Diversity of Marine Phototrophs 3.3 Discovery of Ubiquitous Photoheterotrophs (2000–Current Times) 29
Part A | 3.3
mon Synechococcus. The cyanobacterium named as the
genus Prochlorococcus is coccoid to ellipsoidal unicellular cells with 0:40:6 in diameter and 0:50:8
in length. They perform oxygenic photosynthesis using DVChl a=b which can efficiently absorb the blue
light dominating in the ocean. They are the dominant
picophytoplankton in tropical and subtropical regions
between the latitudes of 30
ı north and 30
ı south. In
several marine regions, the amount of DVChl a is three
times higher than that of usual Chl a, indicating that
Prochlorococcus play a significant role as a primary
producer in those environments.
3.3 Discovery of Ubiquitous Photoheterotrophs (2000–Current Times)
In the early 2000s, two significant findings were reported with respect to the fact that microorganisms
other than traditional phytoplankton could possibly
contribute significantly to the transformation of light
energy into chemical energy in marine environments.
One was the wide distribution of aerobic anoxygenic photosynthetic bacteria (AAPB) which contained
bacteriochlorophyll (BChl) a, that had mainly been
recognized as a photosynthetic pigment in anaerobic anoxygenic photosynthetic bacteria (AnAPB) distributed in anaerobic environments [3.7]. The other was
the wide distribution of proteorhodopsin-containing
bacteria [3.8, 9]. Both bacteria are photoheterotrophs
without carbon fixation and had never been recognized
as primary producers.
APB were first reported in 1979 [3.10]. Phylogenetically, AAPB was derived from non-sulfur purple bacteria [3.11], the anaerobic anoxygenic photosynthetic
bacteria (AnAPB) in the ˛-Proteobacteria. AnAPB had
been traditionally understood to distribute in an anaerobic environment and perform photosynthesis without
oxygen evolution. They employ bacteriochlorophylls
(BChls) for their photosynthesis instead the Chls detected in algae and plants. The amount of primary
production by AnAPB reached up to 30% of the total primary production in some specific lakes [3.12].
However, their contribution was negligible on global
scale, since such environments were restricted. On
the other hand, AAPB grow (photo-)heterotrophycally
in an aerobic environment. AAPB are the same with
AnAPB on the point that they contain BChl a. However, AAPB are aerobic and require molecular oxygen
for their growth and BChl a synthesis [3.11]. AAPB
distributes widely in the euphotic zone of marine environments in amounts with from ranging from 124%
of the total bacterial count [3.13]. It had been already
reported that the phototrophic growth of some APPB
was faster than those growing heterotrophically in the
dark [3.14]. Moreover, the in situ growth rate of APPB
in seawater was faster than that of other heterotrophic
bacteria. The detailed physiological role and activity
of photosystems in all AAPB have not clarified yet,
but AAPB must contribute to the transformation of solar energy in marine environments and act as primary
producer.
Proteorhodopsin is the rhodopsin-like protein detected in marine Proteobacteria. It is a retial-opsin
complex in a member of rhodopsin. Opsin is a bundle of seven transmembrane ˛-helices. Retinal, vitamin
A aldehyde binds to the lysine residue at the central pocket of the opsin. Rhodopsin is well known
as a photoreceptor in vertebrates as a light sensor.
One rhodopsin-like protein was known to contribute
to adenosine triphosphate (ATP) synthesis. Bacteriorhodopsin of Halobacteria in Archea can transform
light energy to transmembrane proton electrochemical
gradients .H
C ) for ATP synthesis [3.15]. Conformation of retinal molecule changes by absorbing light
in synchronization with structure changes of ligand in
the central part of opsin. The conformation change
lead a serial proton to deliver from cytoplasmic to
periplasmic space, and results in a formation of H
C
for ATP synthesis. However, such ATP synthesis using bacteriorhodopsin had been thought to be a special mechanism that is restricted in special Archaean
bacterium-distributed extremely salty environments.
However, a large amount and wide variety of
rhodopsin-like genes were detected in seawater by
metagenomic analysis [3.8, 9]. Moreover the gene was
expressed in E. coli and acted to form H
C for
ATP synthesis under light [3.16]. The genes of proteorhodopsin are contained in a diversity of bacterial
taxa [3.17]. The total amount of proteorhodopsincontaining bacteria is equivalent to 1380% of the
total amount of bacteria and Archea in marine environments [3.18, 19]. These results indicate that proteorhodopsin was widely and significantly distributed
and play a significant role in light energy transformation in the marine environments. Actually, lightdependent ATP synthesis using proteorhodopsin was
Part A | 3.3
mon Synechococcus. The cyanobacterium named as the
genus Prochlorococcus is coccoid to ellipsoidal unicellular cells with 0:40:6 in diameter and 0:50:8
in length. They perform oxygenic photosynthesis using DVChl a=b which can efficiently absorb the blue
light dominating in the ocean. They are the dominant
picophytoplankton in tropical and subtropical regions
between the latitudes of 30
ı north and 30
ı south. In
several marine regions, the amount of DVChl a is three
times higher than that of usual Chl a, indicating that
Prochlorococcus play a significant role as a primary
producer in those environments.
3.3 Discovery of Ubiquitous Photoheterotrophs (2000–Current Times)
In the early 2000s, two significant findings were reported with respect to the fact that microorganisms
other than traditional phytoplankton could possibly
contribute significantly to the transformation of light
energy into chemical energy in marine environments.
One was the wide distribution of aerobic anoxygenic photosynthetic bacteria (AAPB) which contained
bacteriochlorophyll (BChl) a, that had mainly been
recognized as a photosynthetic pigment in anaerobic anoxygenic photosynthetic bacteria (AnAPB) distributed in anaerobic environments [3.7]. The other was
the wide distribution of proteorhodopsin-containing
bacteria [3.8, 9]. Both bacteria are photoheterotrophs
without carbon fixation and had never been recognized
as primary producers.
APB were first reported in 1979 [3.10]. Phylogenetically, AAPB was derived from non-sulfur purple bacteria [3.11], the anaerobic anoxygenic photosynthetic
bacteria (AnAPB) in the ˛-Proteobacteria. AnAPB had
been traditionally understood to distribute in an anaerobic environment and perform photosynthesis without
oxygen evolution. They employ bacteriochlorophylls
(BChls) for their photosynthesis instead the Chls detected in algae and plants. The amount of primary
production by AnAPB reached up to 30% of the total primary production in some specific lakes [3.12].
However, their contribution was negligible on global
scale, since such environments were restricted. On
the other hand, AAPB grow (photo-)heterotrophycally
in an aerobic environment. AAPB are the same with
AnAPB on the point that they contain BChl a. However, AAPB are aerobic and require molecular oxygen
for their growth and BChl a synthesis [3.11]. AAPB
distributes widely in the euphotic zone of marine environments in amounts with from ranging from 124%
of the total bacterial count [3.13]. It had been already
reported that the phototrophic growth of some APPB
was faster than those growing heterotrophically in the
dark [3.14]. Moreover, the in situ growth rate of APPB
in seawater was faster than that of other heterotrophic
bacteria. The detailed physiological role and activity
of photosystems in all AAPB have not clarified yet,
but AAPB must contribute to the transformation of solar energy in marine environments and act as primary
producer.
Proteorhodopsin is the rhodopsin-like protein detected in marine Proteobacteria. It is a retial-opsin
complex in a member of rhodopsin. Opsin is a bundle of seven transmembrane ˛-helices. Retinal, vitamin
A aldehyde binds to the lysine residue at the central pocket of the opsin. Rhodopsin is well known
as a photoreceptor in vertebrates as a light sensor.
One rhodopsin-like protein was known to contribute
to adenosine triphosphate (ATP) synthesis. Bacteriorhodopsin of Halobacteria in Archea can transform
light energy to transmembrane proton electrochemical
gradients .H
C ) for ATP synthesis [3.15]. Conformation of retinal molecule changes by absorbing light
in synchronization with structure changes of ligand in
the central part of opsin. The conformation change
lead a serial proton to deliver from cytoplasmic to
periplasmic space, and results in a formation of H
C
for ATP synthesis. However, such ATP synthesis using bacteriorhodopsin had been thought to be a special mechanism that is restricted in special Archaean
bacterium-distributed extremely salty environments.
However, a large amount and wide variety of
rhodopsin-like genes were detected in seawater by
metagenomic analysis [3.8, 9]. Moreover the gene was
expressed in E. coli and acted to form H
C for
ATP synthesis under light [3.16]. The genes of proteorhodopsin are contained in a diversity of bacterial
taxa [3.17]. The total amount of proteorhodopsincontaining bacteria is equivalent to 1380% of the
total amount of bacteria and Archea in marine environments [3.18, 19]. These results indicate that proteorhodopsin was widely and significantly distributed
and play a significant role in light energy transformation in the marine environments. Actually, lightdependent ATP synthesis using proteorhodopsin was