Warming seas impact cell structures that deliver oxygen to coral

High temperatures have been shown to disrupt the function of cellular structures that deliver oxygen to corals. The finding reveals a new stress mechanism linking warming seas to coral death.

Corals play a vital role beyond the ocean ecosystem, but they are disappearing quickly as climate change progresses. Now, scientists seeking to understand the biological mechanisms behind coral stress have shown how rising temperatures affect tiny structures called cilia that coat the surface of cells found in the outer layer of coral tissue.

Cilia are threadlike structures, around 10–15 micrometres in length and 500 times thinner than a human hair. They beat in a synchronous wave-like rhythm, moving trapped particles in the coral’s mucus to clean the surface and deliver food to the mouth opening. They also create local flows of oxygen-rich water around the coral’s surface.

A team led by University of Copenhagen researchers Cesar Pacherres and Michael Kühl was particularly interested in this area of flow, called the concentration boundary layer. They released particles around 500 nanometres in size and coated in an oxygen sensitive dye into the water surrounding the coral and shined a laser on the coral to illuminate the particles. The dye used glows brighter depending on how much oxygen is present.

They captured images with a camera attached to a long-distance microscope and analysed them to determine the movement and concentration of oxygen at the coral surface. Combined with high-speed footage of the cilia beating, the team was able to explore the relationship between movement and oxygen uptake at various temperatures.

It’s a method developed specifically for this study. ‘We had to come up with new ideas and new ways of visualising things because we are talking [about] very, very small scales, and the techniques were non-existent,’ says Pacherres. ‘It was a combined effort of a couple of years at least,’ he adds.

The team found that under moderate temperature increases the cilia beat more frequently, allowing the coral to keep up with increased metabolic oxygen demands. But at higher temperatures, the metabolic demand for oxygen exceeded the rate at which the cilia could generate sufficient oxygen flow, and the boundary layer progressively lost oxygen.

At 37°C the cilia rhythm collapsed, and the oxygen supply to the corals dropped. ‘At that point the coral is in really bad shape … we could see tissue disintegrating,’ says Kühl.

Early warning

Until now, one of the most reliable indicators of coral stress has been coral bleaching. This is when the coral ejects the colourful algae living in its tissues, and which it relies on for energy. After a prolonged period without the algae, the coral will starve. However, bleaching typically occurs during periods of prolonged stress. The new findings show that a different mechanism of stress that can occur much earlier than bleaching, and much more suddenly.

Ilsa Kuffner, a marine biologist at the US Geological Survey, has seen the effect of sudden temperature spikes in her own work on elkhorn and staghorn corals in the Caribbean. Once assumed to be moderately sensitive to bleaching, in the last five to ten years she has noticed a sudden shift in the species’ resilience.

‘When you get to these really high temperatures, then you reach a different level that could have a completely different process, and this [research] puts a mechanism forward that explains some of the observations that we’ve seen,’ she says.

What’s more, she’s seen it occurring at temperatures much lower than those reported in the study. ‘They talk about temperature thresholds of 37°C, but in the field that could be even lower, like when the wind dies and there are doldrum conditions … we are seeing, for some coral species, these acute-stress effects at 32–33°C,’ she says.

The findings could help inform decision makers on where to focus conservation efforts. Lucy Gorman is a coral biologist at the Marine Biological Association in the UK who studies stress in corals. ‘It gets you thinking about currents and water movements,’ she says. ‘If the [corals are] in a place where the water movement is a lot higher … maybe they get saturated with oxygen anyway, and can survive the higher temperatures a little bit better.’

Pacherres and Kühl hope that their findings could provide an early warning sign not just for corals, but other marine species as well. ‘Many other animals have cilia and use cilia to create feeding currents or oxygenate themselves,’ says Kühl. ‘[So] this kind of mechanism could also be relevant for other organisms in other ecosystems.’