Adaptive Mechanisms of the Model Photosynthetic Organisms, Cyanobacteria, to Iron Deficiency

Cyanobacteria are the oldest oxygen-evolving photosynthetic organisms on the Earth. They are widely distributed in marine, freshwater, and terrestrial environments and contribute about 25% of global primary productivity. They are thought to be responsible for the conversion of the Earth’s atmosphere from anaerobic to aerobic about 2.4 billion years ago. This development permitted the evolution of aerobic bacteria, algae, plants, and animals. However, due to the emergence of oxidative environments on the Earth’s surface, soluble ferrous iron (Fe2+) was almost completely oxidized to hardly soluble ferric iron (Fe3+) in aquatic environments. The extremely low bioavailability of iron in the ocean has been considered as an important factor that is limiting global primary productivity. As photosynthetic organisms, cyanobacteria have higher iron demand than other non-photosynthetic organisms to meet the needs of photosynthetic electron transport and chlorophyll synthesis. The nitrogen-fixing cyanobacteria need even more iron to fix the inert dinitrogen gas. The contradiction between the high iron demand of cyanobacteria and their iron-limiting habitats has forced them to evolve special strategies to overcome iron deficiency during the long-period evolution. In this review, we summarized the recent perspectives on the physiological responses and special strategies of cyanobacteria to overcome the changing iron bioavailability in freshwater, coastal, and open-ocean environments.