Bivalves as Biological Sieves: Bioreactivity Pathways of Microplastics and Nanoplastics

Oceanic and coastal sampling programs have indicated extensive plastic pollution of marine habitats and revealed the need for understanding the scope and potential impacts of plastics on marine organisms. Sampling regimes for macroplastics (>5 mm) that can be visually collected for quantification and characterization in marine habitats provide valuable environmental data for the larger plastics. But less is known about the scope or potential impacts of small micron- and nano-sized bits of plastic that result from weathering of macroplastics and inputs of manufactured particles that could profoundly affect marine invertebrates, especially suspension feeders. Essential fundamental information about bivalve biology along with current research and reviews on microplastics, nanoplastics, and engineered nanoparticles were integrated to discuss how filter-feeding bivalves can serve as valuable bioindicators of plastic pollution. Bivalves can serve as important bioaccumulators of plastic particles and exhibit processing pathways that serve as biological sieves. Mesoplastics (1–5 mm) and large microplastics (>25 µm) will have a relatively short transit time (hours to days) and will primarily be concentrated in biodeposits (pseudofeces and feces). Small microplastics (<25 µm) and nanoplastics (<1 µm) are more likely to be accumulated in digestive gland tissues and cells, and also hemocytes, and will have longer retention times. Lysosomes are a common target organelle for uptake and toxicity in both of these cell types. Therefore, bivalves can potentially act as biological sieves for characterizing relative environmental exposures and bioreactivity of microplastics and nanoplastics, based on critical particle capture and processing pathways. This framework highlights the importance of developing diagnostic approaches to characterize potential environmental risks associated with plastic particles as well as potential interactions with other anthropogenic pollutants.