Plants, algae, cyanobacteria, and certain photosynthetic bacteria all convert solar energy into chemical energy through photosynthesis. These organisms, collectively called photoautotrophs, capture sunlight and use it to build organic molecules from carbon dioxide and water. They are the foundation of nearly every food chain on Earth.
Plants: The Most Familiar Converters
Green plants are the organisms most people think of first. Inside their cells, specialized compartments called chloroplasts house the entire photosynthetic operation. Each chloroplast contains an inner membrane system, the thylakoid membrane, where the green pigment chlorophyll sits. Chlorophyll absorbs red and blue light most strongly (between roughly 400 and 700 nanometers) while reflecting green light, which is why plants look green.
When light hits chlorophyll, it energizes electrons that get passed along a chain of proteins embedded in the thylakoid membrane. This electron transport chain generates two key energy carriers that the cell then uses to convert carbon dioxide into sugar. The overall reaction takes six molecules of carbon dioxide and six molecules of water, powered by sunlight, and produces glucose and oxygen. That glucose is chemical energy, stored in bonds the plant can later break apart for fuel or use as building material for growth.
Plants don’t rely on chlorophyll alone. They also carry accessory pigments, including red, brown, and blue varieties, that capture additional wavelengths of light and funnel that energy to chlorophyll. Some of these pigments also protect cells from damage caused by excess light.
Phytoplankton and Algae: The Ocean’s Powerhouses
Tiny floating organisms in the ocean, collectively called phytoplankton, are responsible for a staggering share of global energy conversion. Despite making up only about 1 to 2% of total plant carbon on Earth, phytoplankton fix between 30 and 50 billion metric tons of carbon every year. That accounts for roughly 40% of all the carbon fixation on the planet. In other words, nearly half of the solar-to-chemical energy conversion happening right now is taking place in the ocean, carried out by organisms too small to see.
Microalgae use the same basic photosynthetic chemistry as land plants. They have chlorophyll, absorb light, split water, and produce oxygen and sugars. Many species of algae are single-celled, but others form large multicellular structures like kelp. Whether microscopic or visible, they all share the ability to turn sunlight into stored chemical energy.
Cyanobacteria: Where It All Started
Cyanobacteria are the oldest known photosynthetic organisms, with fossils dating back billions of years. They are the only bacteria that perform the same type of oxygen-producing photosynthesis that plants use, splitting water molecules and releasing oxygen as a byproduct. This is no coincidence: the chloroplasts inside modern plant cells are thought to have descended from ancient cyanobacteria that were engulfed by early cells.
Cyanobacteria live in an enormous range of environments, from oceans and freshwater lakes to hot springs, desert crusts, and even inside rocks. They are single-celled or form simple colonies, yet their collective impact on global carbon and oxygen cycles is enormous. When you see a greenish film on a pond surface, you’re likely looking at cyanobacteria converting sunlight into chemical energy in real time.
Photosynthetic Bacteria That Don’t Produce Oxygen
Not all solar-to-chemical energy conversion involves water and oxygen. Certain bacteria perform a different version called anoxygenic photosynthesis, which takes place without oxygen. Instead of splitting water, these organisms use other substances as electron donors, most commonly hydrogen sulfide.
Two major families stand out. Purple sulfur bacteria (Chromatiaceae) and green sulfur bacteria (Chlorobiaceae) thrive in oxygen-free environments like deep lake sediments, hot springs, and sulfur-rich muds. They contain bacteriochlorophylls instead of standard chlorophyll, giving their cultures vivid colors ranging from shades of green and brown to pink, red, and purple. These pigments absorb different wavelengths of light than plant chlorophyll does, extending into the near-infrared range beyond 700 nanometers.
These bacteria convert hydrogen sulfide and carbon dioxide into organic matter using sunlight as the energy source. While their contribution to global carbon fixation is small compared to plants and phytoplankton, they play important roles in nutrient cycling and can detoxify hydrogen sulfide in their environments.
How the Conversion Actually Works
Photosynthesis happens in two stages. In the first stage, called the light reactions, pigments absorb photons and use that energy to split water molecules. This produces oxygen (released as a gas), along with two energy-carrying molecules. One of these carriers stores energy in a form the cell can spend like currency, while the other carries high-energy electrons needed for the next stage.
In the second stage, called the Calvin cycle, the cell uses those two energy carriers to grab carbon dioxide from the air and rearrange its atoms into sugar molecules. This is where solar energy officially becomes chemical energy, locked into the bonds of carbohydrates that can fuel the organism or anything that eats it. The cycle also regenerates the starting molecule it needs to keep running, making it a true loop.
Plants and algae absorb light between about 400 and 700 nanometers, which covers most of the visible spectrum. Photosynthetic bacteria extend this range to about 900 nanometers, reaching into the near-infrared. Beyond 900 nanometers, light doesn’t carry enough energy per photon to produce stable chemical products, which is why no known organism uses the far-infrared portion of sunlight. That untapped region represents about 30% of the incoming solar energy at Earth’s surface.
How Artificial Systems Compare
Engineers have been working to mimic photosynthesis with manufactured devices that convert sunlight directly into chemical fuels. The most successful approach so far pairs a solar cell with an electrochemical system that reduces carbon dioxide. One benchmark lab system achieved a solar-to-fuel conversion efficiency of 19.9% for turning carbon dioxide into carbon monoxide. That significantly outperforms natural photosynthesis, which typically converts only 1 to 2% of incoming solar energy into chemical energy in plants.
Systems that try to do everything in a single device, without a separate solar cell, are far less efficient. Prototype “photocatalyst sheets” that directly use light to split carbon dioxide and water have achieved efficiencies of only about 0.07 to 0.08%. Scaling these systems up while maintaining performance remains a major engineering challenge. A larger demonstration system with standard silicon solar cells driving the chemistry reached 7.2% efficiency for producing formate from carbon dioxide, sustained over three hours of continuous operation.
Despite these advances, natural photosynthesis still wins on scale. The billions of tons of carbon fixed each year by plants, algae, and bacteria dwarf anything artificial systems currently produce. Nature’s approach has also been running continuously, maintaining itself, and replicating for over three billion years, something no human-built system comes close to matching.