Photosynthetic microorganisms are diverse prokaryotic and eukaryotic organisms capable of converting light energy into chemical energy through photosynthesis, forming the base of many food webs.
Biology and Diversity
Photosynthetic microorganisms include bacteria, archaea, and small eukaryotes that harness light to drive the synthesis of organic molecules. The best known are cyanobacteria, which perform oxygenic photosynthesis using chlorophyll a and release molecular oxygen as a byproduct. Cyanobacteria such as Prochlorococcus and Synechococcus dominate marine picoplankton and fix large quantities of carbon. Microalgae, including green algae and diatoms, are unicellular eukaryotes that also carry out oxygenic photosynthesis and form the basis of aquatic food chains. In addition to oxygenic forms, there are anoxygenic photosynthetic bacteria that use bacteriochlorophylls and accessory pigments to capture light at different wavelengths. Purple sulfur bacteria and green sulfur bacteria use reduced sulfur compounds as electron donors, while purple non-sulfur and green non-sulfur bacteria can switch between phototrophic and heterotrophic metabolism. Heliobacteria are Gram-positive phototrophs that use bacteriochlorophyll g and grow in anaerobic soils. These microorganisms possess specialized membranes, such as thylakoids or intracytoplasmic vesicles, housing photosystems and reaction centres. They respond to environmental factors such as light intensity, temperature, and nutrient availability and may engage in symbioses, as in lichens where cyanobacteria or algae live with fungi.
Ecological Roles and Applications
Photosynthetic microorganisms are crucial in global biogeochemical cycles. Marine cyanobacteria like Prochlorococcus contribute significantly to atmospheric oxygen and carbon sequestration. Filamentous cyanobacteria form harmful blooms in eutrophic lakes but also provide nitrogen to crops when used as biofertilizers. Green algae such as Chlorella and Dunaliella are cultivated as animal feed, nutritional supplements, and potential biofuel feedstocks due to their rapid growth and lipid accumulation. Anoxygenic photosynthetic bacteria have been studied for their ability to produce hydrogen gas and bioplastics under anaerobic conditions. In microbial mats and stromatolites, layered communities of phototrophs and heterotrophs record early life on Earth. Research on photosynthetic microorganisms informs climate models, wastewater treatment, and the search for extraterrestrial life. Photosynthetic microorganisms encompass a wide array of prokaryotes and microalgae that convert light energy into chemical energy. Their metabolic diversity shapes ecosystems and underpins primary production across oceans, lakes, and soils. Understanding their physiology and interactions is essential for ecology, biotechnology, and earth history. Related Terms: Cyanobacteria, Microalgae, Phototroph, Oxygenic photosynthesis, Anoxygenic bacteria