Cyanobacteria are an ancient and highly diversified phylum of bacteria, often informally referred to as blue-green algae. These prokaryotic organisms lack a membrane-bound nucleus and are unique among prokaryotes for their specialized photosynthetic capability. They represent one of the oldest known life forms on Earth, with fossil records extending back at least 2.1 billion years. The evolutionary success of cyanobacteria stems from a single, transformative biochemical innovation that fundamentally altered the geochemistry of the planet. They are recognized as the primary biological architects of the modern, oxygen-rich atmosphere.
The Initial Spark of Oxygenic Photosynthesis
During the Archean eon, the early Earth possessed an anoxic atmosphere, containing virtually no free molecular oxygen. The first photosynthetic organisms were anoxygenic, utilizing compounds like hydrogen sulfide or iron as electron donors without producing oxygen. Ancestors of modern cyanobacteria branched off from other bacteria around 3.4 billion years ago.
The development of oxygenic photosynthesis—the ability to split a water molecule—was a monumental biochemical achievement. This process requires two interconnected reaction centers, Photosystem I and Photosystem II, to extract electrons from water. Utilizing stable water (H₂O) as an electron donor was a significant biological hurdle.
The chemical reaction uses light energy to convert carbon dioxide and water into glucose, releasing free oxygen (O₂) as a waste product. This innovation appeared between 3.4 and 2.9 billion years ago, marking the origin of the crown group of cyanobacteria.
Oxygen production was initially confined to local environments or “oxygen oases.” This oxygen was immediately consumed by various chemical sinks present in the oceans and atmosphere. It took a long period for the volume of oxygen produced to overwhelm these sinks and trigger a global change.
The Great Oxygenation Catalyst
Long-term oxygen production led to the Great Oxidation Event (GOE), a dramatic environmental transformation that began roughly 2.4 billion years ago. Before the GOE, oceans held vast quantities of dissolved ferrous iron (Fe²⁺), which acted as the largest oxygen sink on the planet.
As cyanobacteria released oxygen, it reacted with the dissolved iron, causing it to precipitate as insoluble ferric iron oxide (Fe₂O₃), or rust. This rust settled onto the seafloor, forming massive geological deposits known as Banded Iron Formations (BIFs). The alternating red and grey layers within BIFs record the early, fluctuating oxygen levels in the oceans.
The widespread deposition of BIFs became common between 2.8 and 2.5 billion years ago. Their global disappearance around 1.85 billion years ago signals the saturation of the deep ocean’s iron capacity. Once these sinks were exhausted, oxygen escaped into the atmosphere, marking the true onset of the GOE.
The rising oxygen levels had a catastrophic effect on existing anaerobic life forms. Oxygen was toxic to these organisms, and its accumulation led to a major mass extinction event sometimes termed the “Oxygen Catastrophe.” Surviving organisms were forced to retreat to anoxic niches or evolve mechanisms to utilize oxygen for respiration.
The rise in atmospheric oxygen also caused a significant climate shift. Oxygen reacted with methane, a potent greenhouse gas, reducing the greenhouse effect. This cooling is hypothesized to have triggered the Huronian glaciation, one of the most extensive “Snowball Earth” episodes, occurring around 2.29 to 2.25 billion years ago.
The Origin of Chloroplasts
Following the atmospheric shift, cyanobacteria were involved in the profound evolutionary step known as primary endosymbiosis. This process involved an ancient eukaryotic cell, likely a predator, engulfing a cyanobacterium sometime between 1 and 2 billion years ago.
Instead of being digested, the cyanobacterium was retained in a mutually beneficial relationship. Over time, the engulfed bacterium evolved into the chloroplast, the specialized organelle responsible for photosynthesis in plants and algae. This single event gave rise to the common ancestor of the Archaeplastida supergroup, which includes red algae, green algae, and glaucophyte algae.
The evidence for this bacterial origin is compelling. Chloroplasts possess their own circular DNA genome, distinct from the host cell’s nuclear DNA, characteristic of prokaryotes. They also replicate via binary fission, the method used by free-living bacteria.
The organelle is surrounded by a double membrane structure. The inner membrane is the original bacterial membrane, while the outer membrane derived from the host cell’s engulfing vesicle. Genetic integration occurred as the cyanobacterium transferred most of its genes to the host cell’s nucleus.
This event allowed the eukaryotic lineage to exploit solar energy, leading to the rapid diversification of photosynthetic eukaryotes. The success of terrestrial plants and marine algae traces directly back to this ancient partnership.
Modern Ecological Significance
Today, cyanobacteria remain major players in global biogeochemical cycles. Planktonic species, such as the tiny marine species Prochlorococcus, contribute over half of the total photosynthesis in the open ocean, producing a substantial portion of the world’s oxygen.
Certain filamentous species can fix atmospheric nitrogen (N₂), converting it into biologically usable forms like ammonia. This process occurs in specialized cells called heterocysts, making these cyanobacteria primary producers in nitrogen-limited aquatic and terrestrial environments.
However, cyanobacteria also present modern environmental challenges, primarily through the formation of harmful algal blooms (HABs). These blooms occur in freshwater and marine environments, often in response to nutrient pollution from excess nitrogen and phosphorus. Increased water temperatures associated with climate change also favor their dominance.
During these blooms, certain species produce powerful cyanotoxins, including:
- Microcystins
- Saxitoxin
- Cylindrospermopsin
These compounds pose a serious threat to public health by contaminating drinking water sources and causing illness or death in wildlife and livestock. The growing frequency and intensity of these toxic blooms represent a significant consequence of human impact on aquatic ecosystems.