A strain of Paracoccus bogoriensis secretes 0.4 mg astaxanthin per gram of wet cells
[183]. A variety of other carotenoids such as spirilloxanthin, spheroidene,
demethylspheroidene, demethylspheroidenone, canthaxanthin, and lycopene type
of carotenoid pigments have been reported from different genera of alkaliphiles
including Dietzia, Microbacterium, Thiorhodospira, Ectothiorhodospira,
Roseibacula, etc. (see [184]). Novel carotenoids containing glucoside esters are
also reported from the alkaliphilic strains of Heliobacteria, Heliorestis
acidaminivorans, and Candidatus Heliomonas lunata [185, 186]. Thus, it seems
that alkaliphiles have a great potential to serve as rich sources of carotenoids.
4.4 Siderophores of Alkaliphiles
Chelators have got various applications in food, pharmaceutical, cosmetic, and
chemical industries. In addition, these substances are being used in agriculture, in
water treatment, and in dental and other medical applications. Due to the consumer
inclination to organic natural products and environmental concerns, there has been
an effort to look for efficient organic chelators. Like many other biotechnological
important products, microorganisms have been the primary targets of the search.
Studies have indicated that microorganisms produce low molecular weight chelating
compounds known as siderophores, and so far, more than 500 siderophores have
been reported [176]. Most of these chelators, at least partly, seem evolved to acquire
iron, one of the most vital substances in cellular processes. Iron is involved in the
electron transport chain system that generates ATP and serves as a cofactor for
enzymes that mediate an array of biochemical reactions.
Production of siderophores is known to be affected by iron availability. The lower
the iron, the higher the siderophore production and vice versa. In alkaline environments, the solubility of inorganic iron and other metal ions is extremely low [187]. In
some alkaline habitats, there is another factor that further dwindles iron and other
metal availability, high level of oxygen. Alkaline environments such as soda lakes
are known to be the most productive lakes [188]. The impressive primary productivity is associated with abundant oxygen release mostly by the photosynthetic
cyanobacteria. The oxygen oxidizes the metal ions including iron and precipitates
it out. This precipitation further shrinks the availability of the metal ions far below
than what is optimally required for normal growth of most organisms. Thus,
alkaliphiles which are thriving in these environments, to ensure acquiring enough
iron and other important metal ions, are expected to produce siderophores which are
very efficient. Thus, it is possible that alkaliphiles may be sources of novel
siderophores. Indeed, structural analysis of some siderophores of alkaliphiles supports this expectation. For instance, novel siderophores have been reported from the
alkaliphilic strains of Caldalkalibacillus thermarum [189] and Halomonas
[190]. Thus, as siderophores are potentially applicable in areas where chemical
chelators are in use and alkaliphiles seem capable of producing efficient and novel
24
G. Mamo and B. Mattiasson
[183]. A variety of other carotenoids such as spirilloxanthin, spheroidene,
demethylspheroidene, demethylspheroidenone, canthaxanthin, and lycopene type
of carotenoid pigments have been reported from different genera of alkaliphiles
including Dietzia, Microbacterium, Thiorhodospira, Ectothiorhodospira,
Roseibacula, etc. (see [184]). Novel carotenoids containing glucoside esters are
also reported from the alkaliphilic strains of Heliobacteria, Heliorestis
acidaminivorans, and Candidatus Heliomonas lunata [185, 186]. Thus, it seems
that alkaliphiles have a great potential to serve as rich sources of carotenoids.
4.4 Siderophores of Alkaliphiles
Chelators have got various applications in food, pharmaceutical, cosmetic, and
chemical industries. In addition, these substances are being used in agriculture, in
water treatment, and in dental and other medical applications. Due to the consumer
inclination to organic natural products and environmental concerns, there has been
an effort to look for efficient organic chelators. Like many other biotechnological
important products, microorganisms have been the primary targets of the search.
Studies have indicated that microorganisms produce low molecular weight chelating
compounds known as siderophores, and so far, more than 500 siderophores have
been reported [176]. Most of these chelators, at least partly, seem evolved to acquire
iron, one of the most vital substances in cellular processes. Iron is involved in the
electron transport chain system that generates ATP and serves as a cofactor for
enzymes that mediate an array of biochemical reactions.
Production of siderophores is known to be affected by iron availability. The lower
the iron, the higher the siderophore production and vice versa. In alkaline environments, the solubility of inorganic iron and other metal ions is extremely low [187]. In
some alkaline habitats, there is another factor that further dwindles iron and other
metal availability, high level of oxygen. Alkaline environments such as soda lakes
are known to be the most productive lakes [188]. The impressive primary productivity is associated with abundant oxygen release mostly by the photosynthetic
cyanobacteria. The oxygen oxidizes the metal ions including iron and precipitates
it out. This precipitation further shrinks the availability of the metal ions far below
than what is optimally required for normal growth of most organisms. Thus,
alkaliphiles which are thriving in these environments, to ensure acquiring enough
iron and other important metal ions, are expected to produce siderophores which are
very efficient. Thus, it is possible that alkaliphiles may be sources of novel
siderophores. Indeed, structural analysis of some siderophores of alkaliphiles supports this expectation. For instance, novel siderophores have been reported from the
alkaliphilic strains of Caldalkalibacillus thermarum [189] and Halomonas
[190]. Thus, as siderophores are potentially applicable in areas where chemical
chelators are in use and alkaliphiles seem capable of producing efficient and novel
24
G. Mamo and B. Mattiasson