When exposed to human noise, specimens of the seagrass Posidonia reveal permanent and severe lesions in their sensory organs—whose function is to sense gravity—that threaten their survival. This is the main conclusion of a recent study by the Laboratory of Applied Bioacoustics (LAB) of the Universitat Politècnica de Catalunya · BarcelonaTech (UPC), entitled “Seagrass Posidonia is impaired by human-generated noise”, which was published in Nature Communications Biology. These new findings demonstrate not only that plants have the physiological ability to perceive sounds, but also, and just as importantly, that commonly encountered sources of noise in the ocean can contribute to depleting their populations.
The last hundred years have seen the introduction of many sources of artificial noise in the sea environment that have been shown to negatively affect marine organisms. Many aspects of how noise and other forms of energy may critically impact the natural balance of the oceans remain unstudied. A lot of attention has been devoted to determining the sensitivity to noise of fish and marine mammals, especially cetaceans and pinnipeds, because they are known to possess hearing organs. Recent studies conducted at Laboratory of Applied Bioacoustics (LAB) of the Universitat Politècnica de Catalunya · BarcelonaTech (UPC), have also shown that cephalopods, anemones and jellyfish, while lacking similar auditory receptors, are also affected by artificial sounds. Indeed, marine invertebrates have sensory organs whose main functions allow these species to maintain equilibrium and sense gravity in the water column. But, interestingly, not a single study has yet addressed the sensitivity to noise of sessile marine organisms such as plants or coral reefs, whose immobility makes them highly susceptible to chronic effects since they also present sensory organs specialised in gravity perception that are essential to them in finding their natural substrate. As a matter of fact, Posidonia has already been made more fragile by mechanical human threats, because of the massive use of leisure boat anchors that literally uproot these unique seagrasses.
Seagrasses are considered to be the equivalent of primary forests in their ecological functions. They are higher plants that have adapted to marine environments. They develop vital ecosystems consisting of complex networks that are thousands of years old and are anchored in soft bottom areas. They have a significant effect on both biodiversity and ecosystem functions, minimising hydrodynamic forces, influencing hosted species (invertebrates and fish) and promoting microbiome and bacterial growth. Seagrasses present starch grains in their roots that function as invertebrate statocysts, which are sensory organs responsible for sensing gravity and processing sound vibration. In addition, their horizontal stems, or rhizomes, which act as storage organs, provide a considerable amount of starch grains, a guarantee of energy provision to the plants.
This study, led by Dr Marta Solé, a senior researcher at the LAB-UPC, reports morphological and ultrastructural changes in seagrass after exposure to sounds in a controlled environment. These changes are new in aquatic plant pathology. Not only did low-frequency sounds produce alterations in the root and rhizome statocysts of Posidonia oceanica, they also affected the plants’ nutritional processes by decreasing the number of rhizome starch grains. In addition, degradation in the specific fungal symbionts of P. oceanica roots was observed. Fungus improves the nutrient status of the plant (e.g. mineral nutrition, water absorption) in exchange for carbon provided by Posidonia, which is necessary for fungal growth and reproduction.
Loss of biodiversity.
This sensitivity to artificial sounds revealed how sound can potentially affect the health status of P. oceanica. Moreover, these findings address the question of how much the increase of ocean noise pollution may contribute in the future to the depletion of seagrass populations and to biodiversity loss.
The LAB of the UPC is part of the Vilanova i la Geltrú Campus. The LAB is internationally recognised for its leading research on the effects of ocean noise pollution. The LAB’s director, Professor Michel André, a 2002 laureate of the Rolex Awards for Enterprise, created the Sense of Silence Foundation, in 2014, a charity that deploys passive acoustic technology to contribute to the sustainable cohabitation of humans and nature by giving back an acoustic balance to nature.