Bioenergetic Aspects of Halophilism

Abstract

SUMMARY Examinination of microbial diversity in environments of increasing salt concentrations indicates that certain types of dissimilatory metabolism do not occur at the highest salinities. Examples are methanogenesis for H2 + CO2 or from acetate, dissimilatory sulfate reduction with oxidation of acetate, and autotrophic nitrification. Occurrence of the different metabolic types is correlated with the free-energy change associated with the dissimilatory reactions. Life at high salt concentrations is energetically expensive. Most bacteria and also the methanogenic archaea produce high intracellular concentrations of organic osmotic solutes at a high energetic cost. All halophilic microorganisms expend large amounts of energy to maintain steep gradients of NA+ and K+ concentrations across their cytoplasmic membrane. The energetic cost of salt adaptation probably dictates what types of metabolism can support life at the highest salt concentrations. Use of KCl as an intracellular solute, while requiring far-reaching adaptations of the intracellular machinery, is energetically more favorable than production of organic-compatible solutes. This may explain why the anaerobic halophilic fermentative bacteria (order Haloanaerobiales) use this strategy and also why halophilic homoacetogenic bacteria that produce acetate from H2 + CO2 exist whereas methanogens that use the same substrates in a reaction with a similar free-energy yield do not.

Unveiling microbial life in the new deep‐sea hypersaline Lake Thetis. Part II: a metagenomic study

“…all associated with methanogenic Euryarchaeota, were also found (Table S1). This finding corroborates the general statement that high salinities favour hydrogenoand/or methylotrophic methanogenesis (Oren, 1999). Presence of such Euryarchaeota in the Thetis brine was shown recently by a 16S rRNA survey as well as by direct cultivation (La Cono et al, 2011).…”

Shifts in methanogen community structure and function associated with long‐term manipulation of sulfate and salinity in a hypersaline microbial mat

“…The dominance of methylotrophic methanogens may be particularly characteristic of hypersaline cyanobacterial mats, where rapid mineralization rates below the photic zone lead to high turnover of dead cells, and the subsequent release of their osmoregulatory solutes (King, 1988), such as glycine betaine, which are readily converted to methanogenic precursors, such as trimethylamine (King, 1984;Oren, 1990;1999;Ollivier et al, 1994). The clear increases in methane production following additions of methylated amines strongly suggest this to be the case in these mats, under normal sulfate concentrations.…”

Modifying Post‐Translational Modifications: A Strategy Used by Archaea for Adapting to Changing Environments?

“…Rather than devoting resources to preventing high levels of salt from entering their cytoplasm, the haloarchaeal cell interior instead contains concentrations of salt comparable to the external surroundings. [25] For instance, the cytoplasm of Halobacterium salinarum, best known as the source of the light-driven proton pump bacteriorhodopsin, [26] contains 4-5 m KCl. [27] Normally, such high levels of salt would interfere with protein folding due the excess of ions impeding with the formation of electrostatic interactions between positively and negatively charged amino acid residues that help a polypeptide realize its final 3D conformation.…”

“…Today, it is known that Archaea perform a plethora of post-translational modifications (PTMs), including glycosylation, phosphorylation, and acetylation. [19][20][21][22][23][24][25] Table 2 lists PTMs recognized to date in Archaea and examples of species in which such modifications have been reported. In many instances, the presence of PTMs has been implicated in the ability of archaeal proteins to withstand the extremes that they encounter.…”