Diatoms are single-celled algae found in virtually every body of water on Earth, from open oceans to puddles on a forest floor, and they are responsible for roughly one-fifth to one-quarter of all photosynthesis on the planet. What sets them apart from other microalgae is their cell wall, a glassy shell made of silica that looks, under a microscope, like an intricately etched jewel box. That combination of ecological dominance and structural beauty has made diatoms a subject of intense scientific interest, with applications ranging from forensic pathology to drug delivery to climate modeling.
Living in a Glass House
Every diatom builds itself a cell wall, called a frustule, out of hydrated silicon dioxide, the same basic compound found in glass and quartz. The frustule is not a smooth blob. It is patterned with pores, ribs, and chambers at the nanometer scale, and those patterns are so consistent within a species that taxonomists use them as the primary way to tell diatom species apart. The designs are genetically encoded, meaning the diatom’s DNA dictates the architecture of its shell down to tiny details.1Annual Reviews. Diatoms-from cell wall biogenesis to nanotechnology To build these walls, diatoms pull dissolved silicic acid out of the surrounding water using specialized transporter proteins. In the sunlit upper ocean, silicic acid concentrations average around 10 micromoles per liter, and the rate at which diatoms take it up is tightly coupled to how fast they are growing and depositing new wall material.2PubMed Central. Silicon Uptake in Diatoms Revisited: A Model for Saturable and Nonsaturable Uptake Kinetics and the Role of Silicon Transporters
The frustule is made of two halves that fit together like a petri dish and its lid. One half, the epitheca, slightly overlaps the other, the hypotheca. Girdle bands connect the two halves along the edges. This two-piece design has important consequences for reproduction, which we will get to shortly. Despite being made of glass, the frustule is surprisingly tough for its weight, and its nanoscale pore patterns give it optical and mechanical properties that engineers have struggled to replicate synthetically.
Moving Without Muscles
Many diatom species are not simply passive drifters. A large group of pennate diatoms (the elongated, boat-shaped ones, as opposed to the round centric types) can glide across surfaces at speeds visible under a microscope. They do this by secreting a trail of sticky mucilage through a slit in the frustule called the raphe. The movement depends on an internal cytoskeletal system based on actin, the same protein that helps your own muscle cells contract. Motor proteins beneath the cell membrane push the mucilage backward through the raphe, and the cell slides forward.3PubMed Central. Contribution of frustules and mucilage trails to the mobility of diatom Navicula sp.
Recent work has gotten more specific about which molecular motors are involved. Researchers studying a gliding diatom called Craspedostauros australis identified a set of myosin proteins unique to raphid diatoms. When they tagged these proteins with a fluorescent marker, three of the four myosins moved in coordinated fashion in the opposite direction to the cell’s travel, consistent with the idea that they are the engines generating the force that propels the mucilage and, by reaction, the cell.4Communications Biology. Gliding motility of the diatom Craspedostauros australis coincides with the intracellular movement of raphid-specific myosins Watching a diatom glide across a microscope slide is one of the small delights of introductory biology; knowing that it is powered by custom-evolved molecular motors makes it more impressive.
A Chimeric Evolutionary History
Diatoms did not arise through a simple, clean line of descent. They are the product of a two-step process in which one cell engulfed another, then kept it. The first step happened over a billion years ago when an ancient single-celled organism swallowed a photosynthetic cyanobacterium, eventually domesticating it into a chloroplast. That gave rise to the lineage of red and green algae. The second step came later: a different single-celled organism swallowed one of those red algae and kept its chloroplast, too. That secondary merger produced the ancestors of diatoms.5PubMed Central. Diatom genomics: genetic acquisitions and mergers
This layered history means the diatom genome is a patchwork. When the first full diatom genome was sequenced, it revealed genes traceable to the host cell, to the red algal endosymbiont, and even to bacteria that contributed genes through horizontal transfer over the eons. The result is a metabolic toolkit that is unusually versatile for a single-celled organism, which helps explain how diatoms came to dominate so many aquatic environments.
The Silicon Cycle and the Carbon Pump
Diatoms occupy a unique niche among phytoplankton because they need silicic acid on top of the nitrogen, phosphorus, and other nutrients that all algae require. A modeling study of the global silicon cycle found that while diatoms are competitively superior to many other algae, the availability of silicic acid acts as a check on their population, preventing them from simply outcompeting everything else. The fraction of total ocean primary production carried out by diatoms is ultimately set by how much dissolved silicon is available, while overall production is limited by phosphorus.6Global Biogeochemical Cycles. Role of diatoms in regulating the ocean’s silicon cycle
When diatoms die, their heavy glass shells cause them to sink faster than most other phytoplankton. This sinking carries organic carbon from the surface ocean to the deep, a process central to the “biological pump” that moves carbon dioxide out of the atmosphere and into long-term storage. Diatom aggregates can also scavenge minerals and other organic particles on their way down, affecting how much carbon reaches the deep sea floor. Experiments have shown that adding suspended minerals to tanks of diatom aggregates changed the size and composition of those aggregates, altering the ratio of organic carbon to total material in ways that depend on whether the minerals were opal or calcium carbonate.7Global Biogeochemical Cycles. Interactions between diatom aggregates, minerals, particulate organic carbon, and dissolved organic matter: Further implications for the ballast hypothesis In short, diatoms do not just photosynthesize; they physically drag carbon into the deep ocean.
Beyond carbon, diatoms can be cultivated industrially as a natural sink for atmospheric COâ‚‚, and the carbon they capture can be redirected into valuable products including lipids, omega-3 fatty acids, pigments, and antioxidants.8PubMed Central. Diatoms for Carbon Sequestration and Bio-Based Manufacturing
Symbioses in Low-Nutrient Seas
In the vast stretches of open ocean where nitrogen is scarce, some diatoms have solved the problem by teaming up with cyanobacteria capable of pulling nitrogen gas directly from the water and converting it into a usable form. Researchers used high-resolution imaging to track the flow of nitrogen atoms in these partnerships and directly proved that the cyanobacterial symbionts fix nitrogen and then pass it to their diatom hosts.9The ISME Journal. Nitrogen fixation and transfer in open ocean diatom–cyanobacterial symbioses The diatom, in turn, likely provides the cyanobacterium with organic carbon and a stable physical habitat. These partnerships are ecologically significant because they introduce new nitrogen into surface waters that would otherwise be too nutrient-poor to support much growth.
The Shrinking Problem and Sexual Reproduction
The two-piece frustule creates a peculiar reproductive challenge. When a diatom divides, each daughter cell inherits one half of the parent’s shell and builds a new, slightly smaller half to fit inside it. The daughter that inherits the larger epitheca stays roughly the same size as the parent, but the daughter that inherits the smaller hypotheca ends up a bit smaller. Over many generations of division, one lineage of cells steadily shrinks.
Eventually, the cells reach a minimum viable size and must restore themselves through sexual reproduction. Two small cells fuse to form a zygote, which then develops into a special expanding cell called an auxospore. Observations of the oceanic diatom Fragilariopsis kerguelensis recorded auxospores elongating from about 24 to 91 micrometers, with the largest auxospores containing the initial full-sized cell whose length ranged between 76 and 90 micrometers. The parent cells that fused to create them were as small as 10 to 31 micrometers.10Journal of Phycology. Auxospore formation by the silica-sinking, oceanic diatom Fragilariopsis kerguelensis (Bacillariophyceae) So sexual reproduction in diatoms is not mainly about genetic mixing, though that happens too. It is about escaping a shrinking trap built into their own architecture.
Sleeping for Thousands of Years
When conditions turn harsh, whether from nutrient depletion, darkness under sea ice, or seasonal changes, many diatom species can form resting spores. These are thick-walled dormant cells designed to survive long periods of environmental stress and germinate when conditions improve.11PubMed Central. Gene expression during the formation of resting spores induced by nitrogen starvation in the marine diatom Chaetoceros socialis How long is “long”? Much longer than anyone initially guessed. Researchers revived and germinated resting spores of the diatom Chaetoceros from sub-seafloor sediments at three age ranges: recent (up to 80 years old), roughly 1,250 years old from the Medieval Climate Anomaly, and roughly 6,600 years old from the Holocene Thermal Maximum.12PubMed. Not dead yet: Diatom resting spores can survive in nature for several millennia
The implications are striking. Millennia-old spores buried in marine sediments represent a kind of living seed bank for the ocean. Studying which ancient spores can still germinate and how they perform under modern conditions may give scientists a window into how diatom populations have adapted over long timescales, and whether ancient genetic diversity might help buffer modern populations against environmental change.
When Diatoms Turn Toxic
Not all diatoms are benign. Several species in the genus Pseudo-nitzschia produce domoic acid, a potent neurotoxin that causes amnesic shellfish poisoning in humans and mass die-offs in marine mammals and seabirds.13PubMed Central. Biosynthesis of the neurotoxin domoic acid in a bloom-forming diatom When these diatoms bloom, shellfish filter-feed on them and accumulate the toxin, which then moves up the food chain. People who eat contaminated mussels, clams, or razor clams can experience seizures, permanent short-term memory loss, and in severe cases, death.
What triggers a diatom to ramp up toxin production? Laboratory experiments with Pseudo-nitzschia multiseries and P. australis showed that nutrient stress plays a major role. Under iron-deficient conditions, cellular production of domoic acid jumped about eightfold compared to optimal growth conditions, and under copper stress it increased roughly twentyfold. The vast majority of the toxin, about 95%, was actively released into the surrounding water rather than retained inside the cell.14Limnology and Oceanography. The effect of Fe and Cu on growth and domoic acid production by Pseudo-nitzschia multiseries and Pseudo-nitzschia australis This means blooms in nutrient-stressed waters can be far more toxic per cell than blooms in nutrient-rich areas.
Monitoring for these toxic blooms has become a public health priority along coastlines worldwide. Researchers working in Monterey Bay, California, during a pervasive Pseudo-nitzschia australis bloom used molecular tools to identify the active transcription of domoic acid biosynthesis genes in weekly phytoplankton samples, enabling them to track toxin production at the genetic level rather than waiting for toxin to accumulate in shellfish tissue.15PubMed Central. Molecular Forecasting of Domoic Acid during a Pervasive Toxic Diatom Bloom That kind of molecular forecasting could eventually give coastal managers earlier warning before a bloom becomes a public health crisis.
Diatoms as Water Quality Sensors
Because different diatom species thrive under different chemical conditions, the mix of species found at a given site acts as a biological fingerprint of water quality. Ecologists have used diatom community composition to assess the health of rivers, lakes, and wetlands for decades. A comprehensive review of diatoms as bioindicators in freshwater ecosystems found that electrical conductivity, pH, and temperature are the physical-chemical parameters that most strongly shape diatom communities, followed by dissolved oxygen, phosphate, and nitrate concentrations.16Ecological Indicators. Diatoms as bioindicators for health assessments of ephemeral freshwater ecosystems: A comprehensive review A sudden shift in the diatom species present at a monitoring station can flag pollution events or nutrient runoff before conventional chemical tests catch them.
Diatoms preserved in lake and ocean sediments also serve as archives of past environmental conditions. Because frustules are made of silica and resist decomposition, they accumulate in sediment layers over centuries. By identifying the species present in each layer, paleoecologists can reconstruct past water temperatures, salinity levels, and nutrient conditions with remarkable resolution.
Diatoms in Forensic Investigations
One of the more unexpected roles for diatoms is in forensic pathology. When a person drowns, inhaled water carries diatoms deep into the lungs, and from there they can enter the bloodstream and lodge in organs like the liver, kidneys, and brain. Finding diatoms in internal organs at autopsy has long been used as evidence of drowning, since a body simply placed in water after death would not take diatoms into its circulatory system. A controlled study in veterinary forensic pathology confirmed that both the number and location of diatoms recovered from organ tissue could reliably distinguish animals that had actually drowned from those that died of other causes and were later submerged.17PubMed Central. Diagnosis of Drowning and the Value of the Diatom Test in Veterinary Forensic Pathology The so-called diatom test is not without controversy, since background contamination is always a concern, but it remains one of the few objective tests available to pathologists when the cause of death is unclear and the body was found in water.
Biofuel, Lipids, and the Nitrogen Starvation Trick
Diatoms have attracted attention as a potential feedstock for biofuels because certain species can accumulate large amounts of lipid, the raw material for biodiesel. The trick to boosting lipid content is often surprisingly simple: starve the cells of nitrogen. When diatoms run low on nitrogen, they shift their metabolism away from growth and toward fat storage. In the model diatom Phaeodactylum tricornutum, nitrogen deprivation led to increased expression of genes involved in fatty acid production while dialing down photosynthesis and lipid breakdown. The proportion of saturated fatty acids rose by about 45%, and monounsaturated fatty acids by about 61%.18PubMed Central. Proteomics to reveal metabolic network shifts towards lipid accumulation following nitrogen deprivation in the diatom Phaeodactylum tricornutum
At a cellular level, what happens during nitrogen starvation is a dramatic remodeling of internal membranes. The lipids that make up chloroplast membranes decrease sharply, while storage fats called triacylglycerols increase, suggesting that the cell is essentially cannibalizing its own photosynthetic membranes and converting them to storage lipids.19PubMed Central. Molecular and cellular mechanisms of neutral lipid accumulation in diatom following nitrogen deprivation Two-stage cultivation strategies, where diatoms first grow under normal nutrient conditions and then are switched to nitrogen-limited media, can significantly boost lipid yields without sacrificing initial biomass production.20PubMed Central. Potential of lipid metabolism in marine diatoms for biofuel production
Whether diatom-based biofuel can compete economically with other algal or plant feedstocks remains an open question. But the underlying biology is well understood and the cells grow fast, which keeps them in the conversation.
Frustules as Optical Devices and Drug Carriers
The nanoscale pore patterns in diatom frustules do something engineers find remarkable: they manipulate light. Experiments with the frustule of Coscinodiscus wailesii showed that the valve could focus coherent laser light at a wavelength of 785 nanometers into a spot just 10 micrometers wide, about 100 micrometers away from the valve itself. The focusing effect results from the way individual pores diffract incoming light and those diffraction contributions add up along the central axis. The same phenomenon was observed with incoherent visible light and in several other diatom species, with the position of the light spot shifting depending on wavelength.21Growth and Form. The Diatom Frustule: Morphogenesis and Role in Light Manipulation Some researchers have speculated that this light-focusing ability might help the cell funnel photons to its chloroplasts, though that idea is still debated.
The same structural features that make frustules interesting to physicists also make them attractive for biomedical engineering. The frustule’s high surface area, thermal stability, biocompatibility, and customizable surface chemistry have led researchers to investigate diatom silica as a vehicle for drug delivery. Loaded with a drug and surface-modified to target specific tissues, diatom-derived microparticles could serve as low-cost, naturally produced alternatives to synthetic nanocarriers.22PubMed Central. Diatoms Green Nanotechnology for Biosilica-Based Drug Delivery Systems The work is still largely at the laboratory stage, but the appeal is clear: nature already mass-produces these intricate silica structures at room temperature and neutral pH, which is something no industrial process can match.
Ocean Acidification and the Future of Diatoms
Climate change threatens diatoms through a route that is less obvious than warming alone. As the ocean absorbs more COâ‚‚ and becomes more acidic, the chemistry of dissolved silicon shifts in ways that accelerate the export of silica from surface waters to the deep ocean. Modeling under moderate and high emissions scenarios projects that surface ocean silicic acid concentrations could drop by 11% to 27% by the year 2200, with a corresponding decline in global diatom biomass of 13% to 26%.23Nature. Enhanced silica export in a future ocean triggers global diatom decline Because diatoms are the ocean’s primary biological pump for carbon, a decline of that magnitude would ripple through marine food webs and could weaken the ocean’s capacity to absorb COâ‚‚, creating a feedback loop.
One proposed intervention, ocean alkalinity enhancement, involves dissolving minerals into seawater to increase its COâ‚‚-absorbing capacity. Whether this helps or harms diatoms depends on the mineral used. Experiments testing silicate-based minerals found that the added dissolved silicate boosted silicification in most diatom genera tested, while calcium-based minerals had little effect on shell building. Increases in alkalinity alone, up to 600 micromoles per kilogram above natural levels, affected silicification in only two of six genera studied.24Biogeosciences. Investigating the effect of silicate- and calcium-based ocean alkalinity enhancement on diatom silicification The takeaway for climate engineers is that the type of mineral matters: silicate-based approaches could inadvertently benefit diatoms by replenishing the very nutrient that limits their growth, while calcium-based approaches seem closer to neutral. Either way, the sensitivity of diatoms to changes in ocean chemistry is a reminder that these tiny organisms sit at the intersection of biology, geology, and climate in ways that are easy to overlook.