Zooxanthellae

Zooxanthellae are tiny, single-celled organisms that live inside many sea animals, such as corals, jellyfish, and sponges. These special little creatures can make their own food using sunlight, a process called photosynthesis. Because of this, they share a friendly relationship with their hosts—they give the animals food and, in return, get a safe place to live with plenty of light.

Most zooxanthellae belong to a group called dinoflagellates, especially those in the genus Symbiodinium. They have special green and yellow colors because of the chemicals they contain, like chlorophyll and peridinin. These colors often give the host animals their bright or brown shades. During the day, zooxanthellae use sunlight to make food, which can sometimes supply as much as 90% of what their host animal needs to grow and stay healthy. This partnership helps both the tiny organisms and their animal homes thrive in the ocean.

Morphology and classification

Zooxanthellae can be grouped into different families and types, such as Bacillariophyceae, Cryptophyceae, Dinophyceae, and Rhodophycaeae. The most common type is called Symbiodinium. These tiny organisms have special parts inside them called chloroplasts that help them make food from sunlight. They also have other important structures like a pyrenoid and mitochondria that help them stay alive.

These organisms have DNA packed tightly in their nucleus, and their DNA includes special molecules like ribosomal RNA. This shows that RNA helps organize their DNA. Zooxanthellae also have unique molecules in their DNA that most other living things do not have.

Life history

Zooxanthellae change forms during their life. They can be resting cells called cysts or moving cells in the water. For some zooxanthellae, like those in the genus Gymnodinium, the life starts as a young cyst. This cyst grows and splits to make another young cyst. When it gets older, it is no longer used.

When zooxanthellae can move, the youngest stage is called a zoosporangium. It grows into a zoospore that can move. This moving cell makes and releases gametes to reproduce.

Vegetative phase

The main form of zooxanthellae is called the vegetative phase. In this form, the cell has a thin wall and many chloroplasts. As the cell grows, the number of chloroplasts goes down. The cell can either split into two new cells or change into a cyst.

Cyst stages

After the vegetative phase, zooxanthellae often become cysts. Cysts have a thick wall but keep the same inside material. They give the host a reddish-brown color. Dividing cysts are when two new cells are connected but have their own walls. Degenerate cysts are rare and do not help the host as much because they photosynthesize less. The moving stages, zoosporangium and zoospore, are also rare. The zoospore stays inside the cyst until the wall breaks open. Zooxanthellae can only move when it is a zoospore.

Motility

When zooxanthellae are zoospores, they can move. They move forward by turning on the back part of their tail and pushing through the water. They can also spin in circles by attaching the back part of their tail to a surface.

Ecology

Zooxanthellae are tiny, green organisms that live inside many sea animals like corals, jellyfish, and clams. They help these animals by giving them food and, in return, get things they need to grow.

Young animals can get zooxanthellae in a few ways. Sometimes they are passed from the parent, or the young animal can find them in the water. Corals can even attract zooxanthellae with special chemicals.

When zooxanthellae live in corals, they give the coral food like sugar. But if the water gets too warm or dirty, the coral may lose its zooxanthellae. This makes the coral turn white, a process called coral bleaching.

Clams also have zooxanthellae, and they can lose them too if it gets too hot. But clams can get them back, which helps them and other sea creatures.

Jellyfish also live with zooxanthellae. These tiny organisms help the jellyfish make food from sunlight. Some jellyfish can survive changes in water by eating other food when they lose their zooxanthellae. Scientists are still learning more about how jellyfish and zooxanthellae work together.