Microbial diversity in the deepsea anaerobic hypersaline environments with emphasis on the role of anaerobic haloarchaea in C and S cycles

Abstract

Extremely halophilic archaea represented by classes Halobacteria, Methanonatronarchaeia and candidate division Nanohaloarchaea apparently originated from two distinct clades of methanogenic anaerobes. The members of class Halobacteria within the phylum Euryarachaeota are hypothesized to acquire by a massive lateral LTG event of ~1,000 eubacterial genes (30% of their genomes). As a consequence of this acquisition, most of the known Halobacteria species are aerobic heterotrophs inhabiting oxygenated layers of brines with very few of them possessing the ability for fermentative growth and/or anaerobic respiration. Our recent research, aimed at microbial sulfur
respiration at extreme salinity in anaerobic sediments, uncovered the existence of at least two novel functional groups of strict anaerobic sulfidogenic haloarchaea.
One group is using acetate as the electron donor for elemental sulfur‐dependent respiration ‐ a catabolic route overlooked previously in the whole Archaeal
Kingdom. The second group uses formate and/or H2 as the electron donor and elemental sulfur, thiosulfate or DMSO as the alternative acceptors thus, representing
a first example of lithoheterotrophy in haloarchaea. The discovery of these groups of obligate anaerobic sulfur‐respiring haloarchaea, widely present in anoxic
hypersaline environments, including deep‐sea brine lakes, showed that (i) the carbon and sulfur cycles in anoxic hypersaline ecosystems should be reconsidered
and (ii) the dominant paradigm on the haloarchaeal physiology is far from completeness.