A fluorescence hyperspectral camera mounted on an underwater remotely operated vehicle (ROV) is being developed by scientists at the University of Bristol and Sealife London Aquarium to cost-effectively monitor coral health.

Corals and the zooxanthellae contain naturally fluorescent proteins (FPs) as well as chlorophyll, which has a well-established fluorescence emission. The presence of FPs can be used as a marker of coral health and the presence of the chlorophyll-rich zooxanthellae indicates the absence of bleaching. Work reported in JSI (Journal of Spectral Imaging) offers a possible solution through the use of an underwater ROV equipped with a fluorescence hyperspectral imaging camera.

Researchers have shown that the optimum excitation for corals’ FPs is in the UV and blue (395 to 405 nm and 440 nm). This provides sufficient energy to excite corals at a reasonable distance (approximately 1 m) and high enough power to mitigate against the effects of light absorption by seawater. They propose the use of a portable, fully waterproof, low-cost hyperspectral imaging system that can be mounted on the ROV and can be driven over a reef to provide a map of the natural fluorescence.

Corals live in a symbiosis with photosynthetic algae called zooxanthellae, which benefit from nutrients and protection from the coral while providing it with energy from their photosynthesis. Coral bleaching is triggered by a number of stressors such as increased water temperature and ocean acidification. The main causes are linked with climate change.

The bleaching of corals is an indication of their poor health caused by the stress of increased water temperature and other factors. The Great Barrier Reef is best known for having undergone bleaching, but many other areas where corals live are also subject to the phenomenon.

During the process of bleaching, corals expel the zooxanthellae as a fight-or-flight-type response. The main constituents of coral color are attributed to zooxanthellae, and in their absence the coral turns white (or bleaches), which is the color of the calcium carbonate skeleton being exposed.

Detection of the early onset of coral diseases and bleaching requiring high-resolution surveys and reef monitoring is expensive. High-resolution surveys rely on divers who can cover only relatively small areas. These divers need to be experienced and may not be able to work as often as required. Larger areas can be covered using satellites but at the expense of spatial and spectral resolution. This proposed technique aims to improve on the shortcomings of both approaches and complement them.