Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 29

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.