Certain types of bacteria are capable of photosynthesis, a fundamental biological process that converts light energy into chemical energy. This energy, stored in organic compounds, fuels their growth and metabolic activities. Photosynthesis involves capturing light, transferring energy through electron transport, and converting carbon dioxide into sugars. While plants and algae are widely recognized for this process, various bacterial groups also perform photosynthesis through diverse mechanisms.
Key Groups of Photosynthetic Bacteria
Photosynthetic bacteria are categorized into two groups based on oxygen production: oxygenic and anoxygenic. Cyanobacteria, often called blue-green algae, are oxygenic. They use water as an electron donor, releasing oxygen as a byproduct, similar to plants and algae. Cyanobacteria are abundant in diverse aquatic and terrestrial environments, including hot springs and polar regions.
Anoxygenic photosynthetic bacteria perform photosynthesis without producing oxygen. This group includes purple bacteria, green sulfur bacteria, green non-sulfur bacteria, and heliobacteria. Purple bacteria (purple sulfur or purple non-sulfur) use compounds like hydrogen sulfide or organic acids as electron donors instead of water. Green sulfur bacteria are obligate anaerobes that use reduced sulfur compounds, such as hydrogen sulfide, as electron sources.
Mechanisms of Bacterial Photosynthesis
Bacterial photosynthesis relies on specialized pigments that capture light energy. Unlike plants, which primarily use chlorophyll a, photosynthetic bacteria employ a variety of pigments. Cyanobacteria contain chlorophyll a, along with accessory pigments like phycobilins and carotenoids, which help capture a broader spectrum of light. Anoxygenic photosynthetic bacteria, such as purple and green sulfur bacteria, use bacteriochlorophylls (BChl) instead of chlorophyll. Several types of bacteriochlorophylls absorb light at longer wavelengths, including infrared, allowing these bacteria to thrive where visible light is scarce.
A key distinction in bacterial photosynthesis is the electron donors used. Oxygenic cyanobacteria split water molecules (H₂O) to obtain electrons, releasing oxygen (O₂) as a byproduct. This process requires significant energy because water is a stable molecule. Anoxygenic photosynthetic bacteria use electron donors other than water. Purple sulfur bacteria and green sulfur bacteria commonly use hydrogen sulfide (H₂S), elemental sulfur, or thiosulfate as electron donors, producing sulfur granules or sulfates as byproducts. Some anoxygenic bacteria also utilize organic compounds like succinate or malate, or even ferrous iron, as electron sources. The captured light energy drives an electron transport chain, generating adenosine triphosphate (ATP) and reducing power (NADPH). These are essential for converting carbon dioxide into organic compounds.
Role in Ecosystems
Photosynthetic bacteria are foundational to many ecosystems, serving as primary producers that convert light energy into organic matter. They form the base of food webs in various aquatic and terrestrial habitats, supporting a wide range of other organisms. Their metabolic activities contribute significantly to global biogeochemical cycles, particularly the carbon and nitrogen cycles. Through carbon fixation, these bacteria convert atmospheric carbon dioxide into organic compounds, regulating Earth’s carbon balance.
Some cyanobacteria can also fix atmospheric nitrogen, converting nitrogen gas into forms usable by other life forms. This nitrogen fixation is important in environments where nitrogen is a limiting nutrient. From an evolutionary perspective, ancient cyanobacteria were instrumental in shaping Earth’s early atmosphere. Their oxygenic photosynthesis, which began billions of years ago, gradually released oxygen, leading to the Great Oxidation Event around 2.4 billion years ago. This atmospheric change paved the way for the evolution of oxygen-breathing life forms. Photosynthetic bacteria often inhabit extreme environments, such as hot springs, deep-sea vents, and anoxic layers of stratified lakes, where other photosynthetic organisms cannot survive.