Chapter 7 - Mechanisms of Photoprotection in Cyanobacteria

Abstract

Cyanobacteria represent a unique group of prokaryotes that are able to perform oxygenic photosynthesis and constitute crucial microflora with respect to total biomass and productivity in terrestrial and aquatic ecosystems. They are natural biofertilizers and also serve as potent source of natural products of industrial and medicinal values. Harvesting of light for photosynthesis exposes cyanobacteria to unpredictable changes in light intensity and harmful doses of solar ultraviolet radiation (UVR). High light intensity induces photooxidative damage of the reaction centers due to the generation of reactive oxygen species and hence, is lethal for cyanobacteria. UVR adversely affects several biological processes, such as growth, development, orientation and motility, photosynthesis, pigmentation, CO2 assimilation, enzyme activity, and N2 fixation. DNA shows absorption in the UV range, hence, is one of the prime targets of UVR. Inclusive survival of cyanobacteria in diverse ecological niches and adaptive diversification have compelled them to evolve an array of survival strategies to compete and sustain successfully in different environments with high light intensity together with UV fluxes on the Earth. Photoprotective mechanisms such as avoidance, nonphotochemical quenching to dissipate excess excited-state energy as heat, synthesis of photoactive proteins such as orange carotenoid protein, fluorescence recovery protein, early shock proteins, late acclimation proteins, antioxidative enzymes, sunscreens such as scytonemin and mycosporine-like amino acids, repair, and programmed cell death are adapted by cyanobacteria to counteract the damage caused by high light conditions and UVR. This chapter provides an in-depth account of important mechanisms involved in photoprotection in cyanobacteria.