10.9 Retinal Protein-Driven Photoheterotrophy
Until the end of the twentieth century, it was generally assumed that
photoheterotrophic growth based on light absorption by membrane-bound retinal
pigments is restricted to Halobacterium and a few related genera of extremely
halophilic members of the Euryarchaeota. The function of the light-driven proton
pump bacteriorhodopsin in these archaea was already known in the 1970s. Therefore, the finding of similar membrane-bound retinal-containing proton pumps in
marine members of the domain Bacteria came as a big surprise. Proteorhodopsin, the
retinal protein of marine members of the Proteobacteria, was discovered in
metagenomics studies of marine bacterioplankton. The functioning of this new
rhodopsin pigment resembles that of the archaeal proton-pumping rhodopsins
(Béjà et al. 2000). Genes encoding proteorhodopsin are distributed among bacterial
taxa belonging to phylogenetically divergent lineages of Alphaproteobacteria,
Gammaproteobacteria (de la Torre et al. 2003), Flavobacteria (Gómez-Consarnau
et al. 2007), and others.
While it is clear that proton pumping by proteorhodopsins can lead to the
formation of ATP driven by the transmembrane proton gradient, the importance of
the pigment in the life of marine bacterioplankton is still far from clear.
Proteorhodopsin-mediated photoautotrophic growth has never yet been demonstrated, and there are only very few cases in which light was shown to increase
growth yield of marine bacteria growing heterotrophically. Growth yield of a retinal
protein-containing strain of Dokdonia (Flavobacteria) was increased during light
exposure (Gómez-Consarnau et al. 2007), but it is still not established as a general
phenomenon. However, quantitative estimations of the amounts of retinal pigments
in the marine environment show that microbial rhodopsins are major contributors to
the capturing of solar energy in the Mediterranean Sea and the Atlantic Ocean. The
highest rhodopsin concentrations were found above the deep chlorophyll a maxima,
and proton-pumping proteorhodopsins may absorb as much light energy as
chlorophyll a–based phototrophs, so that proteorhodopsins may be a major
energy-transducing mechanism of harvesting sunlight in the surface ocean
(Gómez-Consarnau et al. 2019).
10.10 Unity in Biochemistry: Diversity in Biochemical
Pathways
The concept of unity in biochemistry (Kluyver and Donker 1926) implies that in all
forms of life, the basic principles of the biochemical reactions are similar. In spite of
this, the same metabolic goals such as aerobic degradation of simple sugars or
autotrophic CO 2 fixation can often be achieved in different ways (Gottschalk
1985). It is not always clear why some microorganisms use a certain biochemical
pathway, while others prefer a different sequence of reactions in spite of the fact that
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A. Oren
Until the end of the twentieth century, it was generally assumed that
photoheterotrophic growth based on light absorption by membrane-bound retinal
pigments is restricted to Halobacterium and a few related genera of extremely
halophilic members of the Euryarchaeota. The function of the light-driven proton
pump bacteriorhodopsin in these archaea was already known in the 1970s. Therefore, the finding of similar membrane-bound retinal-containing proton pumps in
marine members of the domain Bacteria came as a big surprise. Proteorhodopsin, the
retinal protein of marine members of the Proteobacteria, was discovered in
metagenomics studies of marine bacterioplankton. The functioning of this new
rhodopsin pigment resembles that of the archaeal proton-pumping rhodopsins
(Béjà et al. 2000). Genes encoding proteorhodopsin are distributed among bacterial
taxa belonging to phylogenetically divergent lineages of Alphaproteobacteria,
Gammaproteobacteria (de la Torre et al. 2003), Flavobacteria (Gómez-Consarnau
et al. 2007), and others.
While it is clear that proton pumping by proteorhodopsins can lead to the
formation of ATP driven by the transmembrane proton gradient, the importance of
the pigment in the life of marine bacterioplankton is still far from clear.
Proteorhodopsin-mediated photoautotrophic growth has never yet been demonstrated, and there are only very few cases in which light was shown to increase
growth yield of marine bacteria growing heterotrophically. Growth yield of a retinal
protein-containing strain of Dokdonia (Flavobacteria) was increased during light
exposure (Gómez-Consarnau et al. 2007), but it is still not established as a general
phenomenon. However, quantitative estimations of the amounts of retinal pigments
in the marine environment show that microbial rhodopsins are major contributors to
the capturing of solar energy in the Mediterranean Sea and the Atlantic Ocean. The
highest rhodopsin concentrations were found above the deep chlorophyll a maxima,
and proton-pumping proteorhodopsins may absorb as much light energy as
chlorophyll a–based phototrophs, so that proteorhodopsins may be a major
energy-transducing mechanism of harvesting sunlight in the surface ocean
(Gómez-Consarnau et al. 2019).
10.10 Unity in Biochemistry: Diversity in Biochemical
Pathways
The concept of unity in biochemistry (Kluyver and Donker 1926) implies that in all
forms of life, the basic principles of the biochemical reactions are similar. In spite of
this, the same metabolic goals such as aerobic degradation of simple sugars or
autotrophic CO 2 fixation can often be achieved in different ways (Gottschalk
1985). It is not always clear why some microorganisms use a certain biochemical
pathway, while others prefer a different sequence of reactions in spite of the fact that
172
A. Oren