Are corals in trouble?
What attracts the whale sharks to Ningaloo are the coral reefs close to shore. But corals don’t just inhabit shallow waters - in some places on Earth they live at much greater depths, although it was never clear how they survive hundreds of metres down. Now a new paper announced this week at the Royal Society Summer Science Exhibition in London, reveals the answer. From his exhibit at the Royal Society, Southampton University’s Jörg Wiedenmann is with us…
Jörg - Corals are animals and it’s a colony of animals. You have multiple polyps and they live together in various steps in coral reefs. They host little outer cells inside of their tissue and since they're attached to the rocks, they behave a little bit like plants, so they need light for their survival.
Chris - Okay, so we have an animal which has an algal cell which is a kind of plant living with it, one feeding the other. Where does the hard rocky bit of the coral come from?
Jörg - So that is secreted by the animals and they deposit it underneath it, so they sit on top of the skeleton. But the outer part now, they help potentially also with that by facilitating the position of the limestone material.
Chris - Why are people concerned that corals are in jeopardy around the world – these sort of shallow dwelling corals ? On the Barrier Reef around Australia for example, we hear a lot about that. Why?
Jörg - So, when a sea surface temperatures rise, for the synthetic algae, they become malfunctioning and then the symbiotic relationship breaks down and the outer cells are lost from the animal. As a consequence of that, their white skeleton shines through the animal tissue and that gives the coral this ghostly white appearance. This is why this phenomenon is called ‘coral bleaching’. When the corals don’t recover their outer symbionts then they usually die sooner or later.
Chris - But this is the surface dwelling corals. The ones that live deeper are not so much under threat. Is that right?
Jörg - Well, it’s not just the temperature that causes bleaching. It’s multifactorial while temperature is a major driver. Also, light plays a critical role, and also the nutrient content in the water. and so, there is a hypothesis out there that corals in deeper water may actually be more protected from this bleaching because while they might be exposed to temperature stress, the light stress certainly will be lower.
Chris - Now, if the coral relies on exposure to sunlight because those algal cells – the plant type cells that live alongside the coral creatures and feed them by capturing sunlight, and doing photosynthesis – if they depend on light to do that, surely, as you go deeper down in the water, so they're kinds of deep water corals that you’ve been studying down a hundred metres down or so, they don’t have much light down there. So, how do they get enough light for their algal cells to feed the coral?
Jörg - So, this is what the paper was about. We looked at corals that have a particular type of orange for us and pigments and we find that in corals that live in depths between 20 up to 18 metres. These pigments, they take up the blue light that is prevalent at this greater water depths, and they convert it into orange light. Interestingly, this orange light then can travel deeper into the coral colony and make sure that the symbiotic algae in deeper tissue layers can actually photosynthesise despite being exposed to relatively low light levels.
Chris - Gosh! That’s a genius. So the deep water corals are making a chemical that converts the blue light that is present in the water at depth into an orange colour that can drive the algae, make them photosynthesise.
Jörg - Exactly. That’s the mechanism.
Chris - That means that the deeper water corals are doing something quite different from the surface corals then. So, some people have suggested, well, won't it be okay with climate change and rising water temperature because the ones that live near the surface could just migrate to deeper waters. The fact that these deep dwelling corals have evolved this very special way of living would argue that actually, corals couldn’t just move deeper to get away from climate change.
Jörg - Yeah. That is one of the concerns that arises from this study because it shows how sophisticated this symbiosis is, how well adapted they are to the life in this greater depth. And also, previous studies have found that other corals, they use different types of symbiotic algae to adjust to life in greater depth or some of the others change their growth morphology, so they become very broad and form some sorts of light collectors. So, they respond in various different ways, and not all shallow water corals will be able to do the same.