LAUSR

Abstract The impacts of oil pollution, unlike other disturbances, on the structure and activity of microorganisms in biological soil crusts have never been investigated. We exposed crust pieces from Wadi Al-Khoud, Sultanate of Oman to different oil concentrations and studied changes in their chlorophyll a (Chl a ) content, bacterial communities and photosynthetic and respiration activities. Chl a concentrations did not change in dry oil-polluted crusts but increased in the crusts exposed to oil in the presence of water. Nevertheless, extended incubation with oil for ≥3 weeks or in the presence of higher concentrations resulted in a considerable decline in Chl a in the wet polluted crusts. A remarkable shift in the bacterial community composition of oil-polluted crusts was detected, with the replacement of Cyanobacteria by Firmicutes . Sequences affiliated to Firmicutes constituted 86% of total sequences after 96 days of incubation with oil. This community shift was concomitant with alkanes degradation, as assessed by gas chromatography-mass spectrometry (GC–MS) analysis. Using oxygen microsensors, addition of oil to crusts was shown to result in an instantaneous decline in oxygen production but a quick recovery within hours of incubation. The net oxygen production and net photosynthesis decreased whereas the recovery time increased with increasing concentrations of oil. Recovery was not possible when crusts were exposed to ≥2000 mg oil g −1 crust. Incubation of crust pieces at different oil concentrations in air-tight bottles in the dark showed an increase in CO 2 evolution with increasing oil concentration. We conclude that oil pollution has drastic effects on crust microorganisms, and these effects depend on oil pollution levels, hydration conditions and duration of exposure.