From ionizing radiation to photosynthesis

Abstract

This chapter focuses on the driving forces of the origin and the evolution of the early anoxygenic photosynthesis before the onset of the oxygenic cyanobacterial photosynthesis 2.8-2.4 billion years ago. The early Earth conditions were sustained by the global cycling of carbon dioxide among the atmosphere, primordial crust, and the ocean. High-energy ultraviolet radiation played a decisive role in this interaction, maintaining the temperature of the ocean and promoting mildly oxidizing conditions in the atmosphere and the upper layers of the ocean. Constrained by the sterilizing effect of the UV radiation and the evaporating consequences of the Late Heavy Bombardment, chemolithotrophic life was likely to emerge at the seafloor under excess of carbon dioxide and a constant flux of reducing hydrothermal fluids. Trophic diversification had launched the expansion of the chemolithotrophic lineages to the photic zone. Alternative hypotheses of the terrestrial origin of life with implications for the origin of photosynthesis are discussed. The review reconstructs the development of the ancient photosynthetic habitats under the strong selective pressure of the UV radiation and discusses available hypotheses of the origin of photosynthesis with regard to emergence of workable photosynthetic mechanism. The evolution of the homodimeric protophotosystem and its divergence to the homodimeric iron-sulfur-type and the homodimeric quinone-type reaction centers were probably driven by the availability of the exogenous electron donors in a series of ecological successions from the oceanic photic zone to the shallow water habitats and microbial mats. This process had eventually resulted in the advent of independence of the photosynthetic organisms from the geochemical conditions in cyanobacterial lineages that were able to oxidize water.