Let’s Talk About Symbionts and Symbiosis

Symbiosis is a relationship between two or more living organisms in which all the parties benefit from or support each other. There are many types of symbiosis, for example parasitism where the host is actually harmed by the symbiont. But today we will focus on mutualistic symbiosis.

An iconic example of mutualistic symbiosis is between anemones and clownfish. The anemone hosts the clownfish, providing them with a place to live and protection from predators. Clownfish are considered obligate symbionts as they are never found without an anemone. In return, clownfish chase away butterflyfish and other predators that could eat the anemone.

The clownfish will swim constantly between the tentacles of the anemone, allowing the movement of prey, nutrients, oxygen, and water flow. This is at a macroscopic level, but they also help each other at a microscopic level as their microbes are strongly related (both metabolically coupled and functionally related).

In this example, either the anemone or the clownfish can be called symbionts. A symbiont can be any organism that lives in symbiosis, although the term usually refers to the smaller organism in symbiosis.

Anemone and Nemo (Sofia Mendez) marine biologist Maldives

Magnificent anemone (Heteractis magnifica)
📷 Sofia Méndez

Maldivian clownfish (Katherin Christian) marine biologist Maldives

Maldivian endemic clownfish (Amphiprion nigripes)
📷 Katherin Christian

What About Corals, Do They Live in Symbiosis?

Yes! Corals are animals that live in symbiosis with microscopic dinoflagellates, commonly known as zooxanthellae. The dinoflagellates are endosymbionts, meaning that they live inside the tissue of the corals. It is similar to humans and how we have beneficial microbes that live in our guts!

The symbiotic association between the coral and endosymbiotic dinoflagellates (Symbiodiniaceae) is central for the success of the coral. Additionally, there are other microorganisms associated with corals: Bacteria, Archea, Fungi and viruses. They play a complex role in maintaining the equilibrium between corals and its endosymbionts.

The unicellular microscopic dinoflagellates that live inside the coral tissue belong to the family Symbiodiniaceae. Multiple symbiont cells can reside within a single host cell. For our purposes, we will refer to symbionts when talking about the dinoflagellates (zooxanthellae) and coral as the host in this relationship.

Pocillopora damicornis (marine biologist Maldives)

Pocillopora damicornis
📷 Sofia Méndez

Pocillopora damicornis polyps (marine biologist Maldives)

Close-up of the brown polyps (small colourful tentacles)
📷 Sofia Méndez

Why are Endosymbiotic Dinoflagellates so Important for Coral?

Endosymbiotic dinoflagellates provide the majority (75-90%) of nutrients and energy that the corals need to stay healthy, growth and reproduce. In exchange, corals provide inorganic nutrients and a light-rich habitat. Light is important for symbionts as they have chloroplasts, like plants. Similar to plants, the symbionts use light to produce energy via photosynthesis. With the energy that they produce, they are able to share nutrients and organic compounds to their host (the coral). The symbionts also give color to the corals from dark brown to green, purple, yellow, orange, to any crazy color you can imagine. 🏳️‍🌈 How cool is that! 😎 

Diploastrea heliopora (marine biologist Maldives)

Diploastrea heliopora 📷 Sofia Méndez

Acropora digitifera (marine biologist Maldives)

Acropora digitifera 📷 Sofia Méndez

Porites lobara (marine biologist Maldives)

Porites lobara 📷 Sofia Méndez

What Is Coral Bleaching?

Corals are very sensitive to changes in their surrounding environment such as fluctuations in temperature, nutrient levels, ocean pH, and salinity. Increasing water temperatures are especially stressful to corals, which can result in the coral releasing its symbionts. Without their symbionts, they lose their main source of energy. This can lead to starvation which reduces their growth and reproduction capacity, often resulting in coral mortality. They also lose their color, causing them to reveal their ghostly white skeletons, hence the term coral bleaching!

Bleaching polyps (marine biologist Maldives)

This coral is starting to lose symbionts; some polyps are completely translucent, while others have some symbionts remaining at the base of the polyp (small brown ring). 📷 Sofia Méndez

Can Corals Recover From Bleaching?

Coral bleaching does not always result in coral mortality. When a coral is bleached, it has lost its symbionts but it is still alive. Coral polyps are translucent, causing the fully white appearance. However, corals can recover their symbionts (and their colour) if water temperatures return to their normal ranges. But at the rate ocean temperatures are rising, it can take weeks or months for the water temperatures to return to normal.

Acropora millepora bleached (marine biologist Maldives)

This coral (Acropora millepora) is bleached white but still alive. You can see the coral tiny living tentacles, and the coloured fluorescent tips (a sign of stress). 📷 Sofia Méndez

Is There Only One Type of Coral Endosymbionts?

There are many different species of coral endosymbionts! It has been difficult to identify these unicellular organisms, but luckily, scientists have been studying them for decades. At first they were categorized under one genus Symbiodinium. They had designated genetic linages called clades, that were labelled with letters A-I (for example Clade A, Clade C).
Yet as scientists investigated further, their findings revealed differences in morphology (cell size, chloroplast, and mitochondria volumes), biochemistry, physiology (photosynthetic abilities), behaviour (host preference and motility), and genetic divergence. Several new species were then described.
Now, there are seven genuses under the family Symbiodiniaceae. Clade A is Symbiodinium; Clade B is Breviolum: Clade C is Cladocopium.

But why does this matter?

Endosymbionts have many ecological differences. Some prefer different depths, temperatures, light exposure, different hosts or the same hosts in different depths. Endosymbionts can be picky when choosing a host. Some symbionts can’t exist in warm waters, while others are thermo-tolerant. As you can see in the pictures below, some corals are bleached (completely white because they have lost their symbionts) while some still have colour (their symbionts are resistant to warm waters). It is important to understand the role of symbionts, especially during bleaching events, as we can have a better idea of which coral species have the most resilient symbionts and which are more vulnerable.

Sofia Méndez

Sofia Méndez

Assistant Coral Biologist, Kuda Huraa

Sofi grew up in Guatemala surrounded by volcanoes and nature. She always wanted to be a veterinarian, until falling in love with the ocean! She pursued her dream by studying biology at Universidad de San Carlos de Guatemala, and then specialising in coral symbionts with a Master’s at the Northwest Biological Research Center. She has worked with corals from the Tropical Pacific and Hawaii, studying zooxanthellae’s genetic diversity and gene expression.
Sofi is very excited to explore Maldivian reefs, and is curious about the genetic diversity in local corals. “It’s a privilege to study corals in this region, and to contribute to Reefscapers conservation efforts. I’m happy to be the new assistant Coral Biologist at Kuda Huraa!”

I have been in Kuda Huraa for almost three months now! I’m working on several different coral projects from coral spawning, relocating frames, creating new frames, bleaching surveys at different reefs, and learning about coral larval restoration.
It’s been interesting to see how some corals, even completely bleached corals, have spawned. Also, how some corals are still doing great compared to their neighbours at the same location and depth. Every day I learn something new, and our house reef is full of surprises!
We have a great team here at Kuda Huraa, even during difficult days (such as witnessing the 4th mass coral bleaching event) you get to have fun, learn and support each other!

Coral larval restoration workshop (marine biologist Maldives)

Coral larval restoration workshop

Coral bleaching survey (marine biologist Maldives)

Coral bleaching surveys