Molecular Mechanisms of Photosynthesis. Robert E. Blankenship
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target="_blank" rel="nofollow" href="#ulink_f1e4a00c-bdcb-5e74-a89b-2b69ef73b4a5">Figure 2.3 Thin section transmission electron micrograph of the purple bacterium Rhodobacter capsulatus. The diameter of the cell is about 1 μm.
Source: Courtesy of Steven J. Schmitt and Michael T. Madigan.
The purple bacteria are subdivided into sulfur and nonsulfur groups, depending on the range of ability to metabolize reduced sulfur compounds (Frigaard and Dahl, 2009). However, the terms sulfur and nonsulfur are somewhat misleading because all purple bacteria have the capability to carry out extensive sulfur metabolism. Most purple bacteria use the Calvin–Benson cycle, also known as the reductive pentose phosphate cycle for CO2 fixation.
The “purple” name comes from the color found in many of the common species, which results from the combination of bacteriochlorophyll and carotenoids. Representative species include Rhodobacter sphaeroides, Rhodospirillum rubrum, Allochromatium vinosum, and Blastochloris viridis (formerly known as Rhodopseudomonas viridis). Some of these organisms are not in fact purple in color, such as Blc. viridis, which is a greenish color, as its name suggests.
When grown aerobically, most species of purple bacteria derive their energy from aerobic respiration and completely repress pigment synthesis and expression of the structural proteins involved in photosynthetic energy conversion. Photosynthesis is therefore only observed if the cells are grown under anaerobic conditions. Under these conditions, the ultrastructure of the cytoplasmic membrane changes dramatically and invaginates in toward the cell cytoplasm in vesicles, tubes, or lamellae, which are then called intracytoplasmic membranes. The photosynthetic apparatus is localized in these intracytoplasmic membranes.
One group of purple bacteria, known as aerobic anoxygenic phototrophs (AAP), has the opposite pattern, in that they make pigments and carry out photosynthesis only under aerobic conditions (Yurkov and Beatty, 1998). This is counterintuitive, in that they seem to perform photosynthesis only when they don't really need to. It is not yet clear what advantage this pattern of metabolism gives these organisms. They are widely distributed and have been found throughout the open ocean (Kolber et al., 2001). This group of purple bacteria is not capable of autotrophic metabolism and cannot assimilate CO2 using the Calvin–Benson cycle. Instead, they grow using photoheterotrophic metabolism whereby organic matter from the environment is assimilated with the help of light energy.
Most purple phototrophic bacteria are capable of N2 fixation. In fact, certain classes of Rhizobia, the bacteria that live symbiotically in nodules of leguminous plants, contain bacteriochlorophyll and may actually use photosynthesis to supplement the energy requirements of nitrogen fixation (Fleischman and Kramer, 1998). Other Rhizobia do not express photosynthetic characteristics, but since they are proteobacteria, based on 16S rRNA analysis, they are therefore relatives of purple phototrophic bacteria.
2.5.2 Green sulfur bacteria
In contrast to the versatile purple bacteria, the green sulfur bacteria are metabolic specialists (Overmann, 2006). They are almost always obligate anoxygenic photoautotrophs, unable to grow with only organic carbon as a carbon source (Tang and Blankenship, 2010). The green sulfur bacteria do not fix carbon using the Calvin–Benson cycle; instead, they use the reverse tricarboxylic acid cycle to fix CO2 (Fuchs, 2011). They are also strict anaerobes, are incapable of any form of respiration, and are active nitrogen fixers. Green sulfur bacteria can be found in the anaerobic zone at the bottom of lakes or below the chemocline (the transition from aerobic to anaerobic conditions) in a stratified lake. These organisms preferentially utilize H2S as an electron donor, which is abundant in these environments, although they can also use a variety of other donors such as thiosulfate or elemental sulfur (Frigaard and Dahl, 2009). The green sulfur bacteria can be found living in the lowest light intensities of any known phototrophic organisms (Beatty et al., 2005; Overmann, 2006) and contain highly specialized antenna structures known as chlorosomes. These antenna complexes contain bacteriochlorophyll c, d, or e as principal pigments. The chlorosome is attached to the cytoplasmic side of the cell membrane, which does not invaginate as in purple bacteria. The green sulfur bacteria also contain bacteriochlorophyll a, which functions in both antennas and reaction centers and small amounts of chlorophyll a, which functions in reaction centers.
2.5.3 Filamentous anoxygenic phototrophs
The FAP, sometimes called the green nonsulfur bacteria (Hanada and Pierson, 2006), have metabolic characteristics that are very different from those of the green sulfur bacteria, and in most respects, the two groups of organisms are not closely related. This is in contrast to the purple sulfur and purple nonsulfur bacteria, which are very close relatives by comparison. The FAPs are the earliest branching group of bacterial phototrophs according to the 16S rRNA analysis discussed earlier. They are in most cases capable of photoautotrophic, photoheterotrophic, and aerobic respiratory growth. When grown aerobically, the FAPs suppress pigment synthesis and do not express the structural proteins involved in photosynthesis, similar to many purple bacteria. Most use a unique carbon fixation pathway known as the hydroxypropionate pathway for autotrophic growth (Fuchs, 2011), although they grow best by assimilating organic carbon during photoheterotrophic growth. The FAP bacteria contain bacteriochlorophyll a localized in reaction centers and integral membrane antenna complexes. These complexes are generally similar to those found in the purple bacteria. Most FAP bacteria also contain bacteriochlorophyll c, which is located in chlorosome antenna complexes that are generally similar to those found in green sulfur bacteria and is the major characteristic shared with them. The FAP bacteria are often found in microbial mats, where they live in association with cyanobacteria. They are especially widespread in thermophilic environments.
Surprisingly, a newly discovered anoxygenic bacterium that is a member of the FAP phylum contains a reaction center complex that is unlike the complex found in all other FAPs and is more similar to that found in the green sulfur bacteria (Tsuji et al., 2020). This organism, Candidatus Chlorohelix allophototropha also appears to fix carbon using the Calvin–Benson cycle.
2.5.4 Heliobacteria
The heliobacteria are the third most recently discovered of the groups of anoxygenic phototrophs (Gest, 1994; Madigan, 2006). They are also the only group of phototrophs that belong to the Gram‐positive group of bacteria. Heliobacteria are the only group of phototrophic bacteria known to form endospores, a characteristic of many other Gram‐positive bacteria. They are strict anaerobes and are rapidly killed by exposure to oxygen. The heliobacteria do not appear to be capable of photoautotrophic growth and require organic carbon compounds such as pyruvate in the growth medium. They contain bacteriochlorophyll g as their main photopigment, and also small amounts of chlorophyll a. They are active N2 fixers and are often isolated from rice paddies, where they may make a contribution to the nitrogen economy of those environments.
By any measure, the heliobacteria have the least sophisticated photosynthetic metabolism of all known phototrophic bacteria. They have a homodimeric reaction center complex (discussed more in Chapters 6 and 12) with very few protein subunits, no antenna complexes and no autotrophic carbon fixation pathway. They may be important in understanding the origin and early evolution of photosynthesis.
2.5.5 Chloracidobacteria
The chloracidobacteria are the second most recently discovered group of phototrophic bacteria, found in hot spring microbial mats (Bryant et al., 2007). They have a homodimeric reaction center and chlorosome peripheral antenna complexes. They are aerobic organisms and are not capable of autotrophic carbon fixation. They contain bacteriochlorophyll a, c chlorophyll a, and small amounts of Zn bacteriochlorophyll