When we humans dive underwater, we use either a snorkel or oxygen tanks to breathe. The world’s corals are not afforded that luxury.
In daylight hours, they depend on the oxygen produced by the microscopic algae that grow on corals as part of photosynthesis. At night, they have to make do with the oxygen dissolved in seawater in order to breathe.
For that purpose, corals are equipped with thousands of microscopic hairs in the form of cilia that are about 500 times thinner than a human hair. The cilia beat in unison to circulate oxygen-rich seawater across their surface, where the oxygen is absorbed. Put simply, it is the coral’s version of lungs.
Now a striking new study by an international research team led by Danish scientists shows that the growing heat waves in tropical oceans, driven by human-caused climate change, can have a tragic effect on corals’ respiratory system.
Becomes suffocated at higher temperatures
Experiments with the tropical stony coral Porites lutea in darkened test chambers have shown that if you gradually raise the water temperature from 27 to 41 degrees, the stony coral’s cilia will at first beat faster so it can breathe more quickly to match the increased metabolism triggered by rising temperatures.
But once the temperature exceeds 37 degrees, things go wrong. The coral’s cilia begin to beat more slowly and out of sync, and at 41 degrees they stop altogether. That means the coral suffocates and dies from lack of oxygen.
»We are the first in the world to discover that corals can lose their breath when seawater temperatures rise too much, because the cilia that help corals breathe are disrupted by warmer ocean water. Ultimately, that can lead to coral death«, says Professor Michael Kühl of the Marine Biological Section at the University of Copenhagen, next door to the Øresund Aquarium in Helsingør.
He is standing beside the study’s first author, Cesar Pacherres, in front of a large aquarium bathed in blue light, where, for a brief moment, you get the sense that you have been whisked off to the world’s largest coral reef, the Great Barrier Reef, off Australia’s northeastern coast – a place the two men often visit for their research.
In addition to vividly colored corals in a variety of shapes, exotic tropical fish glide through the tank and catch the eye. To keep the corals as comfortable and authentic as possible, the aquarium is filled with nutrient-poor saltwater from the Red Sea, one of the places in the world rich in tropical corals.
Cesar Pacherres points to two glowing corals that could easily be mistaken for two potato-shaped stones.
»That’s the type of stony coral we examined in our study. The species is widespread on many reefs. Come into our laboratory and I can show you how we studied its cilia and oxygen uptake«, he says.
Inside the lab, a stony coral has been placed in a test chamber bathed in blue laser light, making its tentacles – which the coral uses to catch plankton – glow green. Some fluorescent particles have been added to the saltwater. They reveal both how the water moves in swirling eddies along the coral’s surface and how much oxygen is present near the coral, because the particles change color when oxygen is present.
Maps an entirely new mechanism
The stony coral’s tentacles are not cilia. You cannot see cilia with the naked eye; you need a microscope. So Cesar Pacherres shows a video filmed with a high-speed camera through a microscope, where you can see the cilia beating in sync at a temperature of 27 degrees and beating completely out of sync at 39 degrees, when they lose their ability to circulate oxygen through the water.
»When we raise the temperature of the water in the test chamber to above 37 degrees, we can measure how the oxygen concentration drops drastically at the stony coral’s surface, causing it to gasp for air and later die«, says Cesar Pacherres, continuing.
»It’s an entirely new mechanism, and we are the first to describe it. It’s an additional explanation for why corals are stressed by higher ocean temperatures and face a greater risk of dying as global warming advances«, says Cesar Pacherres.
The Australian professor emeritus Ove Hoegh-Guldberg of the University of Queensland, who has Danish roots and is a world leader in coral research, is impressed by the new study, which has just been published in the respected scientific journal Science Advances.
»The Danish researchers and their international partners have discovered that marine heat waves do not only bleach corals. They literally suffocate them. They have also found that rising ocean temperatures trigger a vicious cycle, in which the corals’ increased oxygen consumption in response to heat leads to localized oxygen depletion, which will stress corals even more«, Ove Hoegh-Guldberg writes to Politiken in an email.
Professor emeritus Hans Ulrik Riisgård of the Department of Biology at the University of Southern Denmark calls the study »detailed, good, and serious«. But he would like to challenge the interpretation that corals beat their cilia faster in warmer seawater to meet a greater need for oxygen.
»We have documented in several scientific papers that an increase in cilia beat frequency with rising temperature is due to declining viscosity, because seawater becomes less viscous at high temperatures. The researchers behind this work did not mention that as another possible interpretation in their scientific article. In my view, that is a shortcoming«, says Hans Ulrik Riisgård.
Half of the world’s coral reefs have disappeared
The world’s coral reefs, which harbor about 25 percent of the species found in the ocean, are under pressure. Scientists estimate that our blue planet has lost about half its coral reefs since the 1950s. The main reason is that warmer seawater linked to climate change has led to global mass bleaching.
Warmer seawater causes the algae that live in close symbiosis with corals – supplying them with oxygen and energy in the form of sugar – to abandon the corals once the water passes a critical temperature. That leads to mass bleaching of corals.
»We have now discovered that corals are also being squeezed from another direction by warmer seawater, which makes it harder for them to breathe«, Michael Kühl says.
That is also one of the reasons the coral-research pioneer Ove Hoegh-Guldberg calls the study »significant and elegant«.
»What makes this study brilliant is that it shifts the spotlight onto the coral animal’s own physical plumbing system, in the form of its cilia-based ventilation system«, he writes.
Greenhouse gases are the culprit
It can be heartbreaking to watch the beautiful, colorful corals with their immense biodiversity, the ocean’s answer to rainforests decline so rapidly.
»There are climate models predicting that even if we keep global warming to one and a half degrees, we’ll still lose between 70 and 90 percent of coral reefs. So if we want to save the remaining coral reefs, which are the ocean’s oases, we as a human race have to reduce our greenhouse-gas emissions before it’s too late«, Michael Kühl says.
When Cesar Pacherres finishes analyzing the stony coral in the test chamber, he returns to the aquarium where it belongs and gently lowers it to the bottom, as a black tropical fish looks on and quickly ducks behind a coral.
It makes him sad and melancholic to think about how much pressure tropical corals are under.
»If you watch the 50-year-old TV clips from the legendary broadcasts with Jacques Cousteau, you can see how much more the seafloor was covered with corals compared with today. But it encourages me that with our research we’re trying to do something so we can preserve the corals that remain«, Cesar Pacherres says, as a coral in the aquarium waves its tentacles.
Dokumentation:Acute temperature effects on cilia beating increase coral deoxygenation
