Producers get their energy from one of two sources: sunlight or inorganic chemicals. The vast majority of producers on Earth, including plants, algae, and cyanobacteria, capture energy from sunlight through photosynthesis. A smaller but ecologically important group of producers, found mostly in deep-sea environments, harvest energy by breaking down inorganic compounds like hydrogen sulfide in a process called chemosynthesis.
Both types of producers are classified as autotrophs, meaning they make their own food using inorganic carbon (CO₂) rather than consuming other organisms. They form the foundation of every food web on the planet.
Sunlight: The Dominant Energy Source
Photosynthetic producers, called photoautotrophs, power nearly all life on Earth’s surface. Plants, algae, and photosynthetic bacteria absorb sunlight and use that energy to build glucose from carbon dioxide and water. Oxygen is released as a byproduct. This single process is responsible for both the food and the breathable atmosphere that most living things depend on.
Not all sunlight is equally useful. Producers absorb light in a range called photosynthetically active radiation, which spans wavelengths from 400 to 700 nanometers. Within that range, leaves preferentially absorb red and blue light while reflecting more green light, which is why most plants look green to our eyes.
How Photosynthesis Converts Light to Chemical Energy
The conversion happens in two stages inside chloroplasts, the tiny structures within plant and algae cells that give them their green color.
In the first stage (the light-dependent reactions), pigment molecules act like antennae. Hundreds of pigment molecules are clustered together in each light-harvesting unit, and they collectively absorb photons and funnel that energy toward a central reaction point. When a photon’s energy reaches that center, it excites an electron to a higher energy state, effectively converting light energy into stored chemical energy. That high-energy electron then passes through a chain of protein carriers embedded in a membrane inside the chloroplast. As the electron moves through this chain, it drives protons (hydrogen ions) across the membrane, building up pressure on one side. That pressure powers a molecular turbine called ATP synthase, which produces ATP, the cell’s main energy currency.
Meanwhile, a second light-harvesting system uses additional photons to generate another energy carrier called NADPH. Together, ATP and NADPH are the chemical outputs of the light-dependent stage. The water molecules split during this process are what release oxygen into the atmosphere.
In the second stage (the light-independent reactions, sometimes called the Calvin cycle), the cell uses that ATP and NADPH to stitch carbon dioxide molecules into glucose. This happens in the fluid-filled interior of the chloroplast called the stroma. The glucose can then be used as fuel or as building material for the organism’s growth.
How Efficient Is the Conversion?
Photosynthesis is not particularly efficient at converting sunlight into usable energy. Macroalgae (seaweeds) convert an average of about 6.2% of the photons they absorb into chemical output, while underwater flowering plants average around 4.9%. Most measurements across both groups fall between roughly 3.7% and 7.9%. Terrestrial plants generally fall in a similar range. The rest of the absorbed energy is lost as heat.
Despite this modest efficiency, the sheer scale of photosynthesis is staggering. Terrestrial ecosystems (forests, grasslands) and aquatic ecosystems (oceans, lakes) fix carbon at broadly similar rates overall, though they store it very differently. Forests lock carbon into long-lived wood and roots, while ocean phytoplankton turn over rapidly, storing far less in living tissue at any given moment despite producing enormous amounts of new organic material.
Chemical Energy: Producers Without Sunlight
Deep on the ocean floor, far below the reach of any sunlight, entire ecosystems thrive around hydrothermal vents. The producers here are chemosynthetic bacteria and archaea, called chemoautotrophs, that extract energy from inorganic molecules dissolved in the superheated water pouring from the vents.
The most common energy source at these vents is hydrogen sulfide (H₂S), a compound that smells like rotten eggs at the surface but serves as a lifeline in the deep ocean. Sulfur-oxidizing bacteria break down hydrogen sulfide and related sulfur compounds, capturing the energy released by those chemical reactions. Researchers have isolated multiple distinct types of sulfur-oxidizing bacteria from vents along the Galapagos Rift at depths of 2,550 meters, confirming that chemosynthesis provides a substantial food source for the dense communities of tube worms, clams, and shrimp found clustered around vents.
Other chemoautotrophs use different inorganic fuels. Some oxidize iron compounds, some use ammonia, and others use methane or hydrogen gas. In each case, the underlying principle is the same: the organism breaks apart an inorganic molecule, captures the energy released, and uses it to build organic compounds from CO₂. It mirrors the logic of photosynthesis, just with a chemical reaction substituted for sunlight.
Why Producer Energy Matters for Every Other Organism
Producers sit at the base of every food chain, and the energy they capture sets a hard ceiling on how much life an ecosystem can support. When a primary consumer like a rabbit eats a plant, it doesn’t absorb all the energy stored in that plant tissue. The commonly cited rule of thumb is that only about 10% of energy transfers from one level of a food chain to the next, though the real number varies widely. Warm-blooded animals (mammals, birds) typically pass along only 1 to 5% of the energy they consume, while cold-blooded animals (insects, fish) average 5 to 15%.
This steep drop-off explains why ecosystems support far more plant mass than herbivore mass, and far more herbivores than top predators. A simple example: if plants capture 1,000 kilocalories of energy, herbivores might convert roughly 100 kilocalories into new body tissue, and a predator eating those herbivores might end up with only 10. Every calorie a wolf or a shark uses traces back to the energy a producer originally captured from sunlight or from chemicals seeping out of the Earth’s crust.