The evolution of photosynthesis and its environmental impact

Abstract

Photosynthesis in plants is a very complicated process, utilizing two photosystems in series to carry out the very energy-demanding process of oxidizing water to molecular oxygen and reducing carbon dioxide to organic compounds. The process of carbon dioxide assimilation, today taking place mainly in conjunction with photosynthesis, appears to be even older than photosynthesis itself. The first photosynthetic organisms, living more than 3.4, perhaps even 3.8 Ga (Ga = gigayears = thousand million years) ago, carried out a simpler process than that observed in plants and likely did not result in oxygen production and only utilized one type of photosystem. A great variety of such one-photosystem photosynthesizers are living even today, and by comparing them, and from chemical fossils, researchers are trying to piece together a picture of the course of the earliest evolution of photosynthesis. Chlorophyll a may have preceded bacteriochlorophyll a as a main pigment for conversion of light into life energy. Oxygenic photosynthesis, i.e., photosynthetic production of molecular oxygen, first appeared in ancestors of present-day cyanobacteria more than 2.7, perhaps already 3.7 Ga ago. Cyanobacteria entered into close association with other organisms more than 1.2 Ga ago. There are indications that chloroplasts in green algae and green plants as well as those in glaucophytes and algae on the “red” line of evolution (red algae, cryptophytes, diatoms, brown algae, yellow-green algae, and others) originated from a single early event of endosymbiotic uptake of a cyanobacterium into a heterotrophic organism. A single event of secondary endosymbiosis, in which a red algal-like organism was taken up into another eukaryote, resulted in the diversity of organisms in the “red line” of algae that is also extended by tertiary (third level) endosymbiotic events. At the end of this chapter, a brief review is given of the evolution of the assimilation of carbon dioxide, the adaptation to terrestrial life, and the impact of photosynthesis on the terrestrial environment.