Photosynthesis isn’t limited to plants. A surprisingly wide range of organisms convert sunlight into energy, from towering trees to bacteria so small they’re invisible to the naked eye. In fact, roughly half of all oxygen production on Earth comes from the ocean, driven mostly by microscopic photosynthesizers rather than forests or fields.
Land Plants
Plants are the most familiar photosynthesizers, and they come in three main varieties based on how they capture carbon dioxide. The majority of plant species, including wheat, rice, barley, soybeans, and potatoes, use the standard pathway (called C3 photosynthesis) that works best in cool, moist conditions with plenty of CO₂. These plants are efficient in temperate climates but struggle in heat because they waste energy through a competing chemical process called photorespiration.
A second group, including maize, sugarcane, millet, and sorghum, evolved a more efficient method that shines in hot, dry environments. These plants concentrate CO₂ inside specialized cells before feeding it into the standard cycle, which dramatically reduces energy waste. They close their pores during the hottest part of the day to conserve water while still photosynthesizing effectively.
The third group takes water conservation to the extreme. Cacti, succulents, orchids, and pineapples only open their pores at night, absorbing CO₂ in the dark and storing it as an acid. During the day, they break down that stored acid and use the released CO₂ for photosynthesis with their pores sealed shut. This strategy lets them survive in deserts and other arid regions where other plants would quickly dry out.
Algae and Phytoplankton
Algae are a massive and diverse group of photosynthesizers that live in both freshwater and marine environments. They range from single-celled organisms invisible without a microscope to giant kelp that can grow tens of meters long. In the ocean, microscopic algae called phytoplankton form the base of nearly every marine food web.
The two main classes of phytoplankton are diatoms and dinoflagellates. Diatoms have rigid, interlocking glass-like shells and drift passively with ocean currents. Dinoflagellates propel themselves through water using whip-like tails and are covered in complex shells. Both groups are enormously productive. Green algae, red algae, and brown algae round out the larger categories, each using slightly different pigments to capture light energy at different wavelengths.
Despite their tiny size, phytoplankton collectively produce as much oxygen as all land plants combined. One species of cyanobacterium called Prochlorococcus, the smallest photosynthetic organism on Earth, generates up to 20% of the oxygen in our entire biosphere. That single microbe outproduces all the world’s tropical rainforests.
Cyanobacteria
Cyanobacteria deserve their own category because of their enormous importance. These are bacteria, not plants or algae, yet they photosynthesize in essentially the same way plants do, splitting water molecules and releasing oxygen as a byproduct. This isn’t a coincidence. Cyanobacteria are the evolutionary ancestors of the chloroplasts inside every plant and algal cell. Billions of years ago, an ancient cell engulfed a cyanobacterium, and that partnership became permanent.
Cyanobacteria thrive in oceans, lakes, rivers, soil, and even on bare rock. Some species form the blue-green blooms you sometimes see on ponds in summer. Others live in extreme environments like hot springs or polar ice. A recently studied species called Anthocerotibacter panamensis, discovered in 2021 in Panama, is believed to be an especially ancient strain. Unlike most cyanobacteria, it lacks the stacked internal membrane structures that house the photosynthetic machinery in its more modern relatives. Instead, its photosystems sit directly in the inner cell membrane, offering scientists a window into what early photosynthesis may have looked like.
Anoxygenic Photosynthetic Bacteria
Not all photosynthesis produces oxygen. At least seven major groups of bacteria photosynthesize without splitting water, using hydrogen sulfide, hydrogen gas, or organic compounds as their energy source instead. These organisms use a different light-capturing pigment called bacteriochlorophyll rather than the chlorophyll found in plants and cyanobacteria.
Purple sulfur bacteria and green sulfur bacteria are the best-known examples. Both live in oxygen-free environments where hydrogen sulfide is available, typically at the bottom of lakes and ponds, in sulfur springs, in marine sediments, and in places like the Black Sea. Green sulfur bacteria are particularly restricted in where they can grow because they need the narrow overlap zone where both sulfide and light are present, usually just the top few millimeters of sediment. Purple non-sulfur bacteria, heliobacteria, and filamentous anoxygenic bacteria fill other niches, using various combinations of sulfur compounds, hydrogen, and organic molecules to power their photosynthesis.
These organisms likely represent the oldest forms of photosynthesis on Earth, predating the oxygen-producing version by hundreds of millions of years. Some thrive in hot sulfur springs above 40°C, and researchers have even speculated that certain purple sulfur bacteria could survive in sea ice.
Photosynthetic Partnerships
Some organisms don’t photosynthesize on their own but form tight partnerships with those that do. Lichens are a classic example: a fungus pairs with a photosynthetic partner, usually a green alga from the genus Trebouxia, and sometimes with cyanobacteria. The fungus provides structure and protection while the alga provides sugar from photosynthesis. Some lichens even house two genetically different algal lineages simultaneously, and the relative abundance of each can shift over time depending on conditions.
Coral reefs depend on a similar arrangement. Coral animals host tiny photosynthetic algae called zooxanthellae inside their tissues. These algae provide the coral with energy from sunlight, and in return, the coral gives the algae shelter and nutrients. When water temperatures rise too high, the zooxanthellae’s internal membranes become unstable, the partnership breaks down, and the coral bleaches.
Animals That Steal Chloroplasts
Perhaps the most surprising photosynthesizers are animals. A group of sap-sucking sea slugs called sacoglossans feed on algae but, instead of digesting the chloroplasts, keep them intact and functional inside the cells of their digestive system. This process, called kleptoplasty, allows the slugs to photosynthesize using stolen equipment.
Several species pull off this trick. Elysia chlorotica, Elysia viridis, Elysia crispata, Elysia timida, and Plakobranchus ocellatus all maintain functional stolen chloroplasts for weeks to months. The carbon captured through photosynthesis doesn’t just sit in the digestive system. Within hours, labeled carbon shows up in the slug’s neural tissue, mucus glands, and reproductive organs. In Elysia viridis, photosynthetically acquired carbon accumulates in the egg-producing glands, suggesting that stolen photosynthesis directly fuels reproduction.
Kleptoplasty isn’t exclusive to sea slugs, though they’re the only animals that maintain it long-term. Single-celled organisms like certain foraminiferans, dinoflagellates, and ciliates also steal and use chloroplasts, and researchers recently identified short-term functional kleptoplasts in two species of marine flatworms.