissociated, as well as oxidized, to form SO2, which is typically converted to sulfate aerosol particles that are highly reflective of incoming solar radiation and, therefore, have the capacity to significantly cool the earth as more and more of them collect above the tropopause. This being the case, it is only natural to suspect that biologically-modulated COS concentrations may play a role in keeping earth' surface air temperature within bounds that are suitable for the continued existence of life.

What was done
The authors collected samples of the lichen Ramalina menziesii from an open oak woodland in central California, after which they studied the lichen's uptake of COS in a dynamic cuvette system under controlled conditions in the laboratory.

What was learned
When optimally hydrated, COS was absorbed from the atmosphere by the lichens at a rate that gradually doubled as air temperature rose from approximately 3 to 25C, whereupon the rate of COS absorption dropped precipitously, reaching a value of zero at 35C.

What it means
Once air temperature rises above 25C, the rate of removal of COS from the air by this particular species of lichen declines dramatically. When this happens, more COS remains in the air, which increases the potential for more COS to make its way into the stratosphere, where it can be converted into sulfate aerosol particles that can reflect more incoming solar radiation back to space and thereby cool the earth. And since the consumption of COS by lichens is under the physiological control of carbonic anhydrase - which is the key enzyme for COS uptake in all higher plants, algae and soil organisms - we could expect this phenomenon to be generally operative over much of the earth. Hence, this biological "thermostat" may well be powerful enough to define an upper limit above which the surface air temperature of the planet may be restricted from rising, even when changes in other forcing factors, such as greenhouse gases, produce an impetus for it to do so.