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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. [ABSTRACT FROM AUTHOR]

Abstract: Biological soil crusts of arid and semiarid regions are one of the least explored habitats with respect to the diversity of their fungal communities and the Arabian deserts, in particular, remains mycologically poorly investigated. Here, we investigate the diversity of free-living and lichen-forming fungal communities associated with crusts at two locations in Oman, using intensive cultivation and pyrosequencing, and their role in improving soil stability and hydrology. A total of 226 fungal isolates were recovered and phylogenetic analysis placed 98% of the isolates within the Ascomycota phylum, most of which belonged to Dothideomycetes class and Pleosporales order. The isolates were phylogenetically affiliated to 101 different species within 44 different genera with >55% of the total isolates belonging to Alternaria, Aspergillus, Cladosporium, Cochliobolus, Fusarium, Myrothecium, Phoma and Ulocladium. Using pyrosequencing, a total of 26,998 sequence reads were obtained with Ascomycota, Basidiomycota and Chytridiomycota encompassing >96% of the total sequences. In cyanobacterial crusts, between 67.2 and 70.6% of the total fungal sequences belonged to the classes Dothideomycetes and Eurotiomycetes with the dominance of yeast-like fungi of the genera Sarcinomyces and Aureobasidium. On the other hand, the sequences obtained from the lichen crusts mainly belonged to the classes Lichinomycetes, Lecanoromycetes and Eurotiomycetes. Among the identified lichens were Placidium lacinulatum, Psora decipiens, Peccania fontqueriana, Stromatella bermudana, Verrucaria chiloensis, Pecania arizonica, Lempholemma polyanthes and Lichinella cribellifera. Although detected fungi confirmed earlier trends in fungal diversity in other deserts, quite a number of isolates and sequences representing novel taxa were recovered. The presence of lichen in crusts improved their resistance to erosion and increased their water holding capacity. We conclude that desert crusts of Oman harbor a large diversity of fungal communities that varies with crust type and desert ecoregions, and the presence of lichens in these crusts improves soil characteristics. [Copyright &y& Elsevier]