Cyanobacteria, often referred to as blue-green algae, are ancient single-celled organisms that perform photosynthesis. This process converts light energy into chemical energy, a capability central to life on Earth. As prokaryotic organisms, they lack a membrane-bound nucleus and other complex internal structures found in eukaryotic cells. Their photosynthetic ability defines their ecological significance across various environments.
The Photosynthetic Process in Cyanobacteria
Cyanobacteria perform photosynthesis through a mechanism similar to that of plants, despite being prokaryotes. They utilize chlorophyll a, the primary pigment found in plants, to capture sunlight. They also possess accessory pigments called phycobilins, which allow them to absorb light across a broader spectrum, particularly in low-light conditions.
The photosynthetic machinery is embedded within specialized internal membrane structures called thylakoids, located in the cytoplasm. Chloroplasts are absent in these bacteria. Photosynthesis involves two main stages. Light-dependent reactions capture light energy to split water molecules, releasing oxygen as a byproduct, along with electrons and protons. These energized electrons move through an electron transport chain, generating energy-carrying molecules. Light-independent reactions, also known as the Calvin cycle, use this captured energy to convert carbon dioxide into carbohydrates. This oxygen-producing (oxygenic) photosynthesis is a characteristic of cyanobacteria.
Oxygenation of Earth: A Cyanobacterial Legacy
The photosynthetic activity of cyanobacteria significantly impacted Earth’s early atmosphere. These organisms were the first to evolve oxygenic photosynthesis, appearing at least 3.5 billion years ago. Their continuous oxygen production led to the Great Oxygenation Event (GOE), a rise in atmospheric oxygen concentrations that began approximately 2.4 to 2.3 billion years ago.
Before the GOE, Earth’s atmosphere had little free oxygen. Oxygen released by cyanobacteria initially reacted with elements like iron in the oceans, forming banded iron formations. Once these reactive sinks were saturated, oxygen accumulated in the atmosphere, changing the planet’s chemistry. This atmospheric shift made the environment suitable for the evolution of aerobic life forms, including plants and animals, and is considered a significant evolutionary event.
Cyanobacteria’s Role in Modern Ecosystems
Cyanobacteria continue to play an important role in Earth’s ecosystems today, extending beyond oxygen production. They serve as primary producers, forming the base of many aquatic food webs by converting sunlight into chemical energy. Their photosynthetic activities are responsible for a substantial portion of global primary production, contributing to the carbon cycle.
Many species of cyanobacteria perform nitrogen fixation, converting atmospheric nitrogen gas into usable forms like ammonia, nitrites, or nitrates. This process is essential for nutrient cycling, especially in aquatic environments like oceans, where fixed nitrogen can be a limiting nutrient for other organisms. Despite their benefits, rapid growth can lead to harmful algal blooms (HABs), particularly in freshwater. These blooms can deplete oxygen, produce unpleasant odors, and some species can release toxins, impacting aquatic life and posing health risks to humans and animals.