Alkaliphiles: some applications of their products for biotechnology

Abstract

The term "alkaliphile" is used for microorganisms that grow optimally or very well at pH values above 9 but cannot grow or grow only slowly at the near-neutral pH value of 6.5. Alkaliphiles include prokaryotes, eukaryotes, and archaea. Many different taxa are represented among the alkaliphiles, and some of these have been proposed as new taxa. Alkaliphiles can be isolated from normal environments such as garden soil, although viable counts of alkaliphiles are higher in samples from alkaline environments. The cell surface may play a key role in keeping the intracellular pH value in the range between 7 and 8.5, allowing alkaliphiles to thrive in alkaline environments, although adaptation mechanisms have not yet been clarified. Alkaliphiles have made a great impact in industrial applications. Biological detergents contain alkaline enzymes, such as alkaline cellulases and/or alkaline proteases, that have been produced from alkaliphiles. The current proportion of total world enzyme production destined for the laundry detergent market exceeds 60%. Another important application is the industrial production of cyclodextrin by alkaline cyclomaltodextrin glucanotransferase. This enzyme has reduced the production cost and paved the way for cyclodextrin use in large quantities in foodstuffs, chemicals, and pharmaceuticals. It has also been reported that alkali-treated wood pulp could be biologically bleached by xylanases produced by alkaliphiles. Other applications of various aspects of alkaliphiles are also discussed.

FIG. 1

pH dependency of alkaliphilic microorganisms. The typical pH dependency of the growth of neutrophilic and alkaliphilic bacteria is shown by open squares and solid circles, respectively.

FIG. 2

Distribution of alkaliphilic microorganisms in environments at various pHs.

FIG. 3

Schematic representation of cytoplasmic pH regulation.

FIG. 4

Physical and genetic map of the chromosome of B. halodurans C-125. The outer and inner circles shows the AscI and Sse83871 physical maps, respectively. The locations of several genes are indicated on the maps. Dashed lines indicate the approximate positions of the genes.