Unexpected diversity and complexity of the Guerrero Negro hypersaline microbial mat

Abstract

We applied nucleic acids-based molecular methods, combined with estimates of biomass (ATP), pigments and microelectrode measurements of chemical gradients, to map microbial diversity vertically on the mm-scale in a hypersaline microbial mat from Guerrero Negro, Baja California Sur, Mexico. To identify the constituents of the mat, small-subunit ribosomal RNA genes were amplified by PCR from community genomic DNA extracted from layers, cloned and sequenced. Bacteria dominated the mat and displayed unexpected and unprecedented diversity. The majority (1336) of 1586 bacterial 16S rRNA sequences generated were unique, representing 752 species (≥97% rRNA sequence identity) in 42 of the main bacterial phyla, including 15 novel candidate phyla. The diversity of the mat samples differentiated according to the chemical milieu defined by concentrations of O2 and H2S. Chloroflexi formed the majority of the biomass by percentage of bulk rRNA and of clones in rRNA gene libraries. This result contradicts the general belief that Cyanobacteria dominate these communities. Although Cyanobacteria constituted a large fraction of the biomass in the upper few mm (>80% of total rRNA and photosynthetic pigments), Chloroflexi sequences were conspicuous throughout the mat. Filamentous Chloroflexi were identified by fluorescent in-situ hybridization within the polysaccharide sheaths of the prominent cyanobacterium Microcoleus chthonoplastes in addition to free-living in the mat. The biological complexity of the mat far exceeds that observed in other polysaccharide-rich microbial ecosystems, such as human and mouse distal guts, and suggests that positive feedbacks exist between chemical complexity and biological diversity.