Question

How is the S layer of the halophile is relevant for adaptation to the saline environment?

Answer 1

Their cellular machinery is adapted to high salt concentrations by having charged amino acids on their surfaces, allowing the retention of water molecules around these components. Most halophiles are unable to survive outside their high-salt native environments

The world of halophilic microorganisms is highly diverse. Microbes adapted to life at high salt concentrations are found in all three domains of life: Archaea, Bacteria, and Eucarya. In some ecosystems salt-loving microorganisms live in such large numbers that their presence can be recognized without the need for a microscope. The brines of saltern crystallizer ponds worldwide are colored pink-red by Archaea (Haloquadratum and other representatives of the Halobacteriales), Bacteria (Salinibacter), and Eucarya (Dunaliella salina).

Hypersaline environments such as saltern pond brines and natural salt lakes present the ecologist with relatively simple ecosystems with low diversity and high community densities. In such systems fundamental questions of biodiversity, selection, biogeography, and evolution in the microbial world can be investigated much more conveniently than in the far more complex freshwater and marine systems. The sediments of such water bodies, however, are often inhabited by extremely diverse, still incompletely explored microbial communities. Different types of halophiles have solved the problem how to cope with salt stress (and often with other forms of stress as well) in different ways, so that the study of microbial life at high salt concentrations can answer many basic questions on the adaptation of microorganisms to their environments. Most known halophiles are relatively easy to grow, and genera such as Halobacterium, Haloferax, and Haloarcula have become popular models for studies of the archaeal domain as they are much simpler to handle than methanogenic and hyperthermophilic Archaea.

Most habitats explored for the presence of halophiles are thalassohaline environments that originated by evaporation from seawater, reflect the ionic composition of seawater, and have a nearly neutral to slightly alkaline pH.In many athalassohaline environments, life at the extremes of high salt is combined with the need to thrive at alkaline pH and elevated temperatures, and organisms growing there do so at the physico-chemical boundary for life.The Dead Sea is a rare example of a low-Na+, high-Mg2+, and high-Ca2+ chloride brine with a slightly acidic pH. Metagenomic studies are now providing information on the microbial diversity in the lake, both at the time of a bloom of microorganisms following dilution of the upper water layers by rain floods in 1992 and during the current drying out of the lake, causing a continuously decreasing ratio of monovalent/divalent cations, making conditions too extreme for even the best salt-adapted microorganisms

Although studies on both archaeal and bacterial S-layer proteins have commonly reported an acidic isoelectric point, a much higher alkaline pI value (9.4–10.4) has been detected in lactobacilli .Other commonly reported S-layer protein features include the occurrence of 50–60% of hydrophobic amino acids and few sulfur-containing amino acid residues . Many known S-layer proteins can be N- or O-glycosylated, usually occurring on Asp and Ser or Thr residues .

This review highlights the various strategies adopted by halophiles to compensate for their saline surroundings and includes descriptions of recent studies that have used these microorganisms for bioremediation of environments contaminated by petroleum hydrocarbons. The known halo-tolerant dehalogenase-producing microbes, their dehalogenation mechanisms, and how their proteins are stabilized is also reviewed. In view of their robustness in saline environments, efforts to document their full potential regarding remediation of contaminated hypersaline ecosystems merits further exploration.

Microorganisms that are not adapted to highly saline environments will lose water, causing the cells to first shrivel and subsequently fatal loss of cellular structure as well as function . To avoid excessive water loss under such conditions, halophiles have evolved two distinct strategies to increase the osmotic activity of their cytoplasm with the external environment, either producingcompatible organic solutes or reaching an equilibrium state in which the overall salt concentration within cells matches that of the environment by accumulating large salt concentrations in their cytoplasm.