1 Definitions
Alkaliphiles are referred to microorganisms growing at alkaline pH range, i.e., at a pH
of the medium higher than 7. A few definitions of alkaliphiles do exist. Despite the
fact that they are known since the end of the nineteenth century, some uncertainty in
their definition still persists, and, at present, Prof. Koki Horikoshi, a pioneer of the
study of alkaliphilic microorganisms, defines alkaliphiles as “bacteria with pH optima
for growth in excess of pH 8, usually pH 9–12” [1]. According to his classification
which is now well established, alkaliphiles can be divided into two distinct groups
based on their pH profiles for growth. Particularly, those that grow starting from pH
7 to 12 have been classified as obligate alkaliphiles and those where the lower limit for
growth is slightly less than 7 (not less than 6.0) as facultative alkaliphiles. Another
division classifies alkaliphiles as obligate alkaliphiles (synonymously to true) which
do not grow at pH 7 or lower, and their optimum pH value for growth is always
greater than 7, and the other group is classified as alkalitolerant, whose growth
optimal pH can be at 7 and slightly lower (not less than 6.0) but the upper limit for
growth extends over 8.0–8.5. In a strict sense, alkalitolerant microorganisms are not
alkaliphiles, they are neutralophiles, the upper limit of which extends over 8, whereas
in common neutralophiles, it usually does not exceed 8.0–8.5. An Extremophiles
Handbook offers even more strict definition: “Alkaliphile: an organism with optimal
growth at pH values above 10” [2].
Alkaline environments in nature, except for some cases, mostly caused by
carbonates (NaHCO 3 /Na 2 CO 3 ) with varying buffering capacity. The evaporation
process can significantly raise the carbonate concentration up to saturation as it is,
for example, in some African or Asian soda lakes. Accordingly, alkaliphiles
isolated out there; besides a requirement for alkaline conditions (high pH), frequently possess a requirement for high sodium content, usually >1 M of Na
+
.
Such alkaliphiles are referred to haloalkaliphiles; they are virtually double
extremophiles and growth ranges both for sodium and pH are equally important
for their characterization. In the dependency of anion associated with sodium,
haloalkaliphiles, in turn, can be subdivided on haloalkaliphiles sensu stricto which
require both carbonate and chloride and natronophiles (in Arabic natrono –
pertaining to soda, sodium carbonate) which require only carbonates, while
sodium chloride can enhance growth or do not affect it. At the moment, the
molecular backgrounds for the discrepancy between haloalkaliphiles and
natronophiles are not clear. Other group of double extremophiles includes the
less numerous thermoalkaliphilic (or alkalithermophilic) microorganisms that are
introduced at the end of the twentieth century by Wiegel [3]. Based on sodium
requirement, alkalithermophiles can also be halophilic, and these organisms are
triple extremophiles or polyextremophiles. Such microorganisms exist in nature,
and the reader can learn more about them in the chapter “Anaerobic alkaliphiles
and alkaliphilic poly-extremophiles” written by Wiegel in the Extremophiles
Handbook [4].
Isolation and Cultivation of Alkaliphiles
55
Alkaliphiles are referred to microorganisms growing at alkaline pH range, i.e., at a pH
of the medium higher than 7. A few definitions of alkaliphiles do exist. Despite the
fact that they are known since the end of the nineteenth century, some uncertainty in
their definition still persists, and, at present, Prof. Koki Horikoshi, a pioneer of the
study of alkaliphilic microorganisms, defines alkaliphiles as “bacteria with pH optima
for growth in excess of pH 8, usually pH 9–12” [1]. According to his classification
which is now well established, alkaliphiles can be divided into two distinct groups
based on their pH profiles for growth. Particularly, those that grow starting from pH
7 to 12 have been classified as obligate alkaliphiles and those where the lower limit for
growth is slightly less than 7 (not less than 6.0) as facultative alkaliphiles. Another
division classifies alkaliphiles as obligate alkaliphiles (synonymously to true) which
do not grow at pH 7 or lower, and their optimum pH value for growth is always
greater than 7, and the other group is classified as alkalitolerant, whose growth
optimal pH can be at 7 and slightly lower (not less than 6.0) but the upper limit for
growth extends over 8.0–8.5. In a strict sense, alkalitolerant microorganisms are not
alkaliphiles, they are neutralophiles, the upper limit of which extends over 8, whereas
in common neutralophiles, it usually does not exceed 8.0–8.5. An Extremophiles
Handbook offers even more strict definition: “Alkaliphile: an organism with optimal
growth at pH values above 10” [2].
Alkaline environments in nature, except for some cases, mostly caused by
carbonates (NaHCO 3 /Na 2 CO 3 ) with varying buffering capacity. The evaporation
process can significantly raise the carbonate concentration up to saturation as it is,
for example, in some African or Asian soda lakes. Accordingly, alkaliphiles
isolated out there; besides a requirement for alkaline conditions (high pH), frequently possess a requirement for high sodium content, usually >1 M of Na
+
.
Such alkaliphiles are referred to haloalkaliphiles; they are virtually double
extremophiles and growth ranges both for sodium and pH are equally important
for their characterization. In the dependency of anion associated with sodium,
haloalkaliphiles, in turn, can be subdivided on haloalkaliphiles sensu stricto which
require both carbonate and chloride and natronophiles (in Arabic natrono –
pertaining to soda, sodium carbonate) which require only carbonates, while
sodium chloride can enhance growth or do not affect it. At the moment, the
molecular backgrounds for the discrepancy between haloalkaliphiles and
natronophiles are not clear. Other group of double extremophiles includes the
less numerous thermoalkaliphilic (or alkalithermophilic) microorganisms that are
introduced at the end of the twentieth century by Wiegel [3]. Based on sodium
requirement, alkalithermophiles can also be halophilic, and these organisms are
triple extremophiles or polyextremophiles. Such microorganisms exist in nature,
and the reader can learn more about them in the chapter “Anaerobic alkaliphiles
and alkaliphilic poly-extremophiles” written by Wiegel in the Extremophiles
Handbook [4].
Isolation and Cultivation of Alkaliphiles
55