The ecophysiological basis of carbonyl sulphide (COS) gas exchange of plants with the atmosphere
The atmospheric concentration of carbonyl sulphide (COS) is around 500 ppt, with high vertical and lateral stability in the atmosphere, and having an atmospheric lifetime of several years. COS originates mainly from emission of marine ecosystems and marshlans. The main antropogenic sources are due to biomass burning and emission from the chemical industry. CS2 is a precursor, and is oxidized to a large proportion to COS in the troposphere. The vegetation of terrestrial is the main sink for COS. The sink strength for COS is proportional to the ecosystem’s photosynthetic uptake of CO2, over the growing season and for diurnal cycles. Plant carbonic anhydrase (CA) is the key enzyme for the uptake of COS in higher plants and catalyses the largely irreversible formation of CO2 and hydrogen sulphide [COS+H2O→H2S+CO2] . The main role of CA in plants is to reversibly catalyse CO2 hydration and dehydration [CO2+H2O↔HCO3-+H+], which permits CO2 to diffuse back to the atmosphere. But according to recent studies (with analyzing range of COS/CO2 uptake rations) the uptake of COS is not a direct function of photosynthesis. Thereby, a detailed analyses of the COS sinks inside the leaves, associated with a description of COS transport from the atmosphere, is need s a basis to unravel the relationship of COS and CO2 uptake by the vegetation. Also a mechanistic model, that explicitly represents the relevant processes involved, would be an important step ahead in validating COS as a tracer of the gross CO2 uptake at the ecosystem level.