such as four [NiFe]-family dehydrogenases that allow for growth with H 2 as an
energy source, and a formate dehydrogenase that uses formate as a proton donor,
contribute to the proton motive force (PMF) used to drive proton-coupled ATP
synthesis [35].
As noted above, alkaline ecosystems also harbour metabolically versatile chemoautotrophic microorganisms that can feed on a variety of organic and inorganic
substrates. Alkalilimnicola ehrlichii MLHE-1, a facultative Gram-negative chemoautotroph isolated from the alkaline and hypersaline Mono Lake in the USA, is
capable of growing both aerobically and anaerobically with inorganic electron
donors such as arsenite, hydrogen, and nitrate as the electron acceptor while also
growing heterotrophically on organic acids [36, 37]. Genome analysis of this
bacterium revealed that, in addition to the RuBisCO genes cbbL and cbbS, which
allow for CO 2 fixation through the Calvin cycle, it also contained the CO dehydrogenase operon coxFEDLSM, which allows for CO metabolism [37]. Three bacterial
strains (A1, B1, and H1) isolated from a serpentinizing site in the USA and
phylogenetically related to the genera Hydrogenophaga and Malikia were also
genetically characterized [38]. Metabolic profiling of these strains revealed that
they have genes required for chemoautotrophic growth on H 2 , specifically group
2b and 3d [NiFe]-hydrogenases, as well as genes encoding for carboxysome shell
proteins, which suggest the ability to employ CO 2 -concentrating mechanisms for
carbon fixation. In addition, strain A1 also contained genes encoding for benzene
and phenylalanine/phenylacetate degradation, which would facilitate growth in
environments with aromatic compounds.
The recent sequencing of the genome of the alkaliphile fungi S. alkalinus,
originally extracted from soda lakes, also contributed to our understanding of the
role that eukaryotic alkaliphiles play in these complex trophic systems. Metabolic
profiling of the genome revealed the capacity to grow on xylan from maize, pectins,
and monosaccharides [24]. In addition, S. alkalinus was found to express a narrow
range of peptidases with strong protease activity at alkaline conditions, which
support the hypothesis that it used protein-rich microscopic crustaceans and prokaryotes as primary food sources [24].
4 The Bioenergetics of Alkaliphiles
The bioenergetics of aerobic alkaliphiles revolves around the control of the PMF and
SMF across the cytoplasmic membrane. pH homeostasis is regulated by coupling the
outwards pumping of protons through the respiratory chain to generate a PMF, with
the export of sodium by Na
+ /H
+ antiporters using the PMF. This interplay maintains
the conditions required for ion gradient-coupled bioenergetics processes while
achieving the acidification of the cytoplasm [39].
Genomics of Alkaliphiles
145
energy source, and a formate dehydrogenase that uses formate as a proton donor,
contribute to the proton motive force (PMF) used to drive proton-coupled ATP
synthesis [35].
As noted above, alkaline ecosystems also harbour metabolically versatile chemoautotrophic microorganisms that can feed on a variety of organic and inorganic
substrates. Alkalilimnicola ehrlichii MLHE-1, a facultative Gram-negative chemoautotroph isolated from the alkaline and hypersaline Mono Lake in the USA, is
capable of growing both aerobically and anaerobically with inorganic electron
donors such as arsenite, hydrogen, and nitrate as the electron acceptor while also
growing heterotrophically on organic acids [36, 37]. Genome analysis of this
bacterium revealed that, in addition to the RuBisCO genes cbbL and cbbS, which
allow for CO 2 fixation through the Calvin cycle, it also contained the CO dehydrogenase operon coxFEDLSM, which allows for CO metabolism [37]. Three bacterial
strains (A1, B1, and H1) isolated from a serpentinizing site in the USA and
phylogenetically related to the genera Hydrogenophaga and Malikia were also
genetically characterized [38]. Metabolic profiling of these strains revealed that
they have genes required for chemoautotrophic growth on H 2 , specifically group
2b and 3d [NiFe]-hydrogenases, as well as genes encoding for carboxysome shell
proteins, which suggest the ability to employ CO 2 -concentrating mechanisms for
carbon fixation. In addition, strain A1 also contained genes encoding for benzene
and phenylalanine/phenylacetate degradation, which would facilitate growth in
environments with aromatic compounds.
The recent sequencing of the genome of the alkaliphile fungi S. alkalinus,
originally extracted from soda lakes, also contributed to our understanding of the
role that eukaryotic alkaliphiles play in these complex trophic systems. Metabolic
profiling of the genome revealed the capacity to grow on xylan from maize, pectins,
and monosaccharides [24]. In addition, S. alkalinus was found to express a narrow
range of peptidases with strong protease activity at alkaline conditions, which
support the hypothesis that it used protein-rich microscopic crustaceans and prokaryotes as primary food sources [24].
4 The Bioenergetics of Alkaliphiles
The bioenergetics of aerobic alkaliphiles revolves around the control of the PMF and
SMF across the cytoplasmic membrane. pH homeostasis is regulated by coupling the
outwards pumping of protons through the respiratory chain to generate a PMF, with
the export of sodium by Na
+ /H
+ antiporters using the PMF. This interplay maintains
the conditions required for ion gradient-coupled bioenergetics processes while
achieving the acidification of the cytoplasm [39].
Genomics of Alkaliphiles
145