Alkali tolerance

from Wikipedia, the free encyclopedia

Alkali tolerance is a term from biology and describes the ability of certain organisms to tolerate increasing pH values in the habitat. The pH optimum of such organisms is usually neutral to weakly alkaline . Occasionally, however, these organisms can also be found in environments with significantly increased pH values. They have the ability to adapt and to tolerate pH values ​​over a wide range.

Occurrences and names

Alkali-tolerant organisms can be found, for example, in lakes with periodically fluctuating water levels or drying out soils whose pH values ​​are subject to fluctuations depending on the precipitation situation and salt concentration. Many alkali-tolerant organisms are therefore also halo- tolerant .

Organisms that are bound to an alkaline environment are called alkaliphilic . However, the boundary between alkali-tolerant and alkali-philic organisms is often blurred.

Organisms that prefer a neutral pH value are called neutrophils ; if they require a neutral pH value, they are obligatory neutrophils, such as the bacterium Escherichia coli .

Organisms that tolerate low pH levels are called acidotolerant or acid tolerant .

Mechanisms

Different membrane-based metabolic mechanisms can cause alkali tolerance:

The bacterium Escherichia coli can only achieve alkali tolerance up to pH 10.0 through gene expression of the membrane-bound transport protein MdfA . MdfA is a multidrug resistance transporter (Mdr, English multidrug-resistance transporter ). MdfA can transport protons ( cations ) of sodium (Na + ) and potassium (Ka + ) through the membrane and then also conveys resistance to various poisons . The gene for MdfA is on a plasmid . The absence of the plasmid with the MdfA gene puts E. coli in its normal state of obligate neutrophilicity.

Some yeasts achieve alkali tolerance through the use of membrane-bound glucosylceramides , which result from glycosylation of a sphingolipid by the enzyme ceramide glucosyltransferase ( EC  2.4.1.80 ). The presence of glucosylceramides in their membrane can even make yeasts alkaliphilic, so that they cannot grow below pH 8.5, without glucosylceramides they are not alkaliphilic, not even alkali-tolerant.

Individual evidence

  1. ↑ a b c d e Oded Lewinson, Etana Padan, Eitan Bibi: Alkalitolerance: a biological function for a multidrug transporter in pH homeostasis. (PDF) In: Proceedings of the National Academy of Sciences of the United States of America 101, No. 39, 2004, pp. 14073-14078, doi: 10.1073 / pnas.0405375101 .
  2. ↑ a b Katsuichi Saito, Naoya Takakuwa, Masao Ohnishi, Yuji Oda: Presence of glucosylceramide in yeast and its relation to alkali tolerance of yeast. In: Applied Microbiology and Biotechnology 71, No. 4, 2006, pp. 515-521, doi: 10.1007 / s00253-005-0187-3 .