Plant Physiology - Musiyenko M.M. 2001

Photosynthesis: physiological, biochemical, and ecological aspects
Bacterial photosynthesis

Phototrophic Nutrition of halobacteria. A group of prokaryotes (halobacteria — Halobacterium), classified as archaebacteria, occupies a unique position regarding phototrophic nutrition.

Morphologically and functionally, these Bacteria are the simplest phototrophic organisms. Their photosynthetic apparatus is restricted to specific Regions of the cytoplasmic membrane, known as the purple membrane, which consists almost exclusively of a single protein species arranged in a hexagonal lattice Structure. This protein is Bacteriorhodopsin, which forms seven transmembrane α-helical segments oriented perpendicularly to the plane of the purple membrane. Much like the visual pigment in the vertebrate retina, it contains retinal, albeit in a different configuration. Bacteriorhodopsin absorbs predominantly quanta in the green region of the spectrum, with each bacteriorhodopsin molecule being homologous to a reaction center. Residing within the purple membrane, it Functions as a light-driven proton pump that converts solar energy—with the aid of a proton-translocating ATPase—into the ATP energy required for the metabolic activities of halobacteria. This elementary process of phototrophic nutrition involves neither electron transport nor the generation of NADPH. Nonetheless, halobacteria are capable of performing reductive CO₂ fixation.

Photoautotrophic and photoheterotrophic bacteria. There are two MAIN TYPES OF photosynthetic bacteria: photoautotrophs (e.g., green and purple sulfur bacteria) and photoheterotrophs (e.g., certain purple non-sulfur bacteria). Cyanobacteria perform Photosynthesis in a manner analogous to higher plants. Photosynthetic bacteria are capable of converting light energy into chemical energy, yet their photosynthesis occurs under anaerobic conditions and without the evolution of oxygen. This is because, unlike Higher Plants and cyanobacteria, they possess only a single photosystem (PSI). Furthermore, in these organisms, the light-dependent reduction of CO₂ is coupled with The oxidation of organic or Inorganic Compounds.

Purple bacteria and green sulfur bacteria contain bacteriochlorophyll in both the reaction center and the light-harvesting complex. Bacterial photosynthesis, much like that of higher plants, relies on a light-induced transfer of electrons and protons from a substrate molecule to an acceptor molecule, resulting in The formation of in vivo NADH+H⁺. In bacterial photosynthesis, NADH (unlike NADPH in higher plants) acts as a specific hydrogen carrier. Its reduction proceeds via a flavoprotein through a reaction terminally linked to Cyclic electron transport. Alongside this, cyclic electron transport coupled with ATP synthesis is also observed.

Of particular importance is the Hydrogenase reaction (also identified in unicellular green Algae) for the generation of NADH+H⁺. During this “photoreduction,” molecular hydrogen is activated by hydrogenase in interaction with the photochemical system, allowing the coenzyme to become bound. Due to the reversibility of the hydrogenase reaction, such organisms possess The ability to release molecular hydrogen when reducing equivalents are in excess and reducing compounds are deficient. This not only maintains the proper equilibrium between NAD and NADH+H⁺, but also protects the photochemical system from light-induced damage by sustaining electron flow. This principle serves as the basis for developing biological and artificial systems aimed at producing molecular hydrogen from Water using solar energy as an environmentally friendly energy source.

Carbon dioxide assimilation in phototrophic bacteria proceeds via The Calvin Cycle. RUBISCO has been identified as the key enzyme. However, its molecular structure differs from that operating in higher plants and algae (in eukaryotes, this enzyme consists of 8 small and 8 large subunits). For instance, Rhodospirillum rubrum contains only 2 large subunits and no small subunits.

In certain bacteria, such as Chromatium and Chlorobium thiosulfatophilum, an additional pathway for Pyruvate synthesis from acetyl-CoA and CO₂ via ferredoxin is also possible. Other variants of bacterial photosynthesis may occur, which can be attributed to the remarkable diversity of bacterial structures and functions, the considerable phylogenetic age of prokaryotes, and their capacity to occupy ecological niches unexploited by eukaryotes.