Cryogenically milled tire tread induces functional and structural changes in natural periphyton communities

Tire wear particles (TWP) are produced due to abrasion and, consequently, deposited on the roadside where they are transferred to the surrounding soil and surface waters. Despite the growing interest on TWP and related chemicals' effects on aquatic organisms, their potential interaction with periphytic biofilm communities is not well understood. As these complex assemblages of microorganisms form the base of aquatic food webs, it is critical to assess the potential impact of tire particles to these communities. We used cryogenically milled tire tread (CMTT), as a surrogate for TWP, to (1) expose periphyton to four CMTT concentrations (0, 50, 500, 1000 mg/L) for 14 days, and (2) compare periphyton responses to CMTT particles (1000 mg/L) and associated chemicals in separate 4 day exposures. In both scenarios, about 25-30% of CMTT were found associated with the periphyton assemblage, illustrating its function as a sink. Additionally, photosynthetic efficiency decreased slightly at the highest tested concentration, 1000 mg/L, and/or associated chemicals, while algal biomass (chlorophyll a) at 14 days decreased ∼30% for 500 and 1000 mg/L CMTT. After the 14 days exposure, changes to the community structure and composition were detected for all CMTT concentrations. In particular, the bacteria phylum Bacillota became nearly absent in the presence of CMTT; similarly, diatom abundance decreased by 45, 85 and 70% at 50, 500, and 1000 mg/L CMTT compared to control, respectively. Consequently, other groups, such as Cyanobacteriota and green algae, became more abundant at higher CMTT concentrations. For the 4 days exposure, CMTT-related chemicals alone caused effects comparable to CMTT particles. Overall, this work highlights the importance of investigating periphyton community responses to TWP, as we show they are susceptible to tire-derived toxicity, while also influencing the fate and effects of tire particles and chemicals in aquatic environments.