Effects of biological filtration by ascidians on microplastic composition in the water column

Plastic pollution, a widespread environmental challenge, significantly impacts marine ecosystems. The degradation of plastic under environmental conditions results in the generation of microplastic (MP; <5 mm) fragments, frequently ingested by marine life, including filter-feeders such as ascidians (Chordata, Ascidiacea). These organisms are integral to benthic-pelagic coupling, transporting MP from the water column through marine food web. Here, we explored the effect of filtration and digestion by the solitary ascidian Styela plicata on the composition of MP in the water column and on the sinking rates of faecal matter, focusing on differences between two distinct plastics, polystyrene (PS) and the biodegradable polylactic acid (PLA). The ascidians efficiently removed 2–5 μm particles within 2 h of filtration. Following digestion and secretion process, PS concentrations in water increased while PLA concentration remained stable. Some particles were egested into the water column repackaged inside faecal pellets, which significantly increased the pellets' drag force and sinking velocity. Raman spectral analysis of digested MP revealed distinct spectrum alterations due to coating by organic substances. These findings highlight the role of ascidians — and other filter-feeders— in modifying the structure of MP in their environment. Research into such modifications is crucial for understanding the MP cycle and its consequences in marine environments.