Methodological predicament: Distinguishing between the accumulated and deposited microplastics in lichen thalli

Microplastics are ubiquitous environmental contaminants that have significant adverse effects on ecosystems and human health. Lichens, recognized as reliable pollution bioindicators, have been proposed as effective monitors of atmospheric microplastic contamination. This study investigated the dynamics of microplastic deposition and accumulation in lichen thalli using Ramalina celastri as a model organism. Lichen thalli were exposed to controlled suspensions of microfibers across three size categories (<1, 1, and 5 mm) over a 4-week period. Microplastic particles were extracted using a novel sodium hypochlorite oxidation technique. The results revealed a distinct differentiation between the deposited microfibers, which correlated with the environmental deposition rates, and the accumulated microfibers, which showed temporal increases and greater retention of smaller particles. These findings challenge the existing assumptions in microplastic research that extracted particles solely represent accumulated material, emphasizing the need for refined terminology. To address this, we propose the term “retained” to encompass deposited and accumulated particles. The sodium hypochlorite oxidation technique provides a cost-effective and safer alternative to traditional wet peroxide oxidation, enhancing its feasibility for researchers in resource-limited settings. Furthermore, the use of NaCl and oil extraction proved effective in isolating microplastics across varying densities. This work underscores the importance of precise definitions and robust methodologies in the study of microplastic bioindicators, contributing to a more comprehensive understanding of microplastic dynamics in lichens and their utility in environmental monitoring.