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Scientists listen to the sound of photosynthesising seaweeds

NIWA scientists are hoping they may one day be able to “listen” to kelp forests in the waters around New Zealand to find out how they are faring.

Dr Giacomo Giorli, a NIWA marine acoustician, is one of the authors of a recently published scientific paper that identified the sound generated as a by-product of photosynthesis in marine macroalgae or seaweed.

The study showed how gas bubbles released by seaweeds during photosynthesis produce sounds which correlate to the amount of oxygen released.

“Declining algal cover is one of the major indicators of stress in coral reef ecosystems. Our observation suggests that monitoring the sounds generated by the algal photosynthesis could be potentially used to estimate the amount of algal cover and ecosystem health in coral reefs. “

Dr Giorli’s and his colleagues’ research at The Hawaii Institute of Marine Biology involved collecting a large amount of the algae Gracilaria salicornia and placing it in an experimental tank equipped with an aquarium light.
The algae were exposed to cycles of dark and light and a hydrophone was used to measure the sound in the tank while a high-resolution video camera recorded the bubbles and measured their size.

“Bubbles form on the algal tissue when oxygen is produced by the alga. The bubbles grow with the addition of more oxygen and then float towards the water surface,“ Dr Giorli said.

The sound comes from the tiny movements of the bubbles as they form a spherical shape in the water.

“During release, relaxation of the bubble to a spherical shape creates a sound source that ‘rings’,” the scientists found. “The bigger the bubble, the lower the sound. The whole process was reversed when we turned the light off.”
While measuring the sound produced by the bubbles could be used as a cheap alternative to estimate algal cover and ecosystem health in coral reefs, NIWA scientists say there is also potential to use it to learn more about New Zealand kelp forests.

NIWA marine biologist Dr Roberta D’Archino says the kelp forests are in decline overseas but little baseline data exist about the size and distribution of these forests in New Zealand.

“Some kelp species are very sensitive to increasing temperatures – we know for instance that the giant kelp Macrocystis pyrifera has decreased up to 90% in Tasmania, and that the range of kelps in Western Australia retracted more than 100km after an extreme heat event. However, while there is some monitoring going on in New Zealand, we don’t really know much about how they are being affected on a large scale.

“Kelp provide important habitats for fish, rock lobsters, paua and other species. We have been exploring different ways to map the forests, and if we could work out a way to link oxygen production to the kelp biomass it would be very exciting.”

The paper was written by Dr Giorli together with Simon and Lauren Freeman from the US Naval Undersea Warfare Center, and Andreas F. Haas from the Royal Netherlands Institute for Sea Research.

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