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

Plastic pollution is a widespread environmental challenge, posing significant threats to marine organisms and ecosystems. For example, microplastics (MP; below 5 mm) frequently ingested by marine life, including filter-feeders such as ascidians (Chordata, Ascidiacea). These organisms are integral to benthic-pelagic coupling, often transporting MP from the water column through the marine food web. During the MP transit through the digestive system, particles are subjected to different pH fluctuations and various digestive enzymes, before being excreted back in fecal pellets. These pellets may greatly influence the transport and dispersion of MP within the water column, affecting their availability to other marine organisms. Here, we explore the effect of the solitary ascidian Styela plicata filtration and digestion on the composition of MP in the water column, focusing on differences between bioplastics (polylactic acid, PLA) and conventional plastics (polystyrene, PS). The ascidians successfully removed smaller MP (2-5 µm) within two hours of filtration. However, after digestion and secretion process PS concentrations increased suggesting a release back in the water column. The concentration of PLA remained stable, hinting of the decreased availability once particles sank. Some of the particles that were egested into the water column were repackaged within fecal pellets. Raman spectral analysis of the digested MPs revealed distinct alteration, with particles exhibiting a changed spectrum post-digestion. This alteration of the particle's surface, along with rapid filtration and egestion rate of MPs by ascidians, highlights the potential role of ascidians—and other filter-feeding organisms—in modifying the structure of MPs in their environment. Also see: https://micro2024.sciencesconf.org/559447/document