High Diversity of Culturable Prokaryotes in a Lithifying Hypersaline Microbial Mat

In the hypersaline Lake 21 on the Kiritimati Atoll (Kiribati, Central Pacific) laminated photosynthetic microbial mats have been found that were located above a layer of reticulate microbialites and characterized by large mineral deposits in the suboxic zone of the gelatinous mat matrix. The microbial population inhabiting these mats was studied using cultivation-dependent methods. For the enrichment of strains representing functional groups that are known to predominate in hypersaline microbial mats various cultivation techniques were applied including the use of highly selective defined media as well as more universal complex media. A total of 62 strains were cultured and identified by 16S rRNA gene sequencing. The obtained isolates represented a broad spectrum of metabolic types and could be affiliated to several major phylogenetic lineages of Bacteria and Archaea, including the classes Actinobacteria, Alphaproteobacteria, Bacilli, Bacteroidia, Chlorobia, Clostridia, Cyanobacteria, Deltaproteobacteria, Deferribacteres, Flavobacteriia, Gammaproteobacteria, Halobacteria, Methanomicrobia, Phycisphaerae, Spirochaetia as well as a novel phylogenetic lineage within the Planctomyces-Verrucomicrobia-Chlamydia superphylum, represented by the isolate L21-Fru-AB. Besides photoautotrophic cyanobacteria, most of the aerobic strains cultured could be assigned to a respiratory chemoheterotrophic or mixotrophic type of metabolism. A majority of the anaerobic isolates represented either saccharolytic fermentative bacteria or incompletely-oxidizing sulfate reducers that likely drive the anaerobic decomposition processes in this habitat. The obtained isolates will be useful for the reconstruction of various defined microbial communities in the laboratory in order to test hypotheses about the relevance of certain microbial clades on mineral precipitation in hypersaline mats.