Semi-arid regions and deserts

Abstract

Summary: Phototrophic microorganisms are mostly endolithic or hypolithic in the more extreme arid environments and are here restricted to situations where sufficient moisture is retained for occasional growth to occur. Slightly less extreme environments frequently have biological soil crusts. In both cases cyanobacteria are the phototrophs most likely to be found and in some cases the only ones. In most cases of crust development Microcoleus vaginatus is one of the first cyanobacteria to occur. The crusts play an important role in maintaining soil and sand surfaces in arid regions, so it is important to understand how environmental factors influence communities at a site. In addition to light, water, temperature, salinity, nutrients, and carbon dioxide, these include wind action and physical and chemical features of the underlying substratum. Experimental studies have confirmed that some species, such as the semi-desert Nostoc flagelliforme, are extremely resistant to damage by high light and UV levels. N. flagelliforme and at least some other species require a regular cycle of hydration and dehydration. Cyanobacterial extracellular polysaccharide not only helps cells to withstand desiccation, but aids the development of crust and soil structure. Understanding of crust structure and succession has proved important in planning reclamation programmes in semi-arid regions of China using cyanobacterial inocula. Details of the procedure are described, which sometimes includes techniques to minimize the effects of wind, such as the use a checker-board arrangement of protective straw to prevent the inocula from being blown away. Reclamation of semi-arid regions in other parts of the world will require similar understanding of the ecology of the cyanobacteria involved.