Cyanobacteria, often mistakenly called blue-green algae, are a phylum of ancient, single-celled prokaryotes. Fossil evidence, such as stromatolites, dates their presence back 3.5 billion years ago. They are globally distributed and thrive in nearly every habitat, from oceans and freshwater to desert crusts. Cyanobacteria were the first to develop oxygenic photosynthesis, a process that fundamentally altered the chemistry of the planet’s atmosphere and oceans. Their success in harnessing sunlight to split water molecules set the stage for all complex life that followed.
The Unique Photosynthetic Process
Cyanobacteria perform oxygenic photosynthesis, meaning they use water as an electron donor and release free molecular oxygen (\(\text{O}_2\)) as a waste product. This process is carried out within internal membrane structures called thylakoids, which are distinct from the cell’s outer membrane. The initial, light-dependent stage is powered by two protein complexes: Photosystem I (PSI) and Photosystem II (PSII).
Photosystem II is responsible for the water-splitting reaction. Located within the thylakoid membrane, PSII contains the oxygen-evolving complex (OEC). This OEC is a cluster of four manganese ions and one calcium ion (\(\text{Mn}_4\text{CaO}_5\)) that acts as the site for water oxidation.
The OEC uses light energy to sequentially strip four electrons from two molecules of water. This yields four hydrogen ions (protons) and a single molecule of diatomic oxygen (\(\text{O}_2\)). The electrons are then passed through an electron transport chain to Photosystem I, where they are re-energized by light before being used to create energy-storing molecules.
To capture light, cyanobacteria utilize the primary pigment chlorophyll \(a\), similar to plants, along with accessory pigments called phycobilisomes. These phycobilisomes form antennae structures on the thylakoid surface, absorbing light from the green and yellow parts of the spectrum that chlorophyll \(a\) absorbs poorly. This contrasts with earlier anoxygenic photosynthesis, which used compounds like hydrogen sulfide and did not produce oxygen.
The Great Oxygenation Event
The continuous production of oxygen by cyanobacteria led to the first major environmental shift in Earth’s history. For the first two billion years of Earth’s existence, the atmosphere was anoxic, dominated by gases like methane and carbon dioxide. The initial oxygen released by cyanobacteria did not immediately accumulate in the air but was consumed by chemical reactions in the oceans.
Dissolved iron (\(\text{Fe}^{2+}\)) was abundant in the ancient oceans. As oxygen reacted with this dissolved iron, it oxidized the iron into insoluble ferric iron oxide (\(\text{Fe}^{3+}\)), which precipitated out of the water and settled on the seafloor. This process formed massive geological deposits known as Banded Iron Formations (BIFs).
These BIFs are characterized by alternating layers of iron-rich rock and silica-rich chert, providing a geological record of the oxygenation process. The layers represent cycles of oxygen release and iron precipitation that continued until the oceans became saturated with oxygen. The global disappearance of BIFs from the geological record around 1.85 billion years ago signals that the oxygen-scavenging capacity of the oceans had been exhausted.
Once the oceans could no longer absorb the oxygen, the gas began to vent into the atmosphere, initiating the Great Oxygenation Event (GOE) approximately 2.4 billion years ago. This atmospheric shift was catastrophic for the anaerobic organisms that had evolved in the absence of oxygen. Free oxygen was toxic to these microbes, leading to a massive extinction event. Only organisms that evolved mechanisms to neutralize or utilize oxygen survived, fundamentally restructuring the biosphere.
Cyanobacteria’s Evolutionary Legacy and Current Ecology
The transformative impact of cyanobacteria extended directly into the evolution of plants through endosymbiosis. This theory posits that an ancient eukaryotic cell engulfed a cyanobacterium over a billion years ago but did not digest it. Instead, the engulfed bacterium survived and established a permanent, symbiotic relationship with its host.
Over time, the cyanobacterium lost most of its genetic material and became the specialized organelle known as the chloroplast. This organelle is the site of photosynthesis in all modern plants and algae, making the cyanobacterium the direct ancestor of the world’s flora. Oxygenic photosynthesis was transferred to the eukaryotic domain, leading to the proliferation of plant life globally.
Beyond this evolutionary link, cyanobacteria perform functions vital to global biogeochemical cycles today. Many species are diazotrophs, meaning they perform nitrogen fixation. They convert inert atmospheric nitrogen gas (\(\text{N}_2\)), which is inaccessible to most organisms, into usable forms like ammonia (\(\text{NH}_3\)).
This process is carried out by the enzyme nitrogenase, which is highly sensitive to oxygen. To solve this paradox, certain filamentous cyanobacteria have evolved specialized, thick-walled cells called heterocysts. These cells lack Photosystem II and maintain an internal anaerobic environment, allowing nitrogen fixation to occur safely while neighboring cells continue to perform oxygenic photosynthesis.
Small, planktonic cyanobacteria like Prochlorococcus are primary producers in the open ocean, contributing a significant fraction of marine photosynthesis and forming the base of oceanic food webs.