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Abstract
Biocrust, the top layer of soil in arid environments hosting diverse microbial assemblages, covers 12% of Earth’s terrestrial surface. Mojave Desert soils, due to extreme dryness and heat, are challenging, polyextreme (i.e.: high temperature, high salinity, extreme pH levels, and low nutrient) habitats. Biocrust, depending on the dominant local photoautotroph lineage, can develop into taxonomically distinct microbial assemblages and, consequently, functionally distinct microhabitats. In other words, bryophyte versus cyanobacterial biocrust dominance, as primary photoautotrophs, significantly changes microbial biodiversity and predicted community functions. In the Mojave Desert additional photic subniches for biocrust occur when these communities happen to be covered by transluminescent quartz stones. These hypolithic environments generally shelter the biocrust organisms from extreme desiccation and heat. In other sunlit extreme environments, for example in oligotrophic ocean gyres and Antarctic Dry Valleys, photoautotrophic and heterotrophic microbial communities may supplement their limited energy quotas via proteorhodopsin-based photoheterotrophy. Proteorhodopsins are small, inner membrane-embedded, proton pumps in prokaryotes that can aid in the generation of chemiosmotic potential resulting in ATP generation by oxidative phosphorylation: a newly described light-driven energy generation pathway known as photo- heterotrophy. Based on the prevalence of photoheterotrophy across light imbued extreme environments, this metabolism may also be a viable strategy in the polyextreme environment of Mojave Desert Biocrusts. To explore this question, I analyzed 60 metagenomes generated from Mojave Desert soil samples spanning bryophyte- and cyanobacteria-dominated surface and hypolithic niches. My large-scale survey employs machine-learning approaches to scan hundreds of predicted proteins as queries against a libraries of profile hidden Markov models of photobiological genes, including proteorhodopsins. Here, I report the presence, prevalence, spectral tunning, and taxonomic provenance of proteorhodopsin gene homologues from various photic soil niches and thereby expand the metabolic repertoire that enables microbial survival in sublithic niches of the Mojave Desert, one of the most extreme environments on Earth.
