Yes, diatoms photosynthesize. They are single-celled algae that use sunlight to convert carbon dioxide and water into energy, just like plants do. In fact, diatoms are remarkably good at it: they generate roughly 20% of all the oxygen produced on Earth each year, which is more than all the world’s tropical rainforests combined.
How Diatom Photosynthesis Works
Diatoms use the same basic process as land plants, capturing light energy and converting it into sugars while releasing oxygen. But they rely on a different set of pigments to do it. Instead of the chlorophyll a and chlorophyll b combination found in plants, diatoms use chlorophyll a paired with chlorophyll c, plus a golden-brown pigment called fucoxanthin. These three pigments form specialized light-harvesting complexes that replace the antenna systems plants use.
Fucoxanthin is the key to diatoms’ distinctive brownish-gold color, and it gives them a real advantage underwater. It absorbs light broadly between 460 and 570 nanometers, covering much of the blue-green spectrum that chlorophyll alone misses. This means diatoms can capture wavelengths of light that penetrate deeper into water, making them efficient photosynthesizers in ocean environments where red light fades quickly. On top of these light-harvesting pigments, diatoms also carry a set of protective pigments (carotenoids) that shield them from damage when light gets too intense.
A Glass Shell That Focuses Light
Diatoms live inside a cell wall called a frustule, made of transparent silica, essentially biological glass. For a long time, scientists viewed the frustule primarily as armor. But research published in Nature has shown it does something far more sophisticated: it acts as a photonic crystal, manipulating light before it reaches the photosynthetic machinery inside.
When researchers studied the frustule of the diatom Coscinodiscus centralis using high-resolution light mapping, they found that the nanoscale pore patterns in the shell selectively enhanced light transmission at 636 and 663 nanometers. Those wavelengths match the absorption peaks of chlorophyll a and c almost exactly. The layered structure of the frustule also traps light inside the cell, preventing it from scattering back out. In other words, the glass shell isn’t just protection. It’s a built-in lens that filters sunlight into the specific colors the cell needs most, boosting photosynthetic efficiency in the process.
How Deep Diatoms Can Photosynthesize
Every photosynthetic organism has a compensation depth: the point underwater where the light is so dim that photosynthesis barely produces enough energy to keep the cell alive. Below that depth, the organism burns more energy through respiration than it creates. Diatoms push this boundary further than most other algae. They can remain photosynthetically active down to the 0.1% light level, meaning they function where only one-thousandth of the surface sunlight remains. By comparison, dinoflagellates (another major group of algae) generally need about ten times more light, hitting their limit around the 1% level.
In clear open-ocean water, this translates to diatoms photosynthesizing as deep as 100 meters. In murkier coastal waters, the compensation depth can shrink to just a few meters. This ability to work in very low light, combined with their efficient pigment system, helps explain why diatoms thrive across such a wide range of ocean conditions.
Nutrients That Fuel Diatom Growth
Light alone isn’t enough. Diatoms need dissolved nutrients to build cells and keep their photosynthetic machinery running. Nitrogen, phosphorus, and silicon are the three big ones. Nitrogen and phosphorus are essential for the proteins and energy molecules involved in photosynthesis, while silicon is required to construct the glass frustule. When any of these nutrients runs low, diatom growth stalls, even if light and temperature are ideal. This is why diatom blooms tend to explode in spring and after storms, when mixing brings nutrient-rich deep water to the surface.
Why Diatom Chloroplasts Look Different
Diatom chloroplasts didn’t come from the same evolutionary path as plant chloroplasts. Plants acquired their chloroplasts when an ancient cell engulfed a photosynthetic bacterium, an event called primary endosymbiosis. Diatoms took a more roundabout route. Their ancestor swallowed another alga that already had chloroplasts, a process known as secondary endosymbiosis. The result is a chloroplast wrapped in extra membranes, a telltale sign of its more complex origin.
Genetic analysis has found that diatom genes related to photosynthesis trace back to both red and green algae in roughly equal proportions. One explanation, called the shopping bag model, suggests that the ancestor of diatoms cycled through multiple algal partners over evolutionary time, picking up useful genes from each one. Eventually it settled on a single endosymbiont that became the permanent chloroplast. This patchwork genetic history is one reason diatom photosynthesis has some features that look plant-like and others that don’t.
Their Outsized Role in Earth’s Carbon Cycle
Diatoms are responsible for about 40% of all marine primary production, the process of turning CO₂ into organic carbon using sunlight. Globally, that works out to roughly 20% of all photosynthesis on the planet, putting them on par with all terrestrial rainforests combined. Estimates of diatom species diversity range from about 12,000 to 30,000 known species, with a recent large-scale ocean survey identifying around 4,748 distinct types of marine planktonic diatoms alone.
What makes diatoms especially important for the climate is what happens after they die. Their heavy glass shells cause them to sink faster than most other phytoplankton. As they fall, they carry carbon from the surface ocean to the deep seafloor, a process called the biological pump. The world’s oceans already hold an estimated 39 trillion metric tons of carbon, and diatoms are one of the major vehicles transporting it there. Once buried in sediment, that carbon can stay locked away for millions of years, which is why diatom photosynthesis isn’t just a biological curiosity. It’s one of the planet’s primary mechanisms for pulling CO₂ out of the atmosphere.