Microplastics do not affect bleaching of Acropora cervicornis at ambient or elevated temperatures

Abstract

Microplastic pollution can harm organisms and ecosystems such as coral reefs. Corals are important habitat-forming organisms that are sensitive to environmental conditions and have been declining due to stressors associated with climate change. Despite their ecological importance, it is unclear how corals may be affected by microplastics or if there are synergistic effects with rising ocean temperatures. To address this research gap, we experimentally examined the combined effects of environmentally relevant microplastic concentrations (i.e., the global average) and elevated temperatures on bleaching of the threatened Caribbean coral, Acropora cervicornis. In a controlled laboratory setting, we exposed coral fragments to orthogonally crossed treatment levels of low-density polyethylene microplastic beads (0 and 11.8 particles L−1) and water temperatures (ambient at 28 °C and elevated at 32 °C). Zooxanthellae densities were quantified after the 17-day experiment to measure the bleaching response. Regardless of microplastic treatment level, corals in the elevated temperature treatment were visibly bleached and necrotic (i.e., significant negative effect on zooxanthellae density) while those exposed to ambient temperature remained healthy. Thus, our study successfully elicited the expected bleaching response to a high-water temperature. However, we did not observe significant effects of microplastics at either individual (ambient temperature) or combined levels (elevated temperature). Although elevated temperatures remain a larger threat to corals, responses to microplastics are complex and may vary based on focal organisms or on plastic conditions (e.g., concentration, size, shape). Our findings add to a small but growing body of research on the effects of microplastics on corals, but further work is warranted in this emerging field to fully understand how sensitive ecosystems are affected by this pollutant.

Exposure to global change and microplastics elicits an immune response in an endangered coral

“…In fact, overall gene expression profiles were indistinguishable between the ambient and microplastics treatments, and an effect of microplastics was only apparent when looking at the expression of top 1,000 genes. This result corroborates previous work reporting negligible effects of microplastics on coral physiology, bleaching susceptibility, and mortality (Reichert et al, 2021; Plafcan and Stallings, 2022) and supports the hypothesis that corals that rely more heavily on heterotrophy may be more impacted by this stressor.…”

“…); however, it is more likely due to species-specific responses (Hankins et al, 2021; Mendrik et al, 2021). Previous work on A. cervicornis suggests that exposure to microplastics in the presence of warming does not impact bleaching, likely because this species may not ingest the microplastics under otherwise ideal conditions (Plafcan and Stallings, 2022). Instead, A. cervicornis relies on photosynthetically-derived carbon from its algal symbionts as the primary source of nutrition instead of heterotrophy, thus avoiding the potential pitfalls of microplastics ingestion observed in other aggressively feeding coral species (Reichert et al, 2019; Rotjan et al, 2019; Hankins et al, 2021; Mendrik et al, 2021).…”

Exposure to global change and microplastics elicits an immune response in an endangered coral

“…In fact, overall gene expression profiles were indistinguishable between the ambient and microplastics treatments, and an effect of microplastics was only apparent when looking at the expression of top 1,000 genes. This result corroborates previous work reporting negligible effects of microplastics on coral physiology, bleaching susceptibility, and mortality (Reichert et al, 2021;Plafcan and Stallings, 2022) and supports the hypothesis that corals that rely more heavily on heterotrophy may be more impacted by microplastic pollution.…”

“…), it is more likely due to species-specific responses (Hankins et al, 2021;Mendrik et al, 2021). Previous work on A. cervicornis suggests that exposure to microplastics in the presence of warming does not impact bleaching, likely because this species may not ingest the microplastics under otherwise ideal conditions (Plafcan and Stallings, 2022). Instead, A. cervicornis relies on photosynthetically-derived carbon from its algal symbionts as the primary source of nutrition rather than heterotrophy, thus avoiding the potential pitfalls of microplastics ingestion observed in other coral species that are largely heterotrophic (Reichert et al, 2019;Rotjan et al, 2019;Hankins et al, 2021;Mendrik et al, 2021).…”

“…For example, recent work found that warming led to consistent reductions in fitness-related traits in corals, while microplastics resulted in mixed responses (Reichert et al, 2021). Similarly, another study found that microplastics had no effect on Symbiodiniaceae cell density under ambient or elevated temperatures (Plafcan and Stallings, 2022), suggesting that microplastics may represent a minor stressor when compared to warming. Little is known about the combined effects of ocean warming and microplastics on corals and even less work has explored how ocean acidification may interact with microplastic pollution to impact corals.…”