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 tolerate low pH levels are called acidotolerant or acid tolerant .
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 22.214.171.124 ). 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.
- 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 .
- 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 .