Photosynthesis occurs in a surprisingly wide range of organisms, from towering trees to microscopic ocean bacteria. Plants are the most familiar example, but they share this ability with several groups of algae, multiple types of bacteria, and even a few animals that have found creative workarounds. Across two of life’s three domains (bacteria and eukaryotes), photosynthesis has evolved and spread into nearly every sunlit environment on Earth.
Land Plants
Every land plant photosynthesizes, from tiny liverworts and mosses to ferns, conifers, and flowering plants. They all share the same basic toolkit: chloroplasts packed with chlorophyll that absorb sunlight, pull in carbon dioxide, and release oxygen. These chloroplasts sit inside cells surrounded by a double membrane, and the plant stores the sugars it produces to fuel growth.
Land plants are part of a larger group called Viridiplantae (green plants), which also includes many green algae. What unites them is a specific combination of pigments, chlorophyll a and chlorophyll b, plus cell walls typically made of cellulose. Flowering plants alone account for roughly 300,000 species, making them the most species-rich group of photosynthesizers on the planet.
Algae: A Diverse Catch-All
“Algae” is not a single group but a label applied to several distantly related lineages of photosynthetic eukaryotes. Green algae are the closest relatives of land plants and share many of the same pigments. Red algae and brown algae belong to entirely separate branches of the tree of life, yet they independently acquired the ability to photosynthesize.
Other photosynthetic algae include diatoms (encased in intricate silica shells), golden algae, and yellow-green algae. Some of these groups gained their chloroplasts not by evolving them directly, but through a chain of events in which one eukaryote engulfed another photosynthetic eukaryote and kept its cellular machinery. This layered history is why algal chloroplasts sometimes have three or four surrounding membranes instead of two.
Many algae are economically important beyond their ecological role. Polysaccharides harvested from red and brown algae are widely used in the food industry as gelling agents, stabilizers, and thickeners.
Phytoplankton and the Ocean’s Oxygen
Phytoplankton are the microscopic photosynthesizers drifting in oceans, lakes, and rivers. They include cyanobacteria, diatoms, dinoflagellates, green algae, and coccolithophores (tiny organisms coated in chalk-like plates that can turn seawater milky white or bright blue). Together, these organisms consume carbon dioxide on a scale equivalent to all the forests on land.
Roughly half of Earth’s oxygen production comes from the ocean, according to NOAA. One species alone, a cyanobacterium called Prochlorococcus, is the smallest photosynthetic organism on the planet yet generates up to 20% of the oxygen in the entire biosphere. That single microbe outperforms all tropical rainforests combined. The ocean’s “biological carbon pump,” driven largely by phytoplankton photosynthesis, transfers about 10 gigatonnes of carbon from the atmosphere to the deep ocean each year.
Climate change is already shifting the balance among these groups. Warming oceans are predicted to favor smaller phytoplankton like cyanobacteria at the expense of larger types like diatoms, which could alter marine food webs and carbon cycling.
Photosynthetic Bacteria
Photosynthesis is spread across at least six bacterial groups, and the differences among them are striking. Cyanobacteria are the only bacteria that perform oxygenic photosynthesis, meaning they split water molecules and release oxygen, exactly the way plants do. This is no coincidence: the chloroplasts inside every plant and alga descended from ancient cyanobacteria that were engulfed by a larger cell billions of years ago.
The other five groups perform anoxygenic photosynthesis, capturing light energy without producing oxygen. These include purple bacteria, green sulfur bacteria, heliobacteria, filamentous green nonsulfur bacteria, and acidobacteria. Instead of water, many of these organisms use hydrogen sulfide or other chemicals as their electron source. They tend to thrive in environments where oxygen is scarce, such as deep water columns, hot springs, and sulfur-rich sediments.
Cyanobacteria are also remarkably tough. They form microbial mats in environments ranging from Antarctic ice to continental hot springs. They colonize hypersaline and highly alkaline lakes, tolerate heavy metals, and even survive in desert rock as endolithic communities, living inside tiny pores in the stone itself.
How Photosynthesis First Appeared
The earliest evidence of oxygenic photosynthesis dates to around 2.4 billion years ago, a period known as the Great Oxidation Event, when oxygen first accumulated in Earth’s atmosphere in significant amounts. Cyanobacteria were responsible. The oldest unambiguous cyanobacterial fossils are slightly younger, about 1.89 to 1.84 billion years old, discovered in the Belcher Supergroup in Canada. Some geochemical signals hint that oxygenic photosynthesis may have started even earlier than 2.4 billion years ago, but this remains debated.
Anoxygenic photosynthesis likely predates the oxygenic version, meaning bacteria were harvesting sunlight long before any organism figured out how to split water. The jump to oxygenic photosynthesis was transformative: it flooded the planet with oxygen, reshaped ocean chemistry, and set the stage for complex multicellular life.
Animals That Borrow Photosynthesis
A handful of animals have blurred the line between plant and animal by stealing photosynthetic machinery. The most striking example is Elysia chlorotica, a bright green sea slug found along the eastern coast of North America. When it feeds on a specific species of algae, it extracts the chloroplasts and stores them in cells lining its digestive system. Those stolen chloroplasts (a process called kleptoplasty) remain functional and can sustain the slug for its entire adult lifespan of about 10 months, using nothing but light and air.
The process requires a precise setup. Larvae will only settle and develop if the right algal species is present. After five to seven days of continuous feeding as juveniles, the association becomes permanent. The chloroplasts never divide inside the slug and are not passed to offspring through eggs, so each new generation must acquire them fresh. Several other sea slugs in the genus Elysia pull off similar tricks, though none maintain their borrowed chloroplasts quite as long as E. chlorotica does.
Corals take a different approach: they host entire photosynthetic algae (dinoflagellates) inside their tissues, receiving sugars in exchange for shelter. This symbiotic photosynthesis is what powers reef-building and what breaks down during coral bleaching events when the algae are expelled.