Bacteria give coral orange glow

The coral glows orange in the daytime: the colony is about 0.6 m high

Orange coral commonly found in the warm Caribbean waters is not naturally coloured that way, but has bacteria to thank for its hue, say scientists.

In return for a home in the Caribbean great star coral, the bacteria help convert nitrogen in seawater to feed it, US researchers report in Science.

Existing seawater nitrogen is in a form which the coral cannot use, the bacteria converts it into ammonia.

The fluorescent coral - Montastraea cavernosa - grows in seabed colonies.

Helping hand

The colonies often look like large rocks and usually glow in the blue colour of the water. This typical of algae known as zooxanthellae found co-existing with coral formations.

But the coral can also give off an orangey colour during the day, which comes from pigments that fluoresce, a team of researchers led by Michael Lesser from the University of New Hampshire has found.

The team took fluorescent and non-fluorescent samples of the coral at depths ranging from 15 to 30m in the waters around Lee Stocking Island in the Bahamas.

A spectrofluorometer was used to measure the fluorescence emission spectra within six hours of collection.

The team discovered that one of the fluorescent pigments in the coral is not part of the coral.

The cyanobacteria, blue green algae, absorbs light in one colour, blue, then gives it out in a different hue, orange, the researchers say.

Professor Lesser says the system in which the coral, bacteria and algae live is a symbiotic relationship, one that helps them all.

The nitrogen converted by the cyanobacteria can also help the zooxanthellae, which also gives the cyanobacteria a source of important energy.