10.8 Newly Discovered Possibilities
for Bacteriochlorophyll-Driven Anoxygenic
Photosynthesis
Bacteriochlorophyll-based anoxygenic photosynthesis is found in many lineages of
the domain Bacteria. Purple and green sulfur bacteria are photoautotrophs that use
reduced sulfur compounds as the electron donors for autotrophic CO 2 fixation;
others lead a photoheterotrophic life in which light is used as energy source, while
carbon for cell growth is derived from organic substrates.
Studies of the distribution of phototrophic microorganisms in the water column of
the Black Sea have shown that some green sulfur bacteria (class “Chlorobia”) can
live at extremely low light intensities. In water samples collected in 1988 when the
0.1% light level was estimated at 30–40 m, the interface between oxygen and sulfide
was located at 80-100 m depth, and bacteriochlorophyll e and characteristic carotenoids derived from Chlorobium sp. were found in the water profile between 68 and
121 m depth (Repeta et al. 1989). Brown-colored sulfur bacteria identified as strains
of Chlorobium phaeobacteroides were isolated from the chemocline at 80-m depth
where light transmission was calculated to be 0.0005% of surface irradiance. The
strains are obligate phototrophs and are adapted to growth at extremely low light
intensities: the amount of light-harvesting pigments is increased, and they have a
very low maintenance energy requirement (Overmann et al. 1992).
In his prophetic paper in which Broda predicted the existence of the anammox
bacteria, Broda also asked why photosynthetic, anaerobic, ammonia-oxidizing bacteria do not appear to exist; such bacteria could theoretically oxidize ammonia to
nitrogen gas, analogous to the oxidation of sulfide to elemental sulfur by purple and
green sulfur bacteria (Broda 1977). Such ammonia-oxidizing anoxygenic
phototrophs have still not been found, but nitrite was shown to serve as an electron
donor by a strain of the purple sulfur bacteria genus Thiocapsa
(Gammaproteobacteria) isolated from sewage sludge. Nitrite is oxidized to nitrate,
and it is the highest-potential electron donor known so far to drive anoxygenic
photosynthesis: NO 3
À /NO 2
À
¼ +0.43 V, compare S o /HS
À
¼ À0.27 V and (for
oxygenic photosynthesis) O 2 /H 2 O ¼ +0.82 V (Griffin et al. 2007). The finding of
nitrite-driven autotrophic growth in an isolate of Rhodopseudomonas
(Alphaproteobacteria; a genus of anoxygenic photoorganotrophic bacteria), also
obtained from a municipal sewage treatment plant, shows that the ability to use
nitrite as an electron donor for photosynthesis is more widespread (Schott et al.
2010).
Another recently discovered reduced compound that can drive anoxygenic photosynthesis is arsenite. Isolates of arsenate-oxidizing Ectothiorhodospira
(Gammaproteobacteria) were obtained from a hot spring and from sediment of the
alkaline hypersaline Mono Lake, California, and from Big Soda Lake, Nevada. The
As(III) oxidation may be a common phenomenon among soda lake purple photosynthetic bacteria (Hoeft McCann et al. 2017).
10 The Grand Microbial Variety Show
171
for Bacteriochlorophyll-Driven Anoxygenic
Photosynthesis
Bacteriochlorophyll-based anoxygenic photosynthesis is found in many lineages of
the domain Bacteria. Purple and green sulfur bacteria are photoautotrophs that use
reduced sulfur compounds as the electron donors for autotrophic CO 2 fixation;
others lead a photoheterotrophic life in which light is used as energy source, while
carbon for cell growth is derived from organic substrates.
Studies of the distribution of phototrophic microorganisms in the water column of
the Black Sea have shown that some green sulfur bacteria (class “Chlorobia”) can
live at extremely low light intensities. In water samples collected in 1988 when the
0.1% light level was estimated at 30–40 m, the interface between oxygen and sulfide
was located at 80-100 m depth, and bacteriochlorophyll e and characteristic carotenoids derived from Chlorobium sp. were found in the water profile between 68 and
121 m depth (Repeta et al. 1989). Brown-colored sulfur bacteria identified as strains
of Chlorobium phaeobacteroides were isolated from the chemocline at 80-m depth
where light transmission was calculated to be 0.0005% of surface irradiance. The
strains are obligate phototrophs and are adapted to growth at extremely low light
intensities: the amount of light-harvesting pigments is increased, and they have a
very low maintenance energy requirement (Overmann et al. 1992).
In his prophetic paper in which Broda predicted the existence of the anammox
bacteria, Broda also asked why photosynthetic, anaerobic, ammonia-oxidizing bacteria do not appear to exist; such bacteria could theoretically oxidize ammonia to
nitrogen gas, analogous to the oxidation of sulfide to elemental sulfur by purple and
green sulfur bacteria (Broda 1977). Such ammonia-oxidizing anoxygenic
phototrophs have still not been found, but nitrite was shown to serve as an electron
donor by a strain of the purple sulfur bacteria genus Thiocapsa
(Gammaproteobacteria) isolated from sewage sludge. Nitrite is oxidized to nitrate,
and it is the highest-potential electron donor known so far to drive anoxygenic
photosynthesis: NO 3
À /NO 2
À
¼ +0.43 V, compare S o /HS
À
¼ À0.27 V and (for
oxygenic photosynthesis) O 2 /H 2 O ¼ +0.82 V (Griffin et al. 2007). The finding of
nitrite-driven autotrophic growth in an isolate of Rhodopseudomonas
(Alphaproteobacteria; a genus of anoxygenic photoorganotrophic bacteria), also
obtained from a municipal sewage treatment plant, shows that the ability to use
nitrite as an electron donor for photosynthesis is more widespread (Schott et al.
2010).
Another recently discovered reduced compound that can drive anoxygenic photosynthesis is arsenite. Isolates of arsenate-oxidizing Ectothiorhodospira
(Gammaproteobacteria) were obtained from a hot spring and from sediment of the
alkaline hypersaline Mono Lake, California, and from Big Soda Lake, Nevada. The
As(III) oxidation may be a common phenomenon among soda lake purple photosynthetic bacteria (Hoeft McCann et al. 2017).
10 The Grand Microbial Variety Show
171