Rethinking the Limitations of Photosynthesis in Cyanobacteria

Cyanobacteria (blue-green algae), the progenitors of all eukaryotic photosynthetic organisms, are free-living or symbiotic prokaryotes that are found in a broad range of ecological habitats. Although cyanobacteria mainly populate aquatic (freshwater, marine, and hypersaline) environments in the water column or the benthic zone ¹ , they are also found in terrestrial environments in a variety of moderate and extreme conditions, including the most inhospitable and infertile habitats on Earth ² . Cyanobacteria can inhabit cold and frozen zones ³ but are also found in hot springs ⁴ or on volcanic ash, dry rocks, ⁵ and even soil crusts in deserts, where they face exposure to high irradiance and extreme temperatures ⁶ . Cyanobacteria can live in nutrient-rich or nutrient-depleted waters and soils. This ability to grow in diverse habitats results primarily from metabolic and physiological flexibility. Although they are photoautotrophs that require only water, sunlight, CO₂, and certain minerals for growth, cyanobacteria have also preserved their heterotrophic metabolism, enabling them to live in darkness for extended periods of time and, in a number of strains, enabling a photomixotrophic mode of living. Many cyanobacterial species are facultative diazotrophs that fix gaseous nitrogen to a bioavailable form in the absence of fixed nitrogen in the environment. In fact, these species are the only nitrogen fixers found in aerobic environments ⁷ .