No short-term effect of sinking microplastics on heterotrophy or sediment clearing in the tropical coral Stylophora pistillata

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.