This scenery looks idyllic only at first glance. Because what so glitters green around a coral through the water here is a sign of a real environmental problem. These are microplastic particles.
The accumulation of microplastics in soils and water has long been a worldwide problem. As is well known, a lot of these tiny plastic particles float in the world’s oceans. They arise when, for example, plastic bags slowly break down into smaller and smaller parts. In addition, industrially manufactured microplastics also contribute to the stress. This reaches the wastewater and then the ocean via cosmetic products. Researchers are only just beginning to understand what the plastic particles mean for the animals living there. But one thing is clear: Many sea creatures absorb the particles – with potential health consequences.
Jeremy Axworthy of the University of Washington at Seattle and his colleagues have now confirmed once again that corals “swallow” microplastics. Many of these cnidarians live with algae symbionts, which provide them with sugar and other nutrients. But in addition, they sometimes feed on organisms floating in the water, such as zooplankton – and these often have a size similar to that of microplastics. In an experiment with two coral species native to Hawaii, the researchers therefore tested whether the animals can differentiate between plastic and zooplankton.
It turned out that some corals apparently only record microplastics – shown in the picture as fluorescent green particles – only under certain conditions. The species Pocillopora damicornis only ate the plastic particles if Axworthy and his team added zooplankton to the water. If her plastic was offered alone, she did not accept it. How this happens is still unclear. It is conceivable, however, that the coral perceives certain chemical or physical signals from the food, but cannot distinguish it from the plastic that is also in the water.
“We are only just beginning to understand the effects of microplastics on marine life,” says Axworthy’s colleague Jacqueline Padilla-Gamino. If the uptake of the plastic particles damages the corals, this would be another factor that threatens the survival of the animals. Enemies are already troubling corals like coral-eating starfish, but above all acidic water and rising temperatures due to climate change. If the heat is too high, the animals lose their algae symbionts and literally turn pale – switching to zooplankton food is therefore also a strategy to compensate for this loss.