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We used 16s rDNA sequences to identify novel species of. cyanobacteria beneath translucent quartz pebbles in the desert pavement on an alluvial piedmont of the Coxcomb Mountains in the southern Mojave Desert, California, USA. Transmission of light, as measured with an integrating sphere, was about 0.08% beneath the thickest pieces of quartz (25 mm) harboring these hypolithic autotrophs. The photosynthetic rate ranged from 0.1 to 1.0 mumol(.)m(-2.)s(-1) in the linear range of its response to light (PAR of 0-50 mumol(.)m(-2.)s(-1)), over which the apparent quantum-use efficiency was 0.019. Light-saturated rates of 1.7-2.7 mumol(.)m(-2.)s(-1) were recorded at light intensities of 200-400 mumol(.)m(-2.)s(-1). The hypolithic community had an upper thermal tolerance of >90degreesC in laboratory conditions. The quartz pebbles confer a modest greenhouse effect that may be important for photosynthetic activity during cool, wet, wintertime periods that prevail in the Mojave Desert.