Cyanobacteria, often called blue-green algae, are prokaryotic bacteria and one of the most ancient life forms on Earth. They are the only prokaryotes capable of performing oxygen-evolving photosynthesis, making their primary mode of sustenance photoautotrophy. This strategy involves converting light energy and inorganic carbon into organic compounds for growth. Cyanobacteria are ubiquitous, found in environments ranging from oceans and freshwater to soils and rocks.
Defining Photoautotrophy
Photoautotrophy describes a nutritional mode where an organism uses light energy to synthesize organic molecules from inorganic substances. The prefix “photo-” signifies light as the energy source, while “auto-” indicates the carbon source is simple inorganic carbon, specifically carbon dioxide (CO2). Cyanobacteria absorb light to convert CO2 and water into sugars, releasing oxygen as a byproduct.
This process contrasts with chemoautotrophy, which uses energy from inorganic chemicals, and heterotrophy, where organisms consume pre-formed organic compounds. By relying only on light, water, and CO2, cyanobacteria are highly self-sufficient primary producers at the base of many food chains. Their ability to fix carbon dioxide is enhanced by a CO2 concentrating mechanism, which includes specialized protein compartments called carboxysomes.
The Unique Photosynthetic Machinery
Cyanobacteria perform oxygenic photosynthesis, a process similar to that found in plants due to endosymbiosis. The light-harvesting machinery is organized on internal membrane structures called thylakoids, which contain chlorophyll a, the primary photosynthetic pigment. They also contain accessory pigments called phycobilins.
Phycobilins are organized into large complexes called phycobilisomes that act as light-gathering antennae on the thylakoid surface. These pigments allow cyanobacteria to capture light wavelengths, such as green and blue-green light, that chlorophyll a absorbs less efficiently. The captured light energy splits water molecules, providing electrons for energy synthesis and releasing molecular oxygen (O2).
This photosynthetic electron transport chain is embedded within the thylakoid membrane, sharing components with the respiratory electron transport chain.
Beyond Light: Secondary Nutritional Strategies
While photoautotrophy is the dominant mode, cyanobacteria exhibit metabolic flexibility, allowing for secondary nutritional strategies. Some species are capable of mixotrophy, switching to heterotrophy by taking up and metabolizing simple organic compounds, such as sugars. This provides an alternative carbon or energy source when light levels are too low.
A separate strategy is nitrogen fixation, the ability to convert inert atmospheric nitrogen gas (N2) into bioavailable ammonia (NH3). This process is carried out by the enzyme nitrogenase, which is sensitive to oxygen. Filamentous cyanobacteria overcome this conflict by differentiating specialized, thick-walled cells called heterocysts. Heterocysts shut down oxygen-producing Photosystem II, creating an anaerobic environment that allows nitrogen fixation to occur and sharing the fixed nitrogen with adjacent cells.
The Ecological Significance of Cyanobacterial Nutrition
The unique nutritional mode of cyanobacteria has had profound global consequences, starting with the Great Oxidation Event (GOE). Over two billion years ago, the sustained release of oxygen from their photosynthesis fundamentally transformed Earth’s atmosphere from an anoxic state to an oxygen-rich one. This change paved the way for the evolution of all aerobic life forms.
In modern ecosystems, cyanobacteria remain foundational as primary producers, converting sunlight into biomass that supports aquatic food webs. Their capacity for nitrogen fixation makes them significant contributors to the global nitrogen cycle, especially where fixed nitrogen is a limiting nutrient. By fixing both carbon and nitrogen, cyanobacteria play a large role in biogeochemical cycling, supporting ecosystem productivity.