We often imagine the world beneath the waves to be a silent place. In our imagination, this silence is broken at most by the doleful song of a lonesome whale or by sounds from outside, muffled by the water. A few may have heard that fish also make use of acoustic signals. However the fact that even algae contribute to the marine soundscape is new even to experts: in 2018, Lauren and Simon Freeman, two oceanographers working with the US Naval Undersea Warfare Center in Rhode Island, first noticed the phenomenon in the waters of Hawaii. They recorded a faint hiss that had long been dismissed as caused by snapping shrimp. However, they observed that the amount of sound correlated with the quantity of macroalgae covering the coral reefs. In order to verify whether the sound was caused by the algae, they set up a tank with controlled conditions and their presumption was confirmed. The sound is caused by the algae performing photosynthesis — as a byproduct, they release oxygen. When an oxygen bubble detaches from he surface of the plant, it creates a short ʻpingʼ sound. The combined pings of many algae merge to form an important part the background noise of their environment. The two oceanographers hope that this finding can open up new opportunities to evaluate and monitor ecosystems through acoustic recordings in the future.
Sources: Freeman, Simon E. et al., “Photosynthesis by marine algae produces sound, contributing to the daytime soundscape on coral reefs”, PLoS ONE, vol.13, issue. 10, 2018.