Photosynthesis is a fundamental biological process where organisms convert light energy into chemical energy, fueling growth and metabolic activities. While commonly associated with plants and algae, certain bacteria also perform photosynthesis. These microorganisms harness sunlight, playing diverse roles in various environments.
Groups of Photosynthetic Bacteria
A diverse array of bacteria engages in photosynthesis. Cyanobacteria, often called blue-green algae, are a prominent group that conducts oxygen-producing photosynthesis, similar to plants. They are found in a wide range of habitats, including freshwater, marine environments, and moist soils, appearing as unicellular forms, filaments, or colonies.
Other significant groups include purple bacteria, which are gram-negative and possess pigments giving them colors from purple to red, brown, or orange. They are broadly categorized into purple sulfur and purple non-sulfur bacteria, typically thriving in aquatic environments where oxygen levels are low. Green sulfur bacteria, another distinct group, are generally non-motile and obligately anaerobic, meaning they require oxygen-free conditions. These bacteria are characterized by their green or brownish hues due to their specific pigments. Heliobacteria, found predominantly in soils, also perform photosynthesis and are strict anaerobes.
Mechanisms of Bacterial Photosynthesis
Bacterial photosynthesis operates through distinct mechanisms, categorized by oxygen production. Oxygenic photosynthesis, carried out by cyanobacteria, utilizes water as the electron donor and releases molecular oxygen as a byproduct, much like plants. These organisms contain chlorophyll a, a pigment also found in plants, along with other accessory pigments that capture light energy. This process is essential for generating energy in the form of ATP and reducing power (NADPH) for carbon fixation.
In contrast, most other photosynthetic bacteria perform anoxygenic photosynthesis, as they do not use water as an electron donor. Instead, anoxygenic phototrophs like purple and green sulfur bacteria use various other compounds as electron sources. These include hydrogen sulfide, elemental sulfur, thiosulfate, or organic compounds. These bacteria utilize various types of bacteriochlorophylls for light absorption, which typically absorb light at longer wavelengths, often in the infrared spectrum, allowing them to thrive in light conditions unavailable to oxygenic organisms.
Ecological Roles
Photosynthetic bacteria play fundamental roles in Earth’s ecosystems, serving as primary producers at the base of many food webs. They convert light energy and carbon dioxide into organic matter. This carbon fixation is crucial for the global carbon cycle, influencing organic carbon availability.
Cyanobacteria had a transformative impact on Earth’s early atmosphere through the Great Oxidation Event. Around 2.4 to 2.1 billion years ago, their oxygen-producing photosynthesis led to the accumulation of free oxygen, profoundly changing the planet and enabling oxygen-breathing life. Beyond oxygen production, some cyanobacteria also contribute to nutrient cycling by fixing atmospheric nitrogen, important in nutrient-limited habitats. Anoxygenic photosynthetic bacteria contribute significantly to elemental cycling in anaerobic environments, participating in the sulfur cycle by oxidizing sulfur compounds or utilizing organic acids. Their diverse metabolic capabilities allow them to inhabit a wide range of niches, from deep anoxic waters and sediments to hot springs.