Molecular Mechanism of Photosynthesis Driven by Red-Shifted Chlorophylls

Abstract

Photosynthesis is the process of light-driven production of organic molecules needed as starting components for whole cellular processes and as the energy source. It is carried out by primary producers including land plants, algae, and oxygenic/anoxygenic photosynthetic bacteria. Oxygenic photosynthesis involves two stages: light-dependent reactions generating NADPH and ATP molecules with oxygen as a by-product and light-independent reactions involving utilization of NADPH and ATP as the energy source to convert carbon dioxide into organic molecules. Light-dependent processes require light-absorbing pigments categorized as carotenoids, bilins, and chlorophylls. Chlorophylls and carotenoids are ubiquitous to all photosynthetic organisms, while bilins in phycobiliprotein complexes are specific to cyanobacteria, rhodophytes, glaucophytes, and cryptophytes. Each pigment has several variations, and a particular type of organism has a specific set optimized for light-capture and photosynthetic efficiency in the given environment. The common chlorophyll to each oxygenic photosynthetic organism is chlorophyll a, with chlorophyll b, c, d, or f synthesized depending on the organism. Chlorophyll d (3-formyl-chlorophyll a) and chlorophyll f (2-formyl-chlorophyll a) have far-red long-wavelength absorption features, namely, red-shifted chlorophylls. These chlorophyll molecules enable some cyanobacterial species to grow in shaded environments and establish unique habitats. Acaryochloris marina is predominantly a chlorophyll d-containing cyanobacterium with minor (≤ 5%) amounts of chlorophyll a under all culture conditions, while all chlorophyll f-producing cyanobacteria constitute chlorophyll f as a minor chlorophyll ~2-15% of the total chlorophyll pool and only produced under far-red light conditions. This chapter will focus on summarizing the current knowledge of the light-dependent processes with particular attention on structures and functions of far-red absorbing pigments and pigment-binding protein complexes.