The influence of water and light on CO₂ exchange in cyanobacteria, lichen, and moss crusts at five temperature levels under controlled laboratory conditions was explored using a portable photosynthesis system. Across the range of temperatures, average optimum water content values for cyanobacteria, lichen and moss crusts were 0.38 ± 0.17 mm, 0.92 ± 0.06 mm, and 2.10 ± 0.02 mm (mean ± SE), respectively, whereas the respective average light saturation points were 900 ± 23, 870 ± 45, and 1200 ± 32 µ mol m⁻² s⁻¹. Optimum temperatures for photosynthesis were 20–27°C, 15°C, and 20°C, respectively, with maximum photosynthetic rates for the three types of crust of 2.67 ± 0.09, 3.06 ± 0.08, and 6.62 ± 0.06 µ mol m⁻² s⁻¹, respectively, under these optimum temperatures. Based on the observed data and climate information, the potential net photosynthetic carbon sequestration rates were estimated at 5.16, 3.46, and 6.05 g C m⁻² a⁻¹, respectively. These results indicate that the range of environmental conditions required for photosynthetic carbon sequestration in biological soil crusts is wide. Furthermore, moss crusts covering the smallest distribution area made the greatest contribution of C to the soil ecosystem, followed by cyanobacteria crusts, which covered the largest area.