Endolithic Microbial Colonization of Limestone in a High-altitude Arid Environment
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The morphology of endolithic colonization in a limestone escarpment and surrounding rocky debris (termed float) at a high-altitude arid site in central Tibet was documented using scanning electron microscopy. Putative lichenized structures and extensive coccoid bacterial colonization were observed. Absolute and relative abundance of rRNA gene signatures using real-time quantitative polymerase chain reaction and phylogenetic analysis of environmental phylotypes were used to characterize community structure across all domains. Escarpment endoliths were dominated by eukaryotic phylotypes suggestive of lichenised associations (a Trebouxia lichen phycobiont and Leptodontidium lichen mycobiont), whereas float endoliths were dominated by bacterial phylotypes, including the cyanobacterium Chroococcidiopsis plus several unidentified beta proteobacteria and crenarchaea. Among a range of abiotic variables tested, ultraviolet (UV) transmittance by rock substrates was the factor best able to explain differences in community structure, with eukaryotic lichen phylotypes more abundant under conditions of greater UV-exposure compared to prokaryotes. Variously pigmented float rocks did not support significantly different communities. Estimates of in situ carbon fixation based upon 14C radio-labelled bicarbonate uptake indicated endolithic productivity of approximately 2.01 g C/m2/year−1, intermediate between estimates for Antarctic and temperate communities.
KeywordsPhotosynthetically Active Radiation Archaea Solar Noon Bacterial Phylotypes Float Rock
The authors are grateful to the Tibet Ministry of Geology for fieldwork assistance and to the Hong Kong Research Grants Council (grant numbers HKU 7375/05M and HKU 7733/08M) and Stephen Hui Trust Fund for financial support.
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