Abstract
Investigations of encounters between corals and microplastics have, to date, used particle concentrations that are several orders of magnitude above environmentally relevant levels. Here we investigate whether concentrations closer to values reported in tropical coral reefs affect sediment shedding and heterotrophy in reef-building corals. We show that single-pulse microplastic deposition elicits significantly more coral polyp retraction than comparable amounts of calcareous sediments. When deposited separately from sediments, microplastics remain longer on corals than sediments, through stronger adhesion and longer periods of examination by the coral polyps. Contamination of sediments with microplastics does not retard corals’ sediment clearing rates. Rather, sediments speed-up microplastic shedding, possibly affecting its electrostatic behaviour. Heterotrophy rates are three times higher than microplastic ingestion rates when corals encounter microzooplankton (Artemia salina cysts) and microplastics separately. Exposed to cysts-microplastic combinations, corals feed preferentially on cysts regardless of microplastic concentration. Chronic-exposure experiments should test whether our conclusions hold true under environmental conditions typical of inshore marginal coral reefs.
Highlights
Investigations of encounters between corals and microplastics have, to date, used particle concentrations that are several orders of magnitude above environmentally relevant levels
Not exclusively, yet more often produced by corals exposed to particles compared to control conditions, trapped both sediments and microplastics and removed them from the corals more immediately as these floated up (Supplementary Fig. S1g, h)
By comparing behavioural responses of corals to microplastics, sediments, and microzooplankton, applied in moderate amounts, our study contributes a new layer to the growing field of knowledge on biota-microplastic interactions (Fig. 5)
Summary
Investigations of encounters between corals and microplastics have, to date, used particle concentrations that are several orders of magnitude above environmentally relevant levels. Whether the consequences of the interactions between marine biota and microplastics are overstated or warrant social and political action remains d ebated[8,9]. This is at least partially due to the paucity of experiments using ecologically realistic microplastic concentrations to test whether organisms react to these and natural particles, and whether key ecological functions are impaired[9]. The extent to which microplastics pose a serious threat to key marine tropical ecosystem engineers (e.g. reef-building corals) remains incompletely resolved. Coral reef sediments can contain up to 48-820 microplastic particles k g−116,19
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