Sylvie Marylène Gaudron

Giant sulfur-oxidising bacteria belonging to the Beggiatoaceae family are well known to occur in cold and temperate waters, as well as at hydrothermal vents, where they form dense mats on the seafloor. Moreover, they have been found in mud volcanoes, shallow water, and tropical marine mangroves, often linked to sulfur cycling and organic matter degradation. Here, we report for the first time the presence of extensive white mats in Montevideo Canyon at a depth of ~1738–1748 m, along with a unique chemosymbiotic fauna, highlighting the existence of a novel deep-sea ecosystem in the Southwestern Atlantic with specialized microbial and molluscan communities adapted to this extreme environment. Mat bacteria were identified as belonging to Candidatus Isobeggiatoa.

Aggregation behaviour is a key process shared by numerous organisms, providing benefits such as predator protection, resistance to abiotic stressors and enhancing reproductive success. In coastal waters, mussel aggregations shape ecosystems and increase biodiversity; however, many aspects of this ecologically key behaviour remain unexplored. Here, we investigated the potential link between aggregation rate and reproductive maturity stages in the blue mussel Mytilus edulis over a seasonal cycle. We showed that the seasonality of M. edulis aggregation rate was a significant sinusoidal function of the mean reproductive maturity stage. Specifically, aggregation increased during gamete maturation until the onset of spawning and subsequently declined, following a cyclical pattern, supporting a reproductive function of this aggregation behaviour in mussels. Given the ubiquity of plastics as marine contaminants and their known harmful effects on organisms, we subsequently assessed how the temporal patterns observed in aggregation behaviour may be impacted by an exposure to leachates from plastic pellets. Thus, an exposure to plastic leachates led to a loss of the cyclical pattern observed in control seawater, potentially compromising mussel reproduction and the stability of mussel bed habitat. Our study showed the relationship between aggregation behaviour, reproduction and contaminants in mussels. Understanding these complex interactions is crucial, given the pivotal role mussels and their aggregations play in shaping marine ecosystems, offering new insights into the resilience of this habitat facing contemporary challenges.

Plastic contamination has become a major scientific and societal concern. While research on this issue has been flourishing over the last decade, several aspects remain insufficiently explored, including the chemical impact of plastic leachates (i.e., a complex mix of chemical compounds released by plastic particles). Furthermore, essential biological factors, such as the reproductive cycle and sex, are still largely overlooked, particularly in marine invertebrates, further limiting our ability to fully assess the ecotoxicological risks posed by pollutants. Here, we investigate the influence of these biological factors on organismal sensitivity to microplastic leachates (MPL) using two mussel species, Mytilus edulis and Mytilus galloprovincialis, which are important ecosystem engineers with significant economic values. The role of reproductive cycle was assessed in M. edulis through aggregation assays, while sex-based differences in sensitivity were assessed in M. galloprovincialis through byssal thread production assays. M. edulis showed increased sensitivity to MPL post-spawning, through a delay in their time to form their first aggregation, while female M. galloprovincialis showed higher sensitivity to MPL through increased byssal thread production. The observed effects may result from the high energetic cost of reproduction, coupled with potential endocrine-disrupting effects of plastic additives, highlighting the need for further studies. Our work stresses the importance of incorporating biological factors, such as the reproductive cycle and sex, into pollution risk assessments to enhance their ecological relevance.

Plastic pollution and ongoing climatic changes exert considerable pressure on coastal ecosystems. Unravelling the combined effects of these two threats is essential to management and conservation actions to reduce the overall environmental risks. We assessed the capacity of a coastal ecosystem engineer, the blue musselMytilus edulis, to cope with various levels of aerial heat stress (20, 25, 30 and 35°C) after an exposure to substances leached from beached and virgin low-density polyethylene pellets. Our results revealed a significant interaction between temperature and plastic leachates on mussel survival rates. Specifically, microplastic leachates had no effect on mussel survival at 20, 25 and 30°C. In turn, mussel survival rates significantly decreased at 35°C, and this decrease was even more significant following an exposure to leachates from beached pellets; these pellets had a higher concentration of additives compared to the virgin ones, potentially causing a bioenergetic imbalance. Our results stress the importance of adopting integrated approaches combining the effects of multiple environmental threats on key marine species to understand and mitigate their potential synergistic effects on ecosystem dynamics and resilience in the face of the changing environment.

Hydrothermal vents are places where seawater exits cracks in the sea floor, having been super-heated and enriched with metals and minerals deep in the underlying bedrock. They are an example of an ecosystem based on chemosynthesis, where life is sustained by energy from chemicals rather than energy from sunlight. The discovery of an abundance of life around deep-sea hydrothermal vents emitting hot and toxic fluids demonstrated that animals and other organisms could thrive in the dark, cold and high-pressure deep oceans. Mussels are among the most studied animals found near hydrothermal vents. Scientists discovered that mussels rely on a close, living relationship—a “symbiosis”—with bacteria for their nutrition. In this symbiosis, bacteria use chemicals from the hydrothermal fluid and seawater to produce organic compounds, while the mussels provide the bacteria with essential compounds and protection. The mussel life cycle is uniquely adapted to finding and colonizing their unusual habitat and then finding suitable symbiotic bacteria, almost immediately. Despite its remoteness, the deep sea is already under threat. Although there is still much work to be done, research into mussels and other animals that have evolved similar symbioses has revealed not only their beauty, but also their fragility.