Adaptation in Haloalkaliphiles and Natronophilic Bacteria

Abstract

AbstractHaloalkaliphiles differ from natronophiles by their requirement for chloride ions in addition to high alkalinity. Natronophilic bacteria grow optimally in soda medium buffered at alkaline pH by a combination of NaHCO3 and Na2CO3. The majority of known haloalkaliphilic and natronophilic prokaryotes are isolated from saline–alkaline ecosystems such as soda lakes and saline–alkaline soils. A great taxonomic and metabolic biodiversity is found in soda systems, enabling the functioning of all the cycles of the essential elements. In spite of the increasing number of haloalkaliphilic and natronophilic isolates, scarce biochemical and functional information on simultaneous adaptation at high salinity and alkalinity is reported. Most of the available data on haloalkaline adaptation can be inferred from the functional characterization of alkaliphilic and halophilic bacterial models as well as from a few haloalkaliphilic and natronophilic genome sequences deposited in databases. At the level of cell envelopes (cell wall and cytoplasmic membrane), the salt and alkaline adaptation strategies are different and relatively conserved between Gram-positive and Gram-negative bacteria. The cell wall of the former group is characterized by the excessive presence of acidic polymers, while cell membranes abound in phospholipids with branched fatty acids. Cell membranes of salt- and alkaline-adapted Gram-negatives contain a large variety of fatty acids as well as significant amounts of nonpolar lipids. Osmotic adaptation mostly depends on the accumulation of organic compatible solutes either by active solute uptake or by combined strategies of importing osmolytes or osmolyte precursors and de novo synthesis of organic compatible solutes. Aerobic and anaerobic haloalkaliphiles are distinguished from each other by very different bioenergetics. Energy conservation in aerobic alkaliphiles and haloalkaliphiles is mainly based on functioning of H+-driven F-type ATP synthase. In spite of the low transmembrane electrochemical proton gradient (equivalent to proton-motive force, pmf) encountered in the alkali-exposed membrane, the energy metabolism remains highly efficient, supporting high growth rate and yield in many aerobic alkaliphiles and haloalkaliphiles. The energetics of haloalkaliphilic anaerobes is less understood, but it seems to involve a greater deal of Na dependency than in their aerobic counterpart. Na+-dependent ATPase activity is reported in a few anaerobic haloalkaliphiles and its role probably deals with active Na+ ejection from the cytoplasm. In haloalkaliphiles and natronophiles, the sodium-motive force (smf) is mainly driving the flagellar movement and sodium/solute symport. Cytoplasmic pH and ion homeostasis in haloalkaliphiles and natronophiles are most probably achieved by a concerted activity of a constellation of alkaline-activated ion transporters, among which Na+/H+ and Mrp-like antiporters have a major contribution.KeywordsCompatible SoluteGlycine BetaineSoda LakeHalophilic BacteriumAlkaliphilic BacillusThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.