Impact of micro-and nanoplastic contamination on reef-building corals

Hermatypic corals form the foundation of tropical reefs, which are considered biodiversity hotspots. The threats they face related to climate change have been extensively studied. However, new threats are emerging, particularly those posed by plastics. Their effects on coral species remain controversial and vary depending on the species studied, the type of plastics, and their size. This is why we focused on two species of tropical corals, Stylophora pistillata and Turbinaria reniformis, in the presence of different particles: micro and nanoplastics. We specifically examined the algal-coral symbiosis through the physiology of the corals and the photosynthetic capacities of their symbionts.

The results show that both the coral species and the size of the plastic particles play a significant role in the observed changes in parameters. Microplastics cause more pronounced effects, particularly on the symbiosis of S. pistillata, leading to the loss of dinoflagellates, a phenomenon known as bleaching. This results from the disruption of photosynthetic activity (decreased photosystem II activity, reduced chlorophyll content, etc.). The metabolism of the corals is also impacted, with a significant decrease in energy reserves (proteins, carbohydrates). Long-term exposure further exacerbates the observed deleterious effects. While nanoplastics seem to show fewer effects on the tested individuals, it is important to consider the concentration of particles used (2.5 x 10¹² particles L⁻¹), which, although higher than that of microplastics (1 x 10⁵ particles L⁻¹), remains lower compared to the actual fragmentation rate of microplastics into nanoplastics. It is crucial, through new studies, to test the effects of higher concentrations that are closer to environmental levels, in order to better understand the harmful effects that such small particles could have.

 

Hermatypic corals are the structural basis of tropical reefs, ecosystems known for their high biodiversity. While the effects of climate change on coral reefs are well documented, there are also newer threats, particularly from plastic pollution. The impact of plastics on corals remains controversial and varies by coral species, plastic type and particle size. This study investigates the effects of micro- and nanoplastics (1 × 10⁵ particles L^-¹ and 2.5 × 10¹² particles L^-¹ for micro-and nanoplastic respectively) on two tropical coral species, Stylophora pistillata and Turbinaria reniformis. We assessed the integrity of the coral-algal symbiosis by analyzing the physiology of the corals and the photosynthetic performance of their algal symbionts.

 

Our results show that both the coral species and the size of the plastic particles significantly influence the physiological responses. Microplastics caused greater effects than nanoplastics, particularly in S. pistillata, which suffered symbiont loss or "bleaching" following a decrease in photosystem II efficiency or in chlorophyll content. The coral metabolism was also impaired, as evidenced by reduced energy reserves (proteins, carbohydrates). Prolonged exposure exacerbated these effects. Although nanoplastics had less noticeable effects on the corals, the particle concentration used in this study is still below environmentally relevant levels resulting from the fragmentation rates of microplastics. Further research at higher nanoplastic concentrations is essential to assess the full extent of the impact of these small particles on coral health.