干旱区生态环境知识资源中心

crusts include those microbial communities living on the surface of the soil and those that live beneath semitranslucent rocks (a. k. a. hypolithic crusts). We examined the distribution, abundance, physiology, and potential soil N contributions of hypolithic, biological crusts in hyperarid ecosystems of the Baja California peninsula and islands in the midriff region of the Gulf of California, Mexico (Sonoran desert). Crusts were limited in distribution to areas with translucent quartz rocks less than 3 cm thick, were not found on areas of islands with seabird guano deposition, but covered as much as 1% (12,750 m(2)) of the surface area of one island. The percent of available rocks colonized by crusts was similar between the mainland (38%) and islands without seabird guano (26%). Carbon fixation rates in the field, which have not been previously reported, ranged between 0 and 1.23 mu mol m(-2) s(-1), and in the lab ranged between 0.66 and 0.94 mu mol m(-2) s(-1). Evidence of low rates of N fixation was inferred from delta N-15 values of crust and soil. Hypolithic crusts were found to have minimal, if any, influence on soil salinity, pH, and NO3-, but may represent up to 14% of the biomass of primary producers on these islands and provide C and N to the belowground and possibly aboveground heterotrophic communities where crusts exist. The results of this study suggest a limited but potentially important contribution of hypolithic soil crusts to hyperarid ecosystems.