Gas bubbles released by marine algae during photosynthesis produce sound whose intensity correlates with the degree of algal cover on coral reefs, according to a study published October 3 in the open-access journal PLOS ONE by Simon and Lauren Freeman, a husband-and-wife team of the US Naval Undersea Warfare Center and colleagues. The finding may allow a simple and rapid way to measure algal cover, which is one of the strongest indicators of stress on coral reefs.