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 Figure 2. Relationship between algae photosynthesis and passive acoustic bubble detection. A: relationship between time of day, bubble detection rate, and bubble frequency. B: relationship between measured sound exposure level (SEL) of bubble pulses and dissolved oxygen level in tank. Reproduced from Freeman et al., 2018.
the alga Gracilaria salicornia showed that photosynthesis causes bubble oscillations between 3 and 35 kHz, with the cumulative sound energy (sound exposure level) tracking the rising and falling of dissolved oxygen levels across day- time and nighttime light levels (Figure 2).
Interestingly, Freeman et al. (2018) found that field measure- ments of daytime ambient noise in this same frequency band appear to correlate with the percentage of algal cover on coral reefs, independent of other reef-based bioacoustic processes. Algal dominance is a key indicator of coral reef ecosystems stress because warmer waters, polluting nutrients from terres- trial runoff, and the removal of herbivorous fishes for human consumption all promote algal growth. These observations suggest that it may be possible to quantify the degree of reef degradation through passive acoustic monitoring.
Beatles and Beetles: Do Plants Respond to Sound or Other Vibration?
Despite casual appearances, evidence abounds that vascu- lar plants respond to changes in environmental conditions. Flowers open during the day and close at night, roots grow in the direction of moisture, and leaves generate natural repel- lents when an insect chews on them. Plants even respond to the actions of other plants via communication by direct touch, light, and chemical compounds. For example, plants near- drought-stressed plants will respond by closing the stomata in their leaves so as not to lose more water.
It is not surprising, therefore, that many have wondered whether plants respond to sound or mechanical vibration
over both short- and long-term scales despite their lack of clear sensory mechanisms for detecting it. Trees do change growth patterns in response to low-frequency mechani- cal vibration. For example, scientific studies of the effect of wind on tree growth go back to 1803, and the impressive (and vaguely Dr. Seussian) term “thigmomorphogenesis” was coined in 1973 to “describe the response of plants to wind and other mechanical perturbations, including mechanical bending or flexing or by touching or brushing by passing animals” (Telewski, 2006, p. 1468). If plants could change growth patterns in response to low-frequency perturbations, why not sound?
The earliest peer-reviewed research on the topic from the 1950s and 1960s seemed focused on how music playbacks influenced plant yields (Klein and Edsall, 1965). There seemed to be little attempt to standardize procedures for music selection, and one can find literature results for Karnatic music (a form of Indian classical music), Gershwin’s “Rhapsody in Blue,” wed- ding music, Gregorian chant, and the Beatles (specifically, “I Want to Hold your Hand”). There was also little-to-no attempt to standardize species selection, playback duty cycle, source level, or other playback variables, and even definitions of plant response were inconsistent. Given the lack of controls and standardization, one would probably not be shocked to real- ize that the reported responses were all over the map (e.g., “we could not observe any stem nutation in plants exposed to the Beatles”; Klein and Edsall, 1965) and generally irreproducible. Work on the topic dropped into the realms of urban folklore and middle-school science fair projects. Highly questionable claims by a 1973 bestselling book, The Secret Lives of Plants, cast a miasma over the entire topic for decades.
A series of papers by Weinberger and various coauthors (e.g., Weinberger and Burton, 1981) seem to be among the first to eschew music in favor of single-frequency tones and other
simple reproducible signals to study plant response. The con- tinuing advent of low-cost electronically customizable and reproducible sound playbacks seems to have led to a spurt of credible research on the topic, with more rigorous controls, beginning in the twenty-first century (Jung et al., 2018).
Several papers have attracted particular attention. Appel and Cocroft (2014) published observations that the play- back of caterpillar feeding sounds (think “munching”) in the absence of caterpillars led rosette plants to release higher levels of chemical defenses (glucosinolates and anthocyanins) once actual caterpillars started to feed on
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