Microplastics in Lichen Thalli: A Photo or a Movie of Local Atmospheric Deposition?

Airborne microplastics (MPs) are a global issue, and there is an urgent need to prevent their spread in the environment. Sensitive and reliable methods are also needed to assess their deposition and effectively evaluate risk in terrestrial ecosystems. Current automated monitoring devices are expensive and do not enable large-scale mapping of MP deposition. As with other persistent atmospheric contaminants, developing accurate, cost-effective and easily applicable biomonitoring methods would therefore be highly beneficial. Cryptogams are among the most suitable biomonitors of airborne contaminants, and preliminary surveys show that epiphytic lichens accumulate higher concentrations of MPs in urban areas and near landfills than in control sites. However, the interaction between lichen thalli and MPs is weak and, as discussed in this review, the anthropogenic fibres and plastic fragments intercepted and retained by lichens probably do not reflect the levels in bulk atmospheric deposition. While emphasizing the need for studies evaluating the effectiveness of cryptogams in accumulating different types of airborne MPs under various meteorological conditions, this review also suggests directing future research efforts toward mosses, which seem to accumulate much higher concentrations of MPs than lichens in both active and passive biomonitoring surveys.