Abstract

Global patterns in diversity were estimated for cyanobacteria-dominated hypolithic communities that colonize ventral surfaces of quartz stones and are common in desert environments. A total of 64 hypolithic communities were recovered from deserts on every continent plus a tropical moisture sufficient location. Community diversity was estimated using a combined t-RFLP fingerprinting and high throughput sequencing approach. The t-RFLP analysis revealed desert communities were different from the single non-desert location. A striking pattern also emerged where Antarctic desert communities were clearly distinct from all other deserts. Some overlap in community similarity occurred for hot, cold and tundra deserts. A further observation was that the producer-consumer ratio displayed a significant negative correlation with growing season, such that shorter growing seasons supported communities with greater abundance of producers, and this pattern was independent of macroclimate. High-throughput sequencing of 16S rRNA and nifH genes from four representative samples validated the t-RFLP study and revealed patterns of taxonomic and putative diazotrophic diversity for desert communities from the Taklimakan Desert, Tibetan Plateau, Canadian Arctic and Antarctic. All communities were dominated by cyanobacteria and among these 21 taxa were potentially endemic to any given desert location. Some others occurred in all but the most extreme hot and polar deserts suggesting they were relatively less well adapted to environmental stress. The t-RFLP and sequencing data revealed the two most abundant cyanobacterial taxa were Phormidium in Antarctic and Tibetan deserts and Chroococcidiopsis in hot and cold deserts. The Arctic tundra displayed a more heterogenous cyanobacterial assemblage and this was attributed to the maritime-influenced sampling location. The most abundant heterotrophic taxa were ubiquitous among samples and belonged to the Acidobacteria, Actinobacteria, Bacteroidetes, and Proteobacteria. Sequencing using nitrogenase gene-specific primers revealed all putative diazotrophs were Proteobacteria of the orders Burkholderiales, Rhizobiales, and Rhodospirillales. We envisage cyanobacterial carbon input to the system is accompanied by nitrogen fixation largely from non-cyanobacterial taxa. Overall the results indicate desert hypoliths worldwide are dominated by cyanobacteria and that growing season is a useful predictor of their abundance. Differences in cyanobacterial taxa encountered may reflect their adaptation to different moisture availability regimes in polar and non-polar deserts.

Highlights

  • Terrestrial ecosystems that experience prolonged moisture deficit are known as deserts or drylands and they comprise the largest terrestrial biome (Laity, 2008)

  • The tropical aquatic hypolithic communities were distinct from all desert samples

  • Resolution of t-RFLP sequence fragments with an extensive hypolithic sequence library (Pointing et al, 2009) allowed identification to at least phylum but in many cases to genus level for 73 of the 89 operational taxonomic units (OTUs)

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Summary

Introduction

Terrestrial ecosystems that experience prolonged moisture deficit are known as deserts or drylands and they comprise the largest terrestrial biome (Laity, 2008). They are categorized in terms of temperature and aridity such that hot, cold, polar tundra and polar frost deserts are differentiated (Peel and Finlayson, 2007). A major focus of research has been the hypolithic microbial communities that develop as biofilms on the ventral surfaces of quartz stones (Chan et al, 2012; Pointing, 2016), since this substrate is commonly encountered as ‘desert pavement’ in extreme deserts worldwide (Laity, 2008; Thomas, 2011). The hypolithic community is clearly distinct from those in other desert niches such as soil, cryptoendolithic communities that colonize weathered porous rocks and chasmoendoliths that colonize cracks and fissures in rock (Chan et al, 2012; Wierzchos et al, 2012)

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