In a recent study, researchers in Germany incubated millimetre-sized polystyrene and polyethylene particles in estuarine and coastal waters and monitored the sinking behaviour of the pellets back in the lab. The sinking velocities of the polystyrene material, which has a density greater than seawater, increased 16% in estuarine water and 81% in marine water after just a few weeks of incubation, the team found.

On the other hand, the pellets of polyethylene - a lower density polymer - remained buoyant following 14 weeks of exposure to estuarine water. However, the material did start to sink after six weeks of incubation in coastal water due to colonization by blue mussels, reported the Leibniz Institute for Baltic Sea Research (IOW) group.

"These results show that biofouling reduces the effect of density differences between particle and fluid over time, and thus exerts a major control over microplastic sinking," wrote the scientists in Environmental Research Letters (ERL).

Scanning electron microscope (SEM) images of the surface of the plastic particles revealed differences in the composition of the biofilm between samples incubated at estuarine and coastal stations.

The team highlights that the attachment of fouling macro-organisms appears to be a key factor, as the development of a microscopic biofilm alone didn't increase the specific density of buoyant microplastic sufficiently to cause samples to sink.

During the experiments, the scientists observed that up to six mussels were connected to individual polyethylene pellets at any one time. Furthermore, the SEM images showed that mussel byssus threads attached to the microplastic particles over an area measuring just 20 microns across.

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