Gas bubbles released by marine algae during photosynthesis produce a distinct “ping” – a discovery that could aid scientists monitoring the health of ocean ecosystems.
New research suggests that the intensity of the ping correlates with the amount of algal cover on nearby coral reefs. This opens the possibility of creating a quick and easy way to monitor algae density, which is one of the strongest indicators of reef stress.
Currently this is done by visual inspection, which is time-consuming and expensive.
The study, published in the journal PLOS ONE, was led by husband-and-wife team Simon and Lauren Freeman, from the US Naval Undersea Warfare Centre in Rhode Island.
Coral reefs are actually quite noisy places, with sounds emanating from water movement, ocean creatures and other sources.
A previous study by the Freemans identified a component of the underwater soundscape that correlated with the abundance of macroalgae – comprising species that form leaf-like structures and filaments – present over the reef, but the mechanism responsible for the sound was unknown.
To determine whether the algae itself was responsible, the authors studied sound emissions from a tank containing the Hawaiian invasive species, Salicornia gracilaria.
They found that the release of gas bubbles – a combination of oxygen produced by photosynthesis, and nitrogen – resulted in a “ringing” sound from the oscillations of the bubbles as they assumed a spherical shape in the water. The sound ranged from two to 20 kiloHertz, matching that from oceanic coral reefs.
Bubble production and sound emission is not unique to this algal species, the researchers say, and so the measurement of the sound in marine environments may be a proxy for algal and aquatic plant photosynthetic activity in general.
Things are still in the early stages, however, and the Freemans note in their paper that “a great deal of further work is required before a passive acoustic tool could be developed for quantifying bubble output from photosynthesis, in-situ primary productivity and algal abundance”.
In fine-tuning the system, a number of confounding factors need to be considered, including the influence of water movement on bubble formation and retention — and some annoyingly intrusive crustaceans.
“Nevertheless, before accurate quantification of algal photosynthetic activity can be attempted in the field, the behaviour and acoustic characteristics of snapping shrimp in the area must be understood to create validated acoustic signal processing algorithms capable of differentiating between shrimp noise and algal sound,” the authors write.