Photosynthesis converts light energy into chemical energy, typically as sugars. This intricate process powers most life on Earth, transforming carbon dioxide and water into organic molecules. While often associated with plants, some bacteria also perform this light-driven energy conversion.
Bacterial Photosynthesis: The Basics
Some bacteria are photosynthetic, converting light energy into chemical energy for their metabolism. This process shares the same goal as plant photosynthesis: to synthesize organic compounds from carbon dioxide and water using light. However, the mechanisms and byproducts differ significantly.
Bacterial photosynthesis falls into two main types: oxygenic and anoxygenic. Oxygenic photosynthesis, like in plants, produces oxygen as a byproduct. Anoxygenic photosynthesis does not release oxygen, instead using different electron donors. This distinction highlights the metabolic diversity among photosynthetic bacteria and their varied adaptations to different environments.
Diverse Photosynthetic Bacteria
Cyanobacteria, often called blue-green algae, are a prominent group performing oxygenic photosynthesis. These bacteria are widespread in nearly every terrestrial and aquatic habitat, and are unique among prokaryotes for producing oxygen.
Beyond Cyanobacteria, several groups perform anoxygenic photosynthesis, meaning they do not produce oxygen. These include Purple non-sulfur bacteria, Green sulfur bacteria, and Heliobacteria.
Purple non-sulfur bacteria are metabolically versatile, inhabiting diverse environments like wastewater ponds, lakes, and sediments. They grow under both aerobic and anaerobic conditions, using organic or inorganic electron donors.
Green sulfur bacteria are obligate anaerobes, unable to survive in oxygen, and are found in aquatic environments rich in hydrogen sulfide, such as the Black Sea. Heliobacteria are Gram-positive, anaerobic phototrophs primarily found in soils, including waterlogged paddy fields and hot springs.
Unique Pathways: Anoxygenic vs. Oxygenic
The core difference between oxygenic and anoxygenic photosynthesis lies in the electron donor and byproduct. In oxygenic photosynthesis, primarily carried out by Cyanobacteria, water (H2O) serves as the electron donor. When water molecules are split, oxygen gas (O2) is released as a byproduct, alongside hydrogen ions and electrons. This process uses chlorophyll a as the main photosynthetic pigment, absorbing light in violet-blue and orange-red wavelengths.
Anoxygenic photosynthetic bacteria utilize electron donors other than water. Green sulfur bacteria and Purple sulfur bacteria use hydrogen sulfide (H2S) as an electron donor, releasing elemental sulfur as a byproduct instead of oxygen. Purple non-sulfur bacteria can use various organic compounds or hydrogen gas as electron donors.
These bacteria employ bacteriochlorophylls, which absorb light at longer wavelengths in the near-infrared spectrum, allowing them to thrive where visible light is scarce. Heliobacteria, for example, use bacteriochlorophyll g. The absence of photosystem II in anoxygenic bacteria prevents them from splitting water and producing oxygen.
Ecological Roles and Significance
Photosynthetic bacteria play a significant role in Earth’s ecosystems, particularly in primary production, the foundation of most food webs. They convert carbon dioxide into organic matter, contributing to the global carbon cycle. Photosynthetic microbes, including bacteria, account for roughly half of the planet’s total primary production.
Beyond carbon fixation, these bacteria are involved in various nutrient cycles. Green sulfur bacteria, for instance, oxidize sulfide, participating in the sulfur cycle. Many purple bacteria can fix nitrogen, converting atmospheric nitrogen into forms usable by other organisms, important for soil fertility.
Ancient photosynthetic bacteria, especially Cyanobacteria, were instrumental in shaping Earth’s early atmosphere. Their oxygenic photosynthesis over geological timescales led to oxygen accumulation, transforming the planet’s atmosphere and paving the way for more complex, oxygen-dependent life forms. These bacteria inhabit a wide array of environments, from oceans and freshwaters to extreme habitats like hot springs and anoxic sediments.