Let us make an in-depth study of the mechanism of bacterial photosynthesis. After reading this article you will learn about (A) Light Reaction and (B) Carbon Assimilation.
(A) Light Reaction:
In contrast to algae and higher plants which are oxygenic (i.e., they evolve O2 during photosynthesis and have two photo-systems that act in tandem or series, the photosynthetic bacteria are anoxygenic (i.e., they do not evolve O2 during photosynthesis and have comparatively simple photo transduction machinery with only one type of photosystem and reaction centre.
Purple bacteria have Type II Reaction Centre which passes electrons through bacteriopheophytin (bacteriochlorophyll lacking central Mg2+ ion) to a quinone. Green sulphur bacteria have Type I Reaction Centre that passes electrons to an Fe-s protein.
(1) Type II Reaction Centre (The Bacteriopheophytin – Quinone Reaction Centre):
In purple bacteria, P-870 constitutes the reaction centre of the only one pigment system present. When P-870 (B. Chl.a) receives a photon of light, it get excited (*). An electron with extra energy is ejected from it which is immediately (within pico seconds) captured by bacteriopheophytin a (B. Pheo).
Thus, charge separation occurs with a positive charge on bacteriochlorophyll and negative charge on bacteriopheophytin:
P-870 + Photon (hv) → P-870* (excited)
P-870 + BPheo → P-870+ + BPheo– (Charge separation)
From BPheo–, the electron is transferred within micro-seconds to a tightly bound molecule of quinone (QA), converting it into a semi-quinone radical (QA). The electron from QA is taken by another quinone (QB) which is loosely bound to the membrane. Two such electron transfers convert QB to its fully reduced anionic form QB2-. The latter also takes two protons (2H+) from the cytosol and is converted into fully reduced uncharged form, the hydroquinone (QBH2). The latter now freely diffuse in the membrane bi-layer.
From QBH2, the electrons (one by one) are transferred to Cyt. C2 through Cyt. bC1 complex and finally back to the reaction centre, thus completing the cycle (Fig. 11.31 A) (Cyt bC1 complex is homologous to complex III in mitochondria).
The energy of electrons flow through the Cyt bC1 complex causes proton pumping across the membrane, producing a proton motive force that powers synthesis of ATP from ADP + Pi by ATP synthase (photophosphorylation). The proton electrochemical gradient across the membrane in purple bacteria is from outside (periplasm) to inside (cytosol).
(2) Type I Reaction Centre (The Fe-S Reaction Centre):
In green sulphur bacteria, P-840 constitutes the reaction centre of the only one pigment system present. Contrary to the cyclic photosynthetic electron transport of purple bacteria, the photosynthetic electron transport in green sulphur bacteria appears to involve both cyclic and non-clyclic routes (Fig. 11.31B).
(i) Cyclic photosynthetic electron transport:
Excitation of P-840 in pigment system by a photon of light results in transfer of an electron from the reaction centre to Cyt be1 complex through a quinone (Q) and back to the reaction centre via Cyt C553. The electron transport through the Cyt bc1 complex causes proton pumping across the membrane, producing a proton motive force that powers synthesis of ATP from ADP + Pi by ATP synthase (photophosphorylation).
(ii) Non-cyclic photosynthetic electron transport:
During this process, some electrons flow from excited P-840 to an Fe-S protein Ferredoxin (Fd), which in turn passes electrons to NAD+ through Fd: NAD-reductase and ultimately forming NADH (Fig. 11.31B). The electrons from the reaction centre which reduce NAD+ → NADH, are compensated by electrons coming from oxidation of H2S to elemental S and then to SO42- (not shown in figure). This process is chemically analogous to oxidation of H2O by oxygenic plants.
(B) Carbon Assimilation:
Like algae and higher green plants, photosynthetic bacteria also utilise the assimilatory power (NADH2 + ATP) generated during light reaction to reduce CO2 to synthesize organic matter. However, some photosynthetic bacteria may also reduce simple organic compounds and photosynthesize complex organic matter in the cells.
Three categories of carbon assimilation in photosynthetic bacteria have been recognised:
(i) The Calvin Cycle:
Certain photosynthetic bacteria e.g., Rhodospirillum rubrum make use of this cycle to synthesize carbohydrates by reducing CO2. However, since these bacteria do not store or utilise carbohydrates, lesser amount of sugar photophates have been detected in them during photosynthesis.
(ii) Reductive Carboxylic Acid Cycle:
In some photosynthetic bacteria such as Chlorobium, Chromatium etc. another carbon reduction cycle is known to operate which is called as Reductive Carboxylic Acid Cycle (Fig. 11.32).
Reduced ferredoxin generated during photochemical process has strong reducing potential which drives the reversal of two reactions of Krebs’ Cycle which are otherwise irreversible in aerobic cells:-
1. Acetyl-CoA + CO2 + Fd (Red) → Pyruvate + CoA + Fd (oxi)
2. Succinyl-CoA + CO2 + Fd (Red) → α-ketoglu tarate + CoA + Fd (oxi)
Each turn of this cycle incorporates 4CO2 molecules which results in net synthesis of one oxaloacetate molecule. Oxaloacetate by further metabolism through this cycle provides C2—C6 carbon compounds which are utilised for the synthesis of various amino acids, lipids and other organic compounds in bacterial cells. However, the amino acids are main soluble products of photosynthesis in such bacteria.
(α-keto acids such as pyruvate, oxaloacetate and α-ketoglutarate produced during this cycle after amination may result in the formation of amino acids-alanine, aspartate and glutamate respectively).
Reductive carboxylic acid cycle does not occur in algae and higher green plants.
(iii) Carbon Assimilation in Bacteria Using Organic Compounds:
Certain purple bacteria make use of simple organic compounds such as acetic acid, butyric acid, propionic acid etc. as the major carbon source for photosynthesizing organic matter in the cells. In such cases, photo assimilation of organic compounds usually directly leads to the formation of organic polymers. For instance, in Rho does spirillum rubrum conversion of acetic acid into poly-β-hydroxybutyric acid is a reductive process:-
2nC2H4O2 + 2nH → (C4H6O2)n + 2nH2O
Similarly, photo assimilation of succinic acid, propionic acid etc. leads to the accumulation of glycogen like polysaccharides. Photosynthesis of these carbon reserves inside the bacterial cells appears to be analogous to the formation of starch in algae and higher green plants.