Abiotic or abiogenic photosynthesis is the synthesis of organic compounds with the aid of radiant energy and various inorganic or organic catalysts.
In September 1912, Benjamin Moore suggested at a discussion on the origin of life, held by the joint sections of Zoology and Physiology of the British Association for the Advancement of Science, that “the first step towards the origin of life must have been the synthesis of organic matter from inorganic by the agency of inorganic colloids acting as transformers or catalysts for radiant solar energy” (Moore and Webster 1913).
In spite of Haldane’s well-known idea that UV light may have served as a driving force for formation of the first viruslike organisms (Haldane 1929), the idea of directly driving abiogenesis by solar energy had not won much support at that time, despite the fact that the Sun is by far the most powerful energy source on Earth. The limited acceptance of the idea was partly due to the low quantum yield of abiotic photosynthetic reactions and the poor reproducibility of experimental results. Abiotic photoproduction of hydrogen, in the presence of ions of divalent iron, has been observed (Mauzerall et al. 1993). It has been shown atmospheric photochemistry can produce aldehydes from CO (Bar-Nun and Chang 1983). Only in the 1980s, were robust procedures of producing colloidal nanoparticles of photoactive semiconductors, such as zinc sulfide (ZnS) or cadmium sulfide (CdS), developed (Henglein 1984). These particles (see Fig. 1), due to their high surface-to-volume ratio, provided experimental systems in which the photoreduction of CO2 to diverse organic compounds could be studied. The photoreduction proceeded with high and reproducible quantum yield (up to 80 % for CO2 reduction to formate at the surface of colloidal ZnS particles (Henglein 1984)). Recent studies have demonstrated high-yielding ZnS- and MnS-mediated photosynthesis under simulated primeval conditions (Zhang et al. 2004, 2007; Guzman and Martin 2009). In the modern oceans, ZnS and MnS are found at the sites of the geothermal activity, where minute particles of these minerals continuously precipitate around hot, deep-sea hydrothermal vents; thereby, particles of ZnS and MnS, slowly precipitating sulfides, make rings around black throats of such vents that are covered by promptly precipitating particles of FeS (Tivey 2007). On the primordial Earth, hot metal-enriched geothermal fluids and vapor may have discharged to the surface of the first continents, so that particles of ZnS and MnS could have precipitated within regions exposed to solar radiation (Mulkidjanian 2009). These sulfide minerals could have been present in shallow waters (Guzman and Martin 2009) and should have precipitated around continental thermal springs (Mulkidjanian 2009). Since Zn2+ ions are much more volatile than Fe2+ ions, the vapor of continental geothermal systems would be particularly enriched in ZnS (Mulkidjanian et al. 2012). On the primordial Earth, ZnS could not be oxidized by atmospheric oxygen, so that photosynthesizing and habitable rings may have persisted around terrestrial thermal springs and fumaroles. The development of the first life forms within photosynthesizing, ZnS-containing precipitates at such anoxic geothermal fields, where Zn2+ ions would be continuously released as by-products of abiogenic photosynthesis, might explain cellular enrichments in Zn2+, the equilibrium concentration of which in the primordial ocean should have been extremely low (Mulkidjanian and Galperin 2009; Mulkidjanian et al. 2012). Several proteins shared by all extant organisms and believed to form the core of the last universal cellular ancestor (LUCA) are particularly enriched in Zn and Mn; this may also support the role of abiogenic photosynthesis in the earliest stages of evolution (Mulkidjanian and Galperin 2009; Mulkidjanian et al. 2012). Since these ubiquitous proteins are depleted in iron, it remains to be established whether and to what extent divalent iron, the predominant transition metal in geothermal exhalations, was involved in abiogenic photosynthesis. It has also been shown that titanium dioxide particles can drive photosynthetic organic chemistry inside cell membrane-like vesicles (Summers et al. 2009). Titanium dioxide (both rutile and anatase) particles could have been formed by precipitation or released (directly or from alteration of other titanium minerals) by weathering. This energy transduction would provide pathways to new compounds in a prebiotic system or support early biochemical reactions.
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