The effect of sand grain size on the development of cyanobacterial biocrusts

Biocrusts are critical components of desert ecosystems, significantly modifying the surfaces they occupy. Although the presence of fine soil particles is known to be conducive to biocrust development and recovery from disturbance, their influence on the inceptive development of biocrusts has not been empirically studied. In this study, the effect of substrate granulometry on the development of biocrusts was explored, under controlled laboratory conditions of light, soil humidity, and temperature. A cyanobacterial inoculum of Microcoleus vaginatus was applied to five sand fractions in the range of 1–2000 μm. The results showed that the biocrusts developed more rapidly on the fine fraction (<125 μm) than on the coarser fractions. While the biocrust cover on the fine fraction was spatially homogenous, it was patchy and discontinuous on the coarse fractions. The difference in the pore size between the different fractions is suggested to be the reason for these discrepancies in biocrust development, since large pores between the particles of coarse soil restrict and regulate the filaments’ spreading. It was found that the spectroscopic indices, the Normalized Difference Vegetation Index and the Brightness Index, were more sensitive to the biocrust development than the bio-physiological parameters of the biocrusts (polysaccharides, protein, and chlorophyll contents). The faster biocrust development on the fine fractions can explain various biophysical phenomena in aeolian environments.