Gulf coral study finds bacteria shield reefs from heat

A Gulf coral study has found that bacteria living inside corals in the Arabian Gulf produce steroid hormones that help the animals cope with extreme heat. The work, published in Nature Microbiology, examined Endozoicomonas, a group of bacteria found with corals around the world, and its effect on the immune response of corals under thermal stress.

The Arabian Gulf is one of the hottest seas on Earth, with surface water regularly exceeding 36°C in summer. Corals there live close to limits that would damage reefs elsewhere, which is why researchers chose the region to study heat tolerance.

Heat stress damages corals in part by disrupting the partnership with algae living in their tissues. When stressed, corals can expel those algae, lose colour and starve if temperatures stay high. Disease often follows, which is why immune regulation during a heatwave matters for survival.

The team studied Acropora pharaonis, a reef-building coral, at Saadiyat Reef off Abu Dhabi. Colonies were monitored over six months and sampled in June, when water was 32°C, in August at 34°C, and in October at 27°C, as conditions moved from early stress through severe stress to recovery. Some colonies showed early signs of white syndrome as the water warmed.

When temperatures rise, corals lose part of their ability to produce steroid hormones that help regulate the immune system, according to first author Amin Mohamed, a research associate at New York University Abu Dhabi. The bacterial partners make up the shortfall, the researchers found, producing the hormones themselves so the coral can mount an immune response that helps its cells survive heat.

Senior author Shady Amin, an associate professor of biology at the university, said the bacteria do more than live alongside the coral. In a statement, he described them as supplying molecules the coral needs to regulate its immune response while under significant stress.

How the bacteria step in when heat rises

Steroid hormone production is usually linked with complex organisms. Only a limited number of bacteria had previously been shown to produce these molecules, including bacteria in the human gut. The finding that coral bacteria can produce and pass on the hormones reveals a function of the coral microbiome that had not been documented before.

The team combined field sampling with laboratory experiments. Genomic and metabolomic analysis identified the bacterial genes and the hormones involved. Most steroids measured in the coral fell as temperatures rose. Progesterone and testosterone were exceptions, staying elevated at the highest temperature sampled and at disease lesion sites.

In laboratory cultures, isolates of Endozoicomonas collected from hosts in the Great Barrier Reef, Taiwan and Abu Dhabi converted cholesterol into testosterone and progesterone. In some strains, the conversion was temperature dependent, switching on at higher temperatures.

Across a pangenome of 29 Endozoicomonas genomes, 66 per cent carried the genes for the relevant steroid metabolism. The pattern suggests the capacity is widespread in the group rather than confined to a single strain.

To test the effect directly, the team exposed fragments of Acropora downingi to testosterone or progesterone at a dose of 10 nM and then raised the temperature above the local summer maximum. Gene expression across immune-related pathways shifted in response, according to the study, with some regulators of the immune response increasing and others falling.

The shifts did not last. Within a day of heat exposure beginning, most of the steroid-linked gene signatures had returned to baseline, the researchers reported. The result points to a short-lived adjustment during acute stress rather than a lasting reprogramming of the coral immune system, and it does not by itself show that bacterial hormone production keeps reefs alive in the field.

What the finding could mean for reef conservation

Coral conservation has largely centred on the coral animal itself, through coral nurseries and assisted evolution. The new work suggests beneficial microorganisms could also have a part to play as ocean temperatures keep rising, according to the researchers.

The research was conducted through New York University Abu Dhabi's Center for Genomics and Systems Biology, in collaboration with John Burt and the Marine Microbiomics Lab. The full paper, titled "Steroid hormones produced by coral-associated Endozoicomonas influence coral immune response during thermal stress", is published in Nature Microbiology.

The Arabian Gulf corals in the study already live in water hotter than most reefs tolerate. Understanding how their bacterial partners help them through heat may help explain why some corals endure conditions that damage others, according to the researchers.