Cyanobacteria are ancient, single-celled organisms often called “blue-green algae,” though they are technically bacteria. These microscopic life forms are widely distributed across Earth’s environments, playing a fundamental role in various ecosystems. Cyanobacteria photosynthesize, a process central to their existence and profound impact on the planet.
Mechanism of Cyanobacterial Photosynthesis
Cyanobacteria perform oxygenic photosynthesis, a process that closely resembles that of plants. This complex biochemical machinery is housed within their cells, despite lacking chloroplasts like plants. Photosynthesis begins with light-harvesting pigments, primarily chlorophyll a, which absorbs light energy. They also possess unique accessory pigments called phycobilins, such as phycocyanin and phycoerythrin, organized in structures called phycobilisomes. These phycobilins allow cyanobacteria to capture a broader spectrum of light wavelengths, including green, red, orange, and yellow light, which chlorophyll a does not absorb as efficiently.
The light-dependent reactions of photosynthesis occur on specialized internal membrane folds called thylakoids. These membranes are the site where light energy is used to split water molecules. This water splitting releases electrons, protons, and oxygen as a byproduct. The energy from these reactions is then captured in the form of ATP and NADPH.
The subsequent light-independent reactions, also known as the Calvin cycle, utilize the ATP and NADPH to fix carbon dioxide. This process converts atmospheric carbon dioxide into sugars, which serve as the organism’s food. Many cyanobacteria enhance carbon fixation through specialized protein compartments called carboxysomes. These structures encapsulate the enzyme RuBisCO, effectively concentrating carbon dioxide around it to improve its efficiency in converting CO2 into organic compounds.
Evolutionary and Ecological Impact
Cyanobacteria played a transformative role in Earth’s early history, most notably through the Great Oxygenation Event (GOE). Appearing billions of years ago, they were the first organisms to produce oxygen through photosynthesis on a large scale. This continuous oxygen production gradually accumulated in the oceans and then the atmosphere, fundamentally altering Earth’s environment between 2.4 and 2.1 billion years ago. The rise of atmospheric oxygen paved the way for the evolution of aerobic life forms and caused a mass extinction of many anaerobic organisms that found oxygen toxic.
Beyond their historical impact, cyanobacteria remain primary producers in aquatic ecosystems worldwide. They form the base of many food webs, converting sunlight into organic matter that supports a vast array of other organisms. Their photosynthetic activity contributes significantly to global carbon and nitrogen cycles.
Many species of cyanobacteria also possess the unique ability to fix atmospheric nitrogen. This process converts atmospheric nitrogen into usable forms through specialized cells called heterocysts. Nitrogen fixation enriches ecosystems, making this nutrient available to other organisms and contributing to Earth’s biogeochemical cycles.
Diverse Roles and Habitats
Cyanobacteria exhibit remarkable adaptability, thriving in nearly every environment on Earth. They are found in oceans, freshwater lakes, rivers, damp soils, and even extreme conditions like hot springs, deserts, and polar regions. Their widespread distribution highlights their efficiency in harnessing solar energy under various conditions.
Beyond free-living forms, cyanobacteria engage in various symbiotic relationships, benefiting other organisms through their photosynthetic capabilities. They form lichens by associating with fungi, providing the fungal partner with nutrients. Some cyanobacteria also live within the cells of plants or other organisms, contributing to the host’s energy supply.
Their efficient growth, particularly in nutrient-rich waters, can lead to cyanobacterial blooms, sometimes called “algal blooms.” These rapid increases in population occur in both freshwater and marine environments, often appearing as green, blue-green, or brownish-green scums on the water surface. While natural, these blooms highlight the growth capacity of cyanobacteria.