Corals trap light for algal lodgers

Corals have struck up a remarkable partnership with minute algae, resulting in some of the most dazzlingly beautiful organisms on the planet. They provide the photosynthesising algal lodgers – which reside sandwiched in the thin layer of animal tissue covering a calcium carbonate skeleton – with shelter and a ready supply of nutritious waste products, which can be scarce in the barren waters surrounding most coral reefs. The algae, in turn, provide their hosts with sugars and oxygen. But there is one more essential factor that the animal host must provide for algae lodging within its tissues, and that is light. Daniel Wangpraseurt and his supervisor Michael Kühl from the University of Technology Sydney, Australia, explain that plant leaves actively trap and redistribute light to improve photosynthesis. Could corals do the same for their algae? Teaming up with Anthony Larkum, Jim Franklin, Milan Szabo and Peter Ralph, the duo set about finding out how light travels through coral tissue (p. 489).

‘We have a research station on the Great Barrier Reef – Heron Island Research Station – and that is where we go to collect corals’, says Wangpraseurt, who brought three types of brain coral back to Sydney where he could begin to investigate them more closely. By shining four different colours of laser light (near infrared, 785 nm; red, 636 nm; green, 532 nm; and violet 405 nm) onto the coral samples, Wangpraseurt measured the amount of light transmitted through the animal to different depths when he gently inserted a minute light sensor into the delicate tissue. He moved the sensor with microscopic precision in 100 μm steps down into the tissue and repeated the measurements through the coral at distances ranging from 2 mm to 2 cm from the light beam. However, working with the corals was far from easy. Wangpraseurt says ‘Once the coral moves its tissue the data cannot be used, so you have to redo the measurement,’ and he adds, chuckling, ‘People that work with plant leaves have it much easier’.

Next the team investigated how red and near-infrared light travel through the coral tissue, and they were pleased to see that red light, which is absorbed by the algal photopigments, was trapped and horizontally transported through the tissue. However, near-infrared light, which is not used by the algae, was mainly reflected back into the animal tissue by the supporting calcium carbonate skeleton below. And when the team analysed how blue and violet light moved through the coral tissue, they found that it was also transported horizontally, increasing the amount of light carried through the tissue to fluorescent pigments in the coral.

But would the captured light improve algal photosynthesis? The team flicked the red laser beam on and off and found that the pulse of light increased the algae's oxygen production by more than 10% at a distance of 6 mm from the laser beam, so horizontal light transfer through the coral tissue can increase the symbiont's photosynthesis.

Having discovered that the thin layer of coral tissue is capable of augmenting the light field that it supplies to its algal lodgers, the team is keen to build a more detailed understanding of the optical properties of each layer of the coral's tissue to find out exactly how they propagate light. And Wangpraseurt adds that corals are not simply static. He says, ‘It seems likely that the animal is able to dynamically modulate the light field’, explaining that they contract to shield the algae in bright light and expand to improve light provision in dim conditions – so the team hopes to found out how much the animal can modify the light field it provides for its algae.