Photosynthesis in Bacteria and Cyanobacteria

Core Idea

Photosynthetic bacteria include anoxygenic purple and green bacteria (which use bacteriochlorophyll) and cyanobacteria (which use chlorophyll a and produce O₂ like plants). All perform light reactions and carbon fixation, but only cyanobacteria evolved oxygenic photosynthesis, fundamentally reshaping Earth's atmosphere and ecology.

Explainer

You already understand the general framework of photosynthesis — light reactions capturing solar energy to generate ATP and NADPH, followed by carbon fixation in the Calvin cycle. You also know the basics of bacterial metabolism. What this topic reveals is that the photosynthesis you learned about in plants is actually a bacterial invention, and the version found in plant chloroplasts represents just one branch of a much older and more diverse family of light-harvesting strategies. Bacterial photosynthesis came first by billions of years, and understanding its variations illuminates how the oxygen-rich atmosphere we breathe came to exist.

The earliest photosynthetic bacteria were anoxygenic — they harvested light energy but did not produce oxygen. Purple bacteria (like *Rhodobacter*) and green sulfur bacteria (like *Chlorobium*) use bacteriochlorophyll instead of chlorophyll a, absorbing light at longer wavelengths (in the infrared range, 800–1000 nm) that penetrate deeper into water and sediments. Crucially, these organisms use only one photosystem (either a Type I or Type II reaction center, but not both) and obtain electrons from donors other than water — hydrogen sulfide (H₂S), hydrogen gas (H₂), or organic compounds like succinate. Because they never split water, they never release O₂. Purple sulfur bacteria, for instance, oxidize H₂S to elemental sulfur, depositing yellow sulfur granules inside or outside their cells. These anoxygenic phototrophs dominated Earth's surface waters for over a billion years before oxygen-producing photosynthesis evolved.

Cyanobacteria changed everything. They are the only prokaryotes that perform oxygenic photosynthesis, and they do so using the same fundamental machinery found in plant chloroplasts: Photosystem II (PSII) and Photosystem I (PSI) linked in series by an electron transport chain. PSII uses light energy to split water (H₂O → 2H⁺ + ½O₂ + 2e⁻), extracting electrons and releasing molecular oxygen as a byproduct. These electrons pass through the cytochrome b₆f complex to PSI, which uses a second photon of light to boost them to a high enough energy level to reduce NADP⁺ to NADPH. This Z-scheme of two linked photosystems — which you may recognize from plant biology — originated in cyanobacteria. In fact, chloroplasts are descendants of ancient cyanobacteria captured by a eukaryotic host cell through endosymbiosis, which is why chloroplast structure, genome, and photosynthetic machinery so closely resemble those of modern cyanobacteria.

The evolutionary consequences of cyanobacterial photosynthesis were staggering. Before cyanobacteria, Earth's atmosphere contained virtually no free oxygen — it was a reducing environment dominated by CO₂, N₂, and trace gases. Beginning around 2.4 billion years ago, the accumulated oxygen output from cyanobacteria triggered the Great Oxidation Event, which transformed atmospheric chemistry, rusted dissolved iron out of the oceans (forming the banded iron formations visible in the geological record), and drove most obligate anaerobes into restricted anoxic habitats. Today, cyanobacteria remain enormously important: marine cyanobacteria like *Prochlorococcus* and *Synechococcus* are responsible for roughly 25% of global net primary productivity and are the most abundant photosynthetic organisms on Earth. Some cyanobacteria can also fix atmospheric nitrogen using specialized cells called heterocysts, which maintain an anaerobic interior to protect the oxygen-sensitive nitrogenase enzyme — making these organisms capable of both carbon and nitrogen fixation, a metabolic versatility unmatched by any plant.