Zooxanthellae is a common name for tiny, single-celled organisms that use sunlight to make their own food. These organisms form partnerships with many marine invertebrates, such as corals, jellyfish, sponges, and nudibranchs. Most zooxanthellae belong to a group of dinoflagellates called Symbiodinium, but some are found in the genus Amphidinium. Other groups of organisms that are not yet fully identified may also form similar partnerships. The name "Zooxanthella," meaning "little yellow animal," was first used in 1881 by Karl Brandt to describe a species called Zooxanthella nutricula, which lives with a type of radiolarian called Collozoum inerme. This species is now classified in the Peridiniales group. Another group of single-celled organisms, called green algae zoochlorellae, also forms similar partnerships in both ocean and freshwater environments.

Zooxanthellae use chlorophyll a and chlorophyll c, along with pigments called peridinin and diadinoxanthin, to give their host organisms their yellow and brown colors. During the day, they produce organic compounds through photosynthesis, which can supply up to 90% of the host's energy needs for growth, metabolism, and reproduction. In return, the host provides nutrients, carbon dioxide, and a location near sunlight for the zooxanthellae to thrive.

Morphology and classification

Zooxanthellae are classified into several groups, including the classes Bacillariophyceae, Cryptophyceae, Dinophyceae, and Rhodophyceae. They also belong to specific genera such as Amphidinium, Gymnodinium, Aureodinium, Gyrodinium, Prorocentrum, Scrippsiella, Gloeodinium, and most commonly, Symbiodinium. The genus Symbiodinium is divided into eight phylogenetic groups labeled A through H. These groups are identified by differences in their nuclear ribosomal DNA and chloroplast DNA.

Zooxanthellae are autotrophs, meaning they produce their own food through photosynthesis. Their chloroplasts contain thylakoids, which are clustered in groups of three. Each chloroplast has a pyrenoid, a structure that is surrounded by a thick, starchy covering. Inside the cell’s cytoplasm, there are lipid vacuoles, calcium oxalate crystals, dictyosomes, and mitochondria. The cell wall of zooxanthellae varies between species. Some have three layers: an outer membrane, a middle layer dense with electrons, and a thin inner layer. Others have a cell wall made entirely of the low-density inner layer. Below the cell wall is the cell membrane, and beneath that are thecal vesicles.

DNA in zooxanthellae is organized into tightly packed chromatin coils. This DNA is condensed in the nucleus and includes an unusual set of histone proteins. The DNA contains ribosomal RNA (rRNA) that is folded in a way similar to rRNA found in archaeobacteria. This similarity suggests that RNA plays a key role in DNA organization within zooxanthellae. Additionally, zooxanthellae, like all dinoflagellates, have 5-hydroxymethyluracil and thymidine in their genomes. These components are not found in the genomes of other eukaryotic organisms.

Life history

Zooxanthellae change between life stages that exist as cysts and as organisms that can move in water. In zooxanthellae of the genus Gymnodinium, one possible life cycle begins with an immature cyst that matures, divides, and forms another immature cyst. As the cell grows older, it becomes less useful. In the life cycle of a moving zooxanthellae cell, the earliest stage is called a zoosporangium, which matures into a zoospore that can move. This moving cell produces and releases gametes for reproduction.

The vegetative phase is the most common form of zooxanthellae. In this form, the single-celled organism has a thin cell wall. Unlike the zoospore, zooxanthellae contain many chloroplasts. As the cell continues to grow, the number of chloroplasts decreases. The vegetative cell either divides into two separate daughter cells or changes into a cyst stage.

After the vegetative phase, the most common stages in zooxanthellae life cycles are cysts, dividing cysts, and degenerate cysts. Cysts have thick cell walls but keep the same cytoplasm composition and make up most of the clusters of zooxanthellae in host tissues. This stage gives the host a reddish-brown color. Dividing cysts make up about one-fourth of zooxanthellae clusters in host tissues and are stages where two daughter cells remain connected but have separate cell walls. Degenerate cysts are rare in clusters and lose much of their helpful role for the host because their ability to perform photosynthesis decreases. The young zoosporangium and moving zoospore stages are present in life cycles but are uncommon in some groups. The zoospore stays inside the zoosporangium until the cyst’s cell wall breaks open. Zooxanthellae can only move if they begin as a zoospore.

Zooxanthellae in the zoospore stage move by either moving forward or spinning in place. When moving forward, the organism spins around the axis of its back flagellum while pushing itself through the water. The zoospore spins in the water by attaching its back flagellum to a surface.

Ecology

Zooxanthellae are commonly found in reef-building corals but also live in other invertebrates and protists. Their hosts include sea anemones, jellyfish, nudibranchs, certain bivalve molluscs like the giant clam Tridacna, sponges, flatworms, and some species of radiolarians and foraminiferans. Many different types of zooxanthellae live in host organisms, each with its own ability to adapt and tolerate different environmental conditions.

A young organism or newly formed colony can gain zooxanthellae through sexual reproduction or directly from the environment. The egg from which the organism develops may already contain zooxanthellae at the time of fertilization, or the symbionts may be passed from the mother to the larva during a period when the parent protects the larva. Alternatively, the new organism may acquire zooxanthellae directly from seawater, where the dinoflagellates live freely during certain life stages. Some stony corals use chemical signals to attract zooxanthellae, which then infect the coral. Infection can also occur when the host eats infected fecal matter or prey that already contains zooxanthellae. This indirect method can lead to the new host being infected by a different species of zooxanthellae than its parent.

• Cross section of the mantle tissue of a giant clam showing the symbiotic protozoa
• A ciliate with green zoochlorellae living inside it endosymbiotically
• Diagram of radiolarian containing zooxanthellae (z)

Zooxanthellae living in coral are found in vacuoles of the host’s gastrodermal cells and belong to the genus Symbiodinium. They provide nutrients to their host cnidarians, such as sugars, glycerol, and amino acids, while receiving carbon dioxide, phosphates, and nitrogen compounds in return. When corals face environmental stress, they may expel zooxanthellae from their tissues. This causes the coral to lose its color, a process called coral bleaching, revealing the white skeleton beneath. Changes in salinity, light, temperature, pollution, sedimentation, and disease can affect zooxanthellae’s ability to photosynthesize or cause them to leave their hosts.

The reasons why zooxanthellae leave their hosts are still being studied. Some theories suggest that zooxanthellae or the host’s gastrodermal cells detach from the coral. During bleaching, entire gastrodermal cells containing zooxanthellae may leave the host. In other cases, the cells remain in the host, but the zooxanthellae inside the vacuoles may become damaged or leave the cells to enter the surrounding environment.

Coral is not the only aquatic organism affected by bleaching and zooxanthellae loss. Clams also experience a similar process when temperatures rise. However, clams often release living zooxanthellae and can later reacquire them. This ability benefits clams and the surrounding ecosystem, as clams are important in the food chain. Giant clams excrete live zooxanthellae, which other organisms consume for nutrients. For clams in their veliger stage, consuming zooxanthellae helps them grow. Zooxanthellae are also found in the mantle tissue of clams, where they absorb ammonia and nitrate, and in the eyes of clams like Tridacna, where they act as a lens. Different groups of zooxanthellae influence clam development. For example, clade E1 of zooxanthellae seems to favor smaller offspring in clams compared to other clades. All five clades appear necessary for larval settlement to occur.

Zooxanthellae and jellyfish have a long history in scientific research, as Symbiodinium was first cultured from the jellyfish Cassiopea, a model species. Many types of zooxanthellae form relationships with jellyfish across different evolutionary groups, and their roles change throughout the jellyfish’s life cycle. As jellyfish age, the variety of zooxanthellae attached to them decreases, suggesting that zooxanthellae compete to form relationships with the host. Not all jellyfish have these relationships, and most are found in tropical and subtropical waters. Unlike coral, jellyfish are affected differently by climate change, even though both belong to the cnidaria family. Some jellyfish and their zooxanthellae may resist the effects of ocean acidification caused by climate change to a degree. However, jellyfish bleaching has been recorded during extreme heat events. While factors affecting coral-zooxanthellae relationships may not apply to jellyfish, light intensity does impact them. Light affects the lipid production of zooxanthellae, which jellyfish use. To maximize light absorption, jellyfish swim near the surface and follow specific migration patterns. These movements also help zooxanthellae access nutrients. Many jellyfish are mixotrophic, meaning they consume live prey and use photosynthesis. This ability may help them survive climate change and bleaching by switching feeding methods instead of relying on recovering lost zooxanthellae. Scientists continue to study the relationship between zooxanthellae and jellyfish to better understand its complexities.