Cyanobacteria are primarily autotrophic. Specifically, they are photoautotrophs, meaning they use sunlight to convert carbon dioxide and water into the organic compounds they need to grow. But the full picture is more interesting: some cyanobacteria species can also switch to heterotrophic or mixotrophic growth under certain conditions, feeding on organic molecules like glucose or glycerol when light is unavailable.
How Cyanobacteria Power Themselves
Cyanobacteria are the only group of bacteria that perform oxygenic photosynthesis, the same type of photosynthesis that plants use. They split water molecules to release oxygen, capture electrons, and run those electrons through two protein complexes called photosystem I and photosystem II. The end result is chemical energy (ATP) and reducing power (NADPH), which they then use to pull carbon dioxide out of the air and build it into sugars and other organic molecules.
What makes cyanobacteria unusual among bacteria is their pigment toolkit. Like plants, they contain chlorophyll a and carotenoids, which absorb blue and red light. But they also carry specialized antenna complexes called phycobilisomes that let them harvest a broader range of wavelengths than most photosynthetic organisms can use. This wider light-harvesting ability helps explain why cyanobacteria thrive in such a wide range of environments, from open oceans to hot springs to the surface of desert rocks.
When Cyanobacteria Act as Heterotrophs
While photoautotrophy is the default, certain cyanobacteria species can grow heterotrophically, meaning they consume pre-made organic molecules for carbon and energy instead of making their own through photosynthesis. In the dark, some strains break down sugars like glucose, fructose, or sucrose, or the alcohol glycerol, using chemical reactions rather than light to generate energy. This mode of growth is called chemoorganoheterotrophy.
There’s also a middle ground called photoheterotrophy. In this mode, cyanobacteria still use light energy through photosystem I, but they can’t split water (photosystem II is inactive), so they rely on organic molecules as their electron source and carbon supply. The light energy alone isn’t enough to fix carbon dioxide, so they need that organic supplement. Most cyanobacteria capable of heterotrophic growth can only use one or two specific organic substrates, and the usable molecules vary between strains. It’s a limited backup system, not a preferred lifestyle.
A third possibility is mixotrophic growth, where cyanobacteria run photosynthesis and simultaneously take up organic carbon from the environment. The well-studied strain Synechocystis PCC 6803, for example, can grow under photoautotrophic conditions (light plus CO₂), mixotrophic conditions (light plus CO₂ plus glucose), or fully heterotrophic conditions (glucose in the dark). This metabolic flexibility lets certain species survive fluctuating environments, but most cyanobacteria in nature rely overwhelmingly on photoautotrophy.
Nitrogen Fixation: Another Autotrophic Trick
Beyond making their own food from CO₂, many cyanobacteria can also fix atmospheric nitrogen, converting N₂ gas into ammonia that they use to build proteins and DNA. This means they can satisfy their need for both carbon and nitrogen from inorganic sources, making them remarkably self-sufficient organisms.
There’s a catch, though. The enzyme that fixes nitrogen, nitrogenase, is destroyed by oxygen, and cyanobacteria produce oxygen as a byproduct of photosynthesis. To solve this problem, cyanobacteria use two strategies. Some separate the two processes in time, photosynthesizing during the day and fixing nitrogen at night, using an internal biological clock. Others separate them in space by developing specialized cells called heterocysts. These thick-walled cells stop performing photosynthesis entirely, creating the oxygen-free interior that nitrogenase needs. Heterocysts then supply fixed nitrogen to neighboring photosynthetic cells in the filament, while those cells send back the carbon compounds the heterocysts need. It’s a division of labor within a single chain of cells.
Why Cyanobacterial Photosynthesis Matters
Cyanobacteria are among the most ancient life forms on Earth, with origins stretching back more than 3 billion years. Their photosynthetic activity was responsible for the Great Oxidation Event roughly 2.5 to 2.3 billion years ago, when atmospheric oxygen levels rose dramatically for the first time. Before cyanobacteria, Earth’s atmosphere contained essentially no free oxygen. Their metabolic activity reshaped the planet’s chemistry and made complex oxygen-breathing life possible.
The connection goes even deeper. Chloroplasts, the organelles that make plant and algal cells photosynthetic, almost certainly descended from an ancient cyanobacterium that was engulfed by a larger cell. The evidence is extensive: chloroplasts and cyanobacteria share the same photosynthetic pigments, the same internal membrane structures (called thylakoids), and nearly all the genes encoded in chloroplast DNA trace back to cyanobacterial ancestors. Chloroplasts even reproduce by splitting in two, just as cyanobacteria do. In a real sense, every plant on Earth runs on cyanobacterial technology.
How Cyanobacteria Differ From Other Photosynthetic Bacteria
Cyanobacteria are not the only bacteria that photosynthesize, but they are the only ones that do it the way plants do. Other photosynthetic bacteria, such as purple bacteria and green sulfur bacteria, perform anoxygenic photosynthesis. They don’t split water and they don’t produce oxygen. Instead, they use hydrogen sulfide, hydrogen gas, or organic compounds as their electron source. They also rely on different pigments called bacteriochlorophylls rather than chlorophyll a.
This distinction is significant. Anoxygenic photosynthetic bacteria are often mixotrophic or heterotrophic, freely using organic compounds alongside or instead of light energy. Cyanobacteria, by contrast, are fundamentally built around oxygenic photosynthesis. Their ability to use water, the most abundant molecule on Earth’s surface, as an electron donor is what made them so ecologically dominant and what ultimately transformed the planet’s atmosphere.
What Fuels Cyanobacterial Blooms
Understanding cyanobacteria as autotrophs helps explain what drives the harmful algal blooms that increasingly plague lakes, reservoirs, and estuaries. Because cyanobacteria build biomass from inorganic ingredients, the limiting factors for their growth are the raw materials they need: primarily nitrogen and phosphorus, plus sunlight and warm temperatures.
Nitrogen enrichment plays a key role in stimulating blooms, with ammonium promoting cyanobacterial growth faster than nitrate in some systems. Phosphorus matters too. When both nutrients are added together, growth often reaches its maximum, suggesting that nitrogen and phosphorus frequently co-limit cyanobacterial production. Stored “legacy phosphorus” in lake sediments can keep fueling blooms for years even after external phosphorus inputs are reduced. Water conditions matter as well: warm temperatures favor cyanobacteria over other algae, and low-flow drought years with long water residence times correlate with higher bloom intensity. Wet years with strong flushing tend to suppress blooms even when nutrient loads are high.