The History of Cyanobacteria’s Evolution

Cyanobacteria are a phylum of prokaryotic bacteria that generate energy through photosynthesis. Often referred to by their common name, “blue-green algae,” they are fundamentally bacteria, not true algae. These ancient microorganisms have profoundly shaped Earth’s history. Their activity over billions of years has driven significant transformations in the planet’s environment, setting the stage for the evolution of all complex life.

Origin and the Early Fossil Record

Life on early Archean Eon Earth existed in an anoxic world, with an atmosphere devoid of free oxygen. In this environment, some of the planet’s first life forms evolved, among which were the ancestors of modern cyanobacteria. While the precise timing is debated, evidence suggests their emergence may have occurred as far back as 3.5 billion years ago. These early microbes likely first evolved in freshwater environments before colonizing marine habitats.

The most compelling physical evidence for their ancient origins comes from stromatolites. These are layered sedimentary structures created by the trapping and binding of sediment grains by vast microbial mats. Fossilized stromatolites provide some of the oldest records of life on Earth, demonstrating the long-standing presence of these microbial communities.

Further evidence is found in microfossils, the preserved remains of individual microscopic organisms. Though interpreting these ancient fossils can be ambiguous, some, like Eoentophysalis belcherensis from the 1.89 billion-year-old Belcher Supergroup in Canada, have been identified with certainty as cyanobacteria. These fossil records help scientists piece together the timeline of their emergence.

The Great Oxidation Event

The evolution of oxygenic photosynthesis by cyanobacteria represents a significant innovation in the history of life. Before this, organisms relied on anoxygenic photosynthesis, which used compounds like hydrogen sulfide instead of water and did not produce oxygen. Cyanobacteria were the first to evolve the ability to use water, a plentiful resource, in a process that released oxygen as a waste product. This metabolic shift set in motion a planetary transformation.

For hundreds of millions of years, the oxygen produced by cyanobacteria was absorbed by the oceans, where it reacted with dissolved iron. This process created massive deposits of rusted iron that settled on the seafloor, forming the banded iron formations that are a distinct geological marker of this era. Once the iron in the oceans was consumed, oxygen began to accumulate in the atmosphere, leading to the Great Oxidation Event (GOE) between 2.4 and 2.1 billion years ago.

This rise in atmospheric oxygen was catastrophic for the planet’s existing anaerobic life, for which oxygen was toxic. The event triggered a mass extinction, wiping out many microbial lineages that could not adapt. The increase in atmospheric oxygen also reacted with and reduced methane levels, potentially triggering the Huronian glaciation. This newly oxygenated environment, however, created the conditions for the evolution of organisms that could use oxygen for respiration, a more efficient way to generate energy.

Endosymbiotic Theory and the Rise of Eukaryotes

Long after the GOE reshaped the planet’s atmosphere, cyanobacteria played another central role in the emergence of eukaryotes through a process known as endosymbiosis. The leading theory proposes that a large, early eukaryotic host cell engulfed a smaller, free-living cyanobacterium.

Instead of being digested, the cyanobacterium survived inside the host cell, and over time, a symbiotic relationship developed. The host provided protection and nutrients, while the cyanobacterium provided the host with energy produced through photosynthesis. This arrangement proved so advantageous that the cyanobacterium became a permanent, integrated part of the host cell, eventually evolving into the specialized organelle known as the chloroplast.

This endosymbiotic event is believed to have happened only once, giving rise to a common ancestor for all plant and algal life on Earth, a group known as Archaeplastida. The chloroplasts within their cells are the direct descendants of that once-independent cyanobacterium.

Modern Adaptations and Diversity

Cyanobacteria are a diverse and ecologically significant group of organisms that continue to evolve today. Their evolutionary success is partly due to the development of traits such as nitrogen fixation. This process allows them to convert inert atmospheric nitrogen into ammonia, a form of nitrogen that other organisms can use, making them important primary producers that fertilize ecosystems.

Their adaptability is demonstrated by their presence in some of the most extreme environments on Earth. As extremophiles, different species of cyanobacteria thrive in hot springs, desert crusts, and even within polar ice. They also form symbiotic relationships with a wide range of other organisms, most notably with fungi to form lichens, which can colonize bare rock and initiate soil formation.

Cyanobacteria remain a component of aquatic food webs, contributing significantly to global carbon and nitrogen cycles. However, under certain conditions, such as nutrient pollution in waterways, their rapid growth can lead to harmful algal blooms (HABs). These blooms can produce toxins that are dangerous to other aquatic life and humans.