Anaerobic Haloalkaliphiles
Authors: Sorokin Dimitry Y 
Published: August 2017

Halo‐alkali‐philes is a type of double extremophiles functioning optimally in saline brines of soda lakes. Soda lakes are a specific type of salt lakes with their brines consisting mostly of alkaline sodium carbonates. With rare exceptions, the haloalkaliphiles are prokaryotes, represented by four major metabolic blocks: aerobic and anoxygenic phototrophs as primary producers and aerobic and anaerobic chemotrophs mostly involved in mineral cycling and mineralisation of organic carbon. The fermentative anaerobes in soda lakes mostly include members of Clostridia involved in polymer hydrolysis and further fermentation of the monomers. The halolalkaliphilic denitrifiers are represented mostly by the members of Gammaproteobacteria from the genera
Halomonas
and
Alkalilimnicola/Alkalispirillum
group. The sulfidogens is the most active group of secondary anaerobes in soda lakes and include two major groups – sulfate/thiosulfate reducers from the Deltaproteobacteria with a prominent capacity for sulfite and thiosulfate disproportionation and sulfur/polysulfide reducers from the phylum Chrysiogenetes. Furthermore, soda lake natronoarchaea can also participate in polysulfide respiration. Methanogenesis in soda lakes is active with all three classical pathway represented by haloalkaliphilic species, but, similar to salt lakes, it is dominated by methylotrophs. So far, little is known about the specific bioenergetic features of the soda lake anaerobes allowing them to thrive at high pH. One of it, however, seems to be in common – the substantial use of sodium‐pumping, both by primary and secondary cation pumps.

Key Concepts

Soda lakes is not the only highly alkaline habitat on Earth, but it is the only one where the extremely high pH is stable.

Not only the high pH, but also a combination of chemical parameters created by extremely high carbonate alkalinity in brines of the soda lakes makes them a unique habitat dominated by Prokaryotic life.

The salinity caused by strongly electrolytic NaCl in pH‐neutral brines differs substantially from the salinity of weakly electrolytic sodium carbonates in its osmotic effect on salt‐tolerant organisms.

Extreme salinity and high pH impose extra energy demands on haloalkalphilic microbes, especially anaerobes with low energy‐producing metabolism. Nevertheless, all major functional groups of anaerobic microbes are present in soda lakes.

The primary organic matter in soda lakes is mostly produced by protein-rich cyanobacteria. However, the identity of haloalkaliphilic anaerobes degrading proteins in soda lake sediments is still mostly unknown.

The denitrifying haloalkaliphilies in soda lakes belong to Gammaproteobacteria but the identity of dissimilatory ammonifyers remains unknown.

Many haloalkaliphilic anaerobes isolated from soda lakes are capable of using toxic metal oxyanions as electron acceptors but very few can use Fe(III).

The most prominent property of the soda lake sulfidogens is the ability to grow by disproportionation of inorganic sulfur compounds.

The methogenesis in soda lakes is dominated by methylotrophic pathways but, in contrast to salt lakes, the other pathways are also active at haloalkaline conditions.

Low sulfide toxicity and high CO
2
and H
2
S‐absorbing capacity of the soda brines makes sulfidogenesis and methanogenesis at haloalkaline conditions attractive for application.