Cyanobacteria, often called blue-green algae, are ancient prokaryotes that fundamentally changed Earth’s atmosphere and biosphere. They initiated the Great Oxygenation Event approximately 2.4 billion years ago, permanently altering the planet’s reducing atmosphere by introducing molecular oxygen. They are ubiquitous, thriving in environments ranging from the open ocean and freshwater lakes to hot springs and polar regions. Their success across diverse habitats stems from a sophisticated photosynthetic apparatus and metabolic flexibility, allowing them to efficiently convert light energy into chemical energy.
The Mechanism of Oxygenic Photosynthesis
Cyanobacteria perform oxygenic photosynthesis using a mechanism similar to plants, but housed within a prokaryotic cell structure. The process begins with light-dependent reactions executed by multi-protein complexes embedded in specialized thylakoid membranes. Unlike plant chloroplasts, these thylakoid membranes are not enclosed by a double membrane and float freely within the cytoplasm.
Light energy powers a sequence of electron transfers. The process initiates at Photosystem II (PSII), where light energy splits water molecules, providing electrons and releasing oxygen as a byproduct. These electrons move through carrier molecules, including the cytochrome b6f complex, which pumps protons across the thylakoid membrane to create an electrochemical gradient.
Electrons pass to Photosystem I (PSI), which absorbs light energy to boost their potential. The energized electrons reduce NADP+ to NADPH, a primary energy carrier. The proton gradient drives the synthesis of Adenosine Triphosphate (ATP) via the ATP synthase enzyme. These two forms of chemical energy, ATP and NADPH, power the subsequent carbon fixation reactions.
Specialized Carbon Acquisition Strategies
A major challenge for photosynthetic organisms is the inefficiency of the primary carbon-fixing enzyme, Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Rubisco fixes carbon dioxide (CO2) but also reacts with oxygen (O2), a competing process called photorespiration that wastes energy. To overcome this limitation, cyanobacteria evolved the Carbon Concentrating Mechanism (CCM).
The CCM actively accumulates inorganic carbon, primarily bicarbonate (HCO3-), within the cell cytoplasm using specialized membrane transporters. Bicarbonate is prevented from leaking out, building a reservoir that is converted into CO2 at the fixation site. This internal concentration can far exceed the surrounding environment, significantly improving photosynthetic efficiency.
The conversion and fixation of this stored carbon occur inside specialized protein microcompartments known as carboxysomes. These polyhedral structures encapsulate nearly all of the cell’s Rubisco, along with the enzyme carbonic anhydrase. Carbonic anhydrase rapidly converts the accumulated bicarbonate into CO2 right at the Rubisco active site.
The carboxysome’s protein shell acts as a selective barrier, allowing bicarbonate to diffuse in while retaining the generated CO2. This creates a localized, highly saturated CO2 environment. This high CO2 concentration effectively outcompetes oxygen for the Rubisco active site, suppressing photorespiration and maximizing carbon fixation. This spatial compartmentalization optimizes photosynthesis in environments where dissolved CO2 is scarce.
Metabolic Adaptation: Nitrogen Fixation and Heterocysts
Many cyanobacteria convert inert atmospheric dinitrogen gas (N2) into usable ammonia (NH3), a process called nitrogen fixation. This function allows them to colonize nitrogen-limited environments. Nitrogen fixation is incompatible with oxygenic photosynthesis because the responsible enzyme, nitrogenase, is destroyed by oxygen.
To reconcile these conflicting demands, filamentous cyanobacteria use spatial separation through cellular differentiation. When fixed nitrogen is scarce, vegetative cells differentiate into specialized heterocysts. This involves morphological and physiological changes to create an internal anaerobic environment for nitrogenase activity.
Within the heterocyst, Photosystem II is deactivated, halting internal oxygen production and ceasing photosynthesis. The cell develops a thickened, multi-layered wall that acts as a diffusion barrier to exclude external oxygen. The cell also increases its respiration rate to scavenge any residual oxygen.
The resulting anoxic microenvironment allows nitrogenase to function safely, converting N2 into ammonia using energy supplied by vegetative cells. This fixed nitrogen, typically glutamine, is transported back to neighboring cells to sustain the filament. This cooperative arrangement demonstrates a sophisticated form of multicellularity, specializing metabolic tasks for organism survival.
Dynamic Responses to Environmental Stressors
Cyanobacteria cope with environmental fluctuations, concerning light and temperature, through physiological changes. When light intensity exceeds utilization capacity, they activate photoprotective mechanisms to dissipate excess energy as heat, known as non-photochemical quenching. This prevents damage to Photosystem II caused by photo-oxidative stress.
Light Quality Adaptation
The composition of the photosynthetic machinery is flexible in response to light quality. This phenomenon, known as chromatic adaptation, involves changing the pigment composition of the phycobilisomes (external light-harvesting antennae). For example, in deeper water, cyanobacteria increase pigments like phycoerythrin to maximize absorption of penetrating green and blue light.
Survival Structures
Certain species cope with extreme environmental fluctuations by forming specialized survival structures. Akinetes are thick-walled, dormant cells filled with storage compounds resistant to desiccation, freezing, and darkness. These resting spores allow the organism to survive harsh, unfavorable conditions until conditions improve.