Three of the six traditionally recognized biological kingdoms contain photosynthetic organisms: Plantae, Protista, and Eubacteria (often grouped under the older term Monera). Photosynthesis is not limited to green plants growing in soil. It occurs across a wide range of life forms, from single-celled ocean bacteria to giant kelp forests, and the boundaries between kingdoms blur in interesting ways when you look at how organisms actually capture light energy.
Kingdom Plantae: The Familiar Photosynthesizers
Every member of the plant kingdom is photosynthetic. This is the defining trait of the group: plants make their own food using sunlight, water, and carbon dioxide. From mosses and ferns to flowering trees, all plants contain chloroplasts, the tiny cellular structures where photosynthesis happens. Plants use chlorophyll a and chlorophyll b as their primary pigments, which is why most plant tissue appears green.
Despite being the most recognizable photosynthesizers, land plants are not the biggest contributors to global oxygen production. More than half of the oxygen in Earth’s atmosphere comes from marine photosynthesizers like phytoplankton and seaweed, not from forests and grasslands.
Kingdom Eubacteria: Where Photosynthesis Began
Bacteria were photosynthesizing billions of years before plants existed. The most important group is cyanobacteria, sometimes called blue-green algae, which perform the same oxygen-producing type of photosynthesis that plants do. A single genus of marine cyanobacterium, Prochlorococcus, is estimated to be the most abundant photosynthesizer on the planet, responsible for roughly 20 percent of atmospheric oxygen on its own.
Cyanobacteria are also the ancestors of chloroplasts. According to the endosymbiotic theory, an ancient single-celled organism engulfed a cyanobacterium, and over time that captured bacterium evolved into the chloroplast found in all plants and algae today. In other words, every green leaf on Earth traces its photosynthetic machinery back to bacteria.
Beyond cyanobacteria, several other bacterial groups photosynthesize without producing oxygen at all. These anoxygenic photosynthetic bacteria include purple sulfur bacteria, green sulfur bacteria, and heliobacteria. Instead of splitting water molecules the way plants do, they use compounds like hydrogen sulfide or organic molecules as their energy source. They also rely on a different pigment called bacteriochlorophyll rather than the standard chlorophyll. These bacteria thrive in environments like deep ocean vents, hot springs, and sulfur-rich sediments where oxygen is scarce.
Kingdom Protista: A Diverse Mix
Protists are single-celled (or simple multicellular) organisms with a nucleus, and many of them photosynthesize. The photosynthetic protists are what most people call “algae,” though that term covers a huge variety of unrelated organisms. Some key groups include diatoms, dinoflagellates, golden algae, green algae, and brown algae.
Diatoms are unicellular organisms encased in intricate glass-like shells made of silicon dioxide. They are among the most productive photosynthesizers in the ocean. Dinoflagellates are another major group, and they can be photosynthetic, feed on other organisms, or do both. Golden algae get their color from carotenoid pigments rather than relying solely on chlorophyll. Brown algae, which include the large seaweeds and kelps you see washed up on beaches, are multicellular and primarily marine.
What makes protists especially interesting is that not all members of the kingdom photosynthesize. Some are predators that engulf food, some absorb nutrients, and some switch strategies depending on conditions. Euglena, for instance, photosynthesizes in sunlight but can also consume organic matter in the dark.
Kingdoms That Don’t Photosynthesize (With Exceptions)
The remaining kingdoms, Fungi, Animalia, and Archaebacteria, are not considered photosynthetic, but the reality is more nuanced than a simple “no.”
Fungi lack chloroplasts entirely and cannot photosynthesize on their own. However, fungi form partnerships with algae or cyanobacteria to create lichens, those crusty patches you see on rocks and tree bark. In a lichen, the fungal partner provides structure and protection while the photosynthetic partner provides food. This relationship is estimated to have originated around 480 million years ago. More recently, researchers discovered that a soil fungus called Mortierella elongata can harbor living algal cells inside its own cells, with the algae actively photosynthesizing from within. This is the first known case of a fungus internalizing a photosynthetic organism inside its cells, distinct from the external arrangement seen in lichens.
Animals are heterotrophs, meaning they eat other organisms for energy. Yet certain animals have evolved clever workarounds. Corals, giant clams, sponges, and some flatworms host symbiotic algae or cyanobacteria within their tissues, effectively becoming solar-powered. The sea slug Elysia chlorotica takes this a step further: it eats algae and steals their chloroplasts, incorporating them into cells lining its digestive system. Those stolen chloroplasts remain functional for the slug’s entire 10-month lifespan, allowing it to survive on nothing but light and air. The chloroplasts are not passed to offspring, though, so each new generation must acquire them by feeding.
Archaebacteria (the kingdom Archaebacteria, or domain Archaea) do not perform true photosynthesis. Some archaea, particularly salt-loving species like Halobacterium salinarum, can harvest light energy using a purple pigment called bacteriorhodopsin. This pigment absorbs light and pumps protons across the cell membrane to generate energy. But this process does not fix carbon dioxide into organic molecules the way photosynthesis does, so biologists classify it as light-driven energy production rather than photosynthesis.
Why Photosynthesis Spans So Many Kingdoms
The spread of photosynthesis across kingdoms is largely explained by one event repeated in different ways: endosymbiosis. About 1.5 billion years ago, a non-photosynthetic cell swallowed a cyanobacterium and, instead of digesting it, kept it alive. That captured cyanobacterium eventually became the chloroplast. This primary endosymbiosis gave rise to the lineage that includes land plants, green algae, and red algae.
Later, some of these early algae were themselves swallowed by other organisms in secondary endosymbiotic events. This is how groups like brown algae, diatoms, and dinoflagellates gained their chloroplasts, even though they are not closely related to green plants. The process involved massive genetic reshuffling: the captured organism lost most of its genome, transferring genes to the host cell’s nucleus, while the host evolved new protein-targeting systems to shuttle supplies back into the chloroplast.
This history explains why photosynthetic organisms don’t cluster neatly into one branch of life. Photosynthesis originated once in bacteria, then spread horizontally across the tree of life through a series of cellular mergers. The kingdom system, which was designed to sort organisms by visible traits, struggles to capture this kind of evolutionary mixing, which is one reason modern classification systems increasingly move away from rigid kingdom boundaries altogether.