Photosynthetic Organisms in Extreme Environments

Various extreme habitats such as thermal springs, deserts, hypersaline environment, polar regions, and acidic pools are present in the Earth, and these habitats harbor extremophilic oxygenic photosynthetic organisms. Photosynthetic organisms play key roles in sustaining the ecosystems of extremophilic habitats by acting as primary producers. Phototrophs thriving in these environments may face multiple extreme factors at the same time, e.g., in acidic thermal springs, both acidic conditions and high temperature are available. Phototrophs inhabiting extreme habitats exhibit a wide range of diversity, i.e., from prokaryotic archaea, bacteria, and cyanobacteria to eukaryotic algae and plants, which have been discussed in the present chapter. To sustain the photosynthetic processes at extreme habitats, these phototrophs possess some special adaptation strategies including morphological alterations, modulations in light absorption machinery, changes in carbon fixation enzymes, and structural and compositional modifications in chloroplasts and cyanobacteria. While the morphological changes, especially in the leaf structure and canopy architecture, govern the survival of higher plants in extreme conditions, adaptations in the composition and structure of phycobiliproteins form the basis of cyanobacterial existence in such conditions. Other notable adaptive responses of cyanobacteria thriving in the extreme environments include substitution of some amino acids in the proteins of photosystem II (PSII) complex and plastocyanin. Sequence analysis of D1 proteins of PSII from different groups of extremophilic phototrophs reflected the occurrence of many isoforms of this protein. Nevertheless, such analysis of plastocyanin synthesized by extremophilic phototrophs has not been performed yet. In this chapter, phylogenetic analysis of plastocyanin showed that extremophilic plants and eukaryotic algae share a similar type of plastocyanin, while the plastocyanin of extremophilic cyanobacteria made separate clade. Besides, the chapter also encompasses elaborate discussion on the pellucid role of compatible solutes and alkanes for unabating photosynthesis in extremophilic phototrophs. Although many research articles, related to photosynthesis behavior of organisms in extreme conditions, have been found in the literature, comprehensive compilation of these studies is still needed to further elucidate the existing lacunae. So, this chapter inclusively has diversity of extremophilic photosynthetic organisms and their adaptation features which make their photosynthesis sustainable in extreme conditions.