Anoxygenic and Oxygenic Photosynthesis

After reading this article you will learn about the anoxygenic and oxygenic photosynthesis.

The Anoxygenic Photosynthesis:

Anoxygenic photosynthesis uses energy from sunlight to couple the reduction of C in CO₂ to the anaerobic oxidation of S in S⁰ or H₂S. If one is accustomed to thinking of S oxidation in a strictly aerobic sense, then anaerobic S oxidation appears contradictory. Anoxygenic photosynthesis would have been compatible with the anoxic (O₂-free) conditions of the earth’s primordial atmosphere.

Dominance of anoxygenic photosynthesis would have favoured anaerobic respiration or fermentative pathways for obtaining energy from the products of photosynthesis. Consequently one might expect, and certainly one finds, a wide representation of anaerobic micro-organisms in soil environments.

Many elements can cycle under entirely anaerobic conditions due to the syntrophic relationships among photolithotrophs and anaerobic chemoorganotrophs.

With the input of electromagnetic radiation to power photosynthesis is photolithotrophs that reduce C and oxidize elements such as S, combined with decomposition by anaerobic chemoorganotrophs to re-oxidize C and re-reduce S, one can summarize the anoxygenic cycle as:

4CO₂ + 2H₂S + 4HOH ←→ 2SO₄^2- + 4CH₂ + 4H⁺.

Energy trapped as organic compounds during an oxygenic photosynthesis could be released by oxidation of the reduced C (e^– donor) coupled to reduction of the oxidized minerals (e^– acceptor) formed during anoxygenic photosynthesis.

The Oxygenic Photosynthesis:

Oxygenic photosynthesis can be carried out by many eukaryotes, but only by the cyanobacteria among the prokaryotes. Consequently, anoxygenic photosynthesis is dominantly prokaryotic and oxygenic photosynthesis dominated by eukaryotes. The oxygenic cycle has two circuits.

First, a photo-chemo circuit consisting of photoaquatrophs to reduce C coupled with chemoorganotrophs, which may be either aerobic or anaerobic, to re-oxidize it. Second, a chemo-chemo circuit consisting of aerobic chemolithotrophs to oxidize minerals coupled with anaerobic (or facultative) chemoorganotrophs to oxidize C and re-reduce minerals (Fig. 18.52).

Electrons flow among the cycles, thereby connecting them, For example, an electron from water may be passed to CH₂O during photosynthesis, proceed through the photo-chemo circuit, and be returned to water through aerobic oxidation.

From there it may again be passed to CO₂ (photosynthesis) to form CH₂O proceed to the chemo-chemo circuit, and under anaerobic conditions be used to reduce SO₄^2- to H₂S. In the presence of O₂, it proceeds through the aerobic part of the chemo-chemo circuit and the electron is again used to reduce CO₂ to CH₂O concurrently with oxidation of (loss of electrons from) S^2-.

As it continues its journey, under anaerobic conditions the electron may again be transferred to H₂O, which bring it to the intersection again with the photo-chemo circuit. Hence, the oxidation and reduction of many elements, although not conducted by photosynthetic organisms, is tied to photosynthesis by transfers of electrons among organisms through the reduced C and O₂ produced by photosynthesis.

Hence, the oxidation of elements is made possible by O₂ from photosynthesis and their reduction by the reduced C from photosynthesis. This alternating oxidation-reduction system involving chemolithotrophs requires that O₂ and CH₂O from photosynthesis travel separately and that there be habitats from which the O₂ is excluded. Soils are uniquely suited to providing such habitats.

The two chemotrophic circuits of Fig. 18.52 can be expanded to distinguish aerobic, facultative, and anaerobic domains based on sensitivity to O₂. Overall control is provided by O₂ because it inhibits anaerobic processes. Consequently the balance among the three domains in Fig. 18.52 is a function of O₂ availability in local environments or microsites.

N₂ fixation is interesting in that it is inhibited by O₂ and mediated by three of the four groups of organisms, with only chemolithtrophs excluded. In addition, the chemoorganotrophs are responsible both for removing N from the pedosphere, by reducing NO₃^– to N₂, and for returning it by further reducing N₂ to NH₃.

As the E_h becomes increasingly negative oxidants become decreasingly effective (Fig. 18.52). The energy available through a redox reaction is directly proportional to the change in oxidation potential between the two couple, less and less energy is released as one moves from aerobic to strictly anaerobic metabolism.

Soil organisms have evolved in an energy-limited environment so communities of soil organisms use the most energetically favourable energy source available to them. Such a strategy favour use of O₂ as an electron acceptor followed by NO₃^–, then SO₄^2-; after depletion of NO₃^– and SO₄^2- a portion of the C in CH₂O is eventually used during methanogenesis.

In other words, CH₂O is allocated first to reduction of O₂, then NO₃^– followed by SO₄^2-, and finally methanogenesis. From a practical perspective, then NO₃^– might be a useful electron acceptor for metabolism of organic contaminants under anaerobic conditions. Indeed NO₃^– and denitrifying populations have been proposed for removal of organic contaminants under anaerobic conditions.

It is not possible for O₂ to accumulate in the atmosphere from photosynthesis unless large quantities of CH₂O are not decomposed. Disrupting the cycle of photosynthesis and decomposition would retain O₂ in the atmosphere rather than consuming it in oxidation of photosynthetic.

In addition, oxidation-reduction reactions over geological time control Fe solubility and, because of its reactivity with P, Fe controls P concentration. Fe and P concentrations in solution, especially in marine environments, often limit photosynthesis and N₂ fixation.

The metabolism of polyaromatic hydrocarbon takes place by soil micro-organisms which are shown in Fig. 18.53.