Anoxygenic Photosynthesis —A Photochemical Reaction That Does Not Contribute to Oxygen Reproduction—

Plants, algae, and cyanobacteria perform photosynthesis with the resultant production of oxygen, which supports all organisms that consume it through their respiration. This type of photosynthesis is an indispensable part of the global oxygen flux. However, another type of photochemical reaction , photosynthesis without oxygen production, exists, and has been designated anoxygenic photosynthesis. A number of ecologists regard anoxygenic photosynthesis as a negligible photochemical reaction because it contributes nothing to the reproduction of oxygen. This type of photosynthesis does not appear to have any beneficial function. The global impact of anoxygenic photosynthesis is considered to be negligible. Nevertheless, organisms in the domain Bacteria perform this type of photosynthesis, with some dilettantes referring to them as anoxygenic phototrophic bacteria. Contrary to all expectations, studies on anoxygenic phototrophic bacteria have a long history in microbiology. Anoxygenic phototrophic bacteria were discovered at the dawn of microbiology more than 100 years ago. The first anoxygenic phototrophic bacterium was identified in 1901 and was subsequently described as Rhodospirillum species by the German botanist, Hans Molisch in 1907 (10). Rhodospirillum species are purple bacteria that belong to the phylum Proteobacteria, and the term " purple bacteria " , which refers to a major group among anoxygenic phototrophic bacteria, was also simultaneously proposed by Molisch. Another major anoxygenic phototrophic group, green sulfur bacteria, which belong to the phylum Chlorobi, was described by the Russian biologist, Georgii A. Nadson in 1906 (13). Martinus W. Beijerinck, a famous microbiologist in the Netherlands, who was a contemporary of Molisch and Nadson, proposed a lactic acid bacterial group, the genus Lactobacillus, in 1901. None of the anoxygenic phototrophic bacteria have the ability to use water as an electron donor (or cannot oxidize water), and, thus, perform photosynthesis using sulfide, hydrogen or organic substrates. Therefore, photosynthesis by these bacteria does not involve oxygen. In the domain Bacteria, oxygenic photosynthesis is limited to only one phylum, i.e., Cyanobacteria. On the other hand, anoxygenic photosynthesis is widely distributed over several bacterial phyla. Purple bacteria in the phylum Proteobacteria and green sulfur bacteria in the phylum Chlorobi described above inhabit various environments such as soil, ocean, lakes, and rivers. Some species have even been discovered in hot springs (7, 25), and a culture-independent survey revealed that a brackish lake was abundant in a number of green sulfur bacteria (12). In addition to these bacteria, there are two groups, i.e., thermophilic or mesophilic phototrophs showing filamentous …