Photosynthesis is the utilization of radiant energy for the synthesis of complex organic molecules. The phototrophic way of life implies the capture of electromagnetic energy (see Light Absorption and Light Energy Transfer in Prokaryotes in this Chapter), its conversion into chemical energy (see Conversion of Light into Chemical Energy in this Chapter), and its use for cellular maintenance and growth (see Efficiency of Growth and Maintenance Energy Requirements in this Chapter). Photosynthesis may encompass the reduction of carbon dioxide into organic molecules, a mode of growth defined as photoautotrophy. The solar electromagnetic energy reaching the Earth's surface (160 W-m-2; see Light energy and the spectral distribution of radiation) surpasses the energy contributed by all other sources by four to five orders of magnitude (electric discharge, radioactivity, volcanism, or meteoritic impacts; ~0.0062 W-m-2 on primordial Earth; Mauzerall, 1992; present day geother-mal energy ~0.0292 W-m-2; K. Nealson, personal communication).
At present the flux of electromagnetic energy supports a total primary production of 172.5 x 109 tons dry weight-year-1 (168 g C-m-2-year-1; Whittaker and Likens, 1975). If this global primary production is converted to energy units (39.9 kJ-g C-1, assuming that all photosynthetic products are carbohydrate), 0.21 W-m-2 and thus 0.13% of the available solar energy flux are converted into chemical energy. Even at this low efficiency, the chemical energy stored in organic carbon still exceeds geothermal energy by at least one order of magnitude. As a consequence, photosynthesis directly or indirectly drives the biogeochemical cycles in all extant ecosystems of the planet. Even hydrothermal vent communities, which use inorganic electron donors of geothermal origin and assimilate CO2 by chem-olithoautotrophy (rather than photoautotro-phy), still depend on the molecular O2 generated by oxygenic phototrophs outside of these systems (Jannasch, 1989).
Several lines of evidence indicate that in the early stages of biosphere evolution, prokaryotic organisms were once responsible for the entire global photosynthetic carbon fixation. Today, terrestrial higher plants account for the vast majority of photosynthetic biomass; the chlorophyll bound in light-harvesting complex LHCII of green chloroplasts alone represents 50% of the total chlorophyll on Earth (Sidler, 1994). In contrast, the biomass of marine primary producers is very low (0.2% of the global value). However, the biomass turnover of marine photosynthetic microorganisms is some 700 times faster than that of terrestrial higher plants. Thus, marine photosynthetic organisms contribute significantly to total primary productivity (55-109 tons dry weight-year-1, or 44% of the global primary production). Because the biomass of cyanobac-terial picoplankton (see Habitats of Phototrophic Prokaryotes in this Chapter) can amount to 67% of the oceanic plankton, and their photosynthesis up to 80% in the marine environment (Campbell et al., 1994; Goericke and Welschmeyer, 1993; Liu et al., 1997; Waterbury et al., 1986), prokaryotic primary production is still significant on a global scale. A single monophyletic group of marine unicellular cyanobacterial strains encompassing the genera Prochloroccoccus and Synechococcus with a global biomass in the order of a billion of metric tons (Garcia-Pichel, 1999) may be responsible for the fixation of as much as 10-25% of the global primary productivity. Additionally, prokaryotic (cyanobacterial) photosynthesis is still locally very important in other habitats such as cold (Friedmann, 1976) and hot deserts (Garcia-Pichel and Belnap, 1996) a nd hyper-trophic lakes.
Today, the significance of anoxygenic photosynthesis for global carbon fixation is limited for two reasons. On the one hand, phototrophic sulfur bacteria (the dominant anoxygenic phototrophs in natural ecosystems) form dense accumulations only in certain lacustrine environments and in intertidal sandflats. The fraction of lakes and intertidal saltmarshes which harbor anoxygenic phototrophic bacteria is unknown, but these ecosystems altogether contribute only 4% to global primary production (Whittaker and Likens, 1975). In those lakes harboring pho-
totrophic sulfur bacteria, an average of 28.7% of the primary production is anoxygenic (Overmann, 1997). Consequently, the amount of CO2 fixed by anoxygenic photosynthesis must contribute much less than 1% to global primary production. On the other hand, anoxygenic photosynthesis depends on reduced inorganic sulfur compounds which originate from the anaerobic degradation of or ganic carbon. Since this carbon was already fixed by oxygenic photosynthesis, the CO2-fixation of anoxygenic phototrophic bacteria does not lead to a net increase in organic carbon available to higher trophic levels. The CO2-assimilation by anoxygenic phototrophic bacteria has therefore been termed "secondary primary production" (Pfennig, 1978). Therefore, capture of light energy by anoxygenic photosynthesis merely compensates for the degradation of organic carbon in the anaerobic food chain. Geo-thermal sulfur springs are the only exception since their sulfide is of abiotic origin. However, because sulfur springs are rather scarce, anoxy-genic photosynthetic carbon fixation of these ecosystems also appears to be of minor significance on a global scale.
The scientific interest in anoxygenic phototro-pic bacteria stems from 1) the simple molecular architecture and variety of their photosystems, which makes anoxygenic phototrophic bacteria suitable models for biochemical and biophysical study of photosynthetic mechanisms, 2) the considerable diversity of anoxygenic pho-totrophic bacteria, which has implications for reconstructing the evolution of photosynthesis, and 3) the changes in biogeochemical cycles of carbon and sulfur, which are mediated by the dense populations of phototrophic bacteria in natural ecosystems.
All known microorganisms use two functional principles (both mutually exclusive and represent two independent evolutionary developments) for the conversion of light into chemical energy. Chlorophyll-based systems are widespread among members of the domain Bacteria and consist of a light-harvesting antenna and reaction centers. In the latter, excitation energy is converted into a redox gradient across the membrane. In contrast, the retinal-based bacte-riorhodopsin system is exclusively found in members of a monophyletic group within the domain Archaea. These prokaryotes lack an antenna system and use light energy for the direct translocation of protons across the cyto-plasmic membrane. In both systems, photosyn-thetic energy conversion ultimately results in the formation of energy-rich chemical bonds of organic compounds.
The advent of modern genetic and biochemical methods has led to a considerable gain in knowledge of the molecular biology of pho totrophic prokaryotes. At the same time, microbial ecologists have found these microorganisms of considerable interest and now frequently use molecular methods to investigate natural populations. The present chapter is limited to the discussion of phototrophic bacteria and attempts to link the physiology, ecology, and evolution of phototrophic bacteria to a molecular basis. Emphasis is laid on those molecular structures or functions that have evident adaptive value. This integrating view may provide a more solid foundation for understanding the biology of photo-synthetic prokaryotes.
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