Coral polyps host an entire community of living organisms inside their tissues, but the most important resident is a microscopic algae commonly called zooxanthellae. These single-celled algae belong to the family Symbiodiniaceae and live packed inside individual cells in the polyp’s inner tissue layer, called the gastrodermis. They provide up to 90 percent of the organic material the coral needs to survive. But zooxanthellae are far from the only tenants. Bacteria, archaea, fungi, and other microbes all colonize different parts of the polyp, from its surface mucus down into the calcium carbonate skeleton beneath.
Zooxanthellae: The Primary Resident
The polyp’s body wall is just two cell layers thick: an outer epidermis and an inner gastrodermis that lines the gut cavity. Zooxanthellae live exclusively in that inner layer, housed not just between cells but inside them. Each algal cell sits within a specialized compartment called a symbiosome, a membrane-bound pocket that isolates the algae from the rest of the cell. The symbiosome is highly acidic, with a pH around 4, and the coral actively controls what nutrients pass through that membrane. Proteins shuttle to and from the symbiosome wall, regulating the delivery of nitrogen compounds to the algae in a system that works much like the way human kidneys concentrate ammonia.
This arrangement gives the coral remarkable control over its photosynthetic partners. The algae photosynthesize using sunlight and transfer the sugars and other organic compounds they produce directly to the host. NOAA estimates that as much as 90 percent of what the zooxanthellae produce gets passed along to the coral. In return, the coral provides the algae with a protected, sunlit environment and a steady supply of carbon dioxide and nitrogen. It is this exchange that allows reef-building corals to thrive in the nutrient-poor tropical waters where they are found.
Not All Zooxanthellae Are the Same
The family Symbiodiniaceae contains multiple genera (formerly grouped into lettered “clades”), and different types bring different capabilities to their coral host. Some handle heat well, while others falter at elevated temperatures. Clade D symbionts, for instance, tend to tolerate warmer water better than Clade A or Clade C types. A single coral colony can harbor more than one type at a time, and the particular mix it carries influences how well it withstands thermal stress. Research on the jellyfish relative Cassiopea andromeda showed that recently isolated symbionts maintained significantly higher photosynthetic efficiency under heat stress than the same genetic lineage cultured in a lab for two and a half years, suggesting that real-world environmental variability keeps the algae primed for stress.
This diversity matters because a coral’s thermal tolerance is partly determined by which symbionts it hosts. Even an increase as small as one degree Celsius above a reef’s long-term summer maximum can trigger bleaching, the process where stressed polyps expel their zooxanthellae and turn white. Corals that carry heat-tolerant symbiont types have a better shot at surviving these events.
How Polyps Get Their Symbionts
Young corals can acquire zooxanthellae in two ways. In many species that release eggs into the water column, larvae pick up free-living algae from the surrounding environment after they settle, a process called horizontal transmission. Brooding corals, which develop larvae internally, often pass symbionts directly from parent to offspring through the eggs, known as vertical transmission. But the reality is messier than either category suggests. Research on the brooding coral Seriatopora hystrix found that larvae carried symbiont communities distinct from their parents, including types not detected in the adult colony at all. This points to a mixed-mode strategy where some symbionts are inherited and others are picked up from the water. That flexibility could give corals more resilience to changing conditions than a purely inherited system would allow.
Bacteria and Archaea in the Tissue
Zooxanthellae get the most attention, but bacteria colonize every microhabitat a polyp offers. The surface mucous layer that coats the polyp is especially rich in microbial life. Some of these bacteria produce antibiotic compounds that help defend the coral against pathogens. Others are diazotrophs, organisms capable of pulling nitrogen gas from the water and converting it into forms the coral can use. Both bacteria and archaea perform this nitrogen fixation, and archaea in the mucous layer also handle ammonia oxidation, recycling nitrogen waste back into usable nutrients.
Together, these microbes form what scientists call the coral holobiont: the animal, its algae, and its full microbial community functioning as a single ecological unit. The bacteria help maintain the holobiont’s health by cycling essential nutrients, filling metabolic gaps the coral and its algae cannot cover on their own.
Life Inside the Skeleton
Even the hard calcium carbonate skeleton beneath the living tissue is inhabited. Cut through a coral skeleton and you will often see distinct color bands: the living tissue on top, a thin white plate just below it, a green layer, and then a deeper white layer. For years, researchers assumed the green band was dominated by green algae, but detailed genetic analysis of the coral Isopora palifera from Taiwan’s Green Island revealed something unexpected. The green layer was actually dominated by anaerobic green sulfur bacteria of the genus Prosthecochloris, outnumbering the expected green microalgae by 94 to 452 times.
These endolithic (skeleton-dwelling) bacteria are not just passive passengers. Isotope-tracing experiments showed that carbon and nitrogen move between the green layer and the living coral tissue above it. Organic matter produced in the skeleton travels outward, contributing to the coral’s carbon and nitrogen budget. Nitrogen compounds originating in the green layer diffuse up through the basal plate and are converted into ammonium, supplying an additional nutrient source for the holobiont. The skeleton bacteria also carry out sulfur metabolism and their own form of photosynthesis using a different biochemical pathway than plants, making them a hidden but meaningful energy contributor, particularly when the coral is under stress and its zooxanthellae are compromised.
Why the Internal Community Matters
The polyp animal is, in a real sense, a habitat. Its tissues, mucus, and skeleton support a layered ecosystem of algae, bacteria, and archaea that collectively handle photosynthesis, nitrogen fixation, nutrient recycling, and pathogen defense. The zooxanthellae in the gastrodermis are the energetic engine, providing the bulk of the coral’s daily calories. The bacteria in the mucus act as a chemical shield and nutrient processor. The microbes buried in the skeleton form a backup supply line for carbon and nitrogen. Remove any one of these communities and the polyp loses a critical support system. Coral bleaching is the most visible example: when zooxanthellae are expelled, the coral loses its primary energy source and, unless conditions improve quickly, starves.