Entrainment of polyester microfibers modifies the structure and function of stream biofilms

As plastics degrade over time, they fragment into smaller particulates known as microplastics (MPs). There is a growing body of literature suggesting that MPs, including polyester microfibers (PMFs), are capable of altering the structure and function of microbial communities. To date, no studies have observed the effects of extended PMF exposure (≥ 1 month) on stream biofilms over an extensive concentration gradient. The purpose of this study was to examine the response of various mature biofilm biological endpoints to increasing PMF concentrations using stream mesocosms. We inoculated 12 recirculating mesocosms with a biofilm slurry sampled from Fishing Creek, PA, and exposed them to increasing concentrations of PMFs ranging from 0 to 22,000 PMF/L. Total biofilm biomass was not altered over the PMF gradient, while we did observe a decrease and increase in algal and bacterial biomass, respectively. Diatom abundance was also reduced in proportion to that of green algae, aligning with observed reductions in light harvesting efficiency. We further observed increased levels of respiration and a switch from net autotrophy to heterotrophy around 10,000-12,000 PMF/L. Increased PMFs also inhibited extracellular β-1,4-glucosidase activities and modified bacterial community composition. While elevated PMF concentrations reduce algal biomass, they may support bacterial growth by substituting algal exudates with a novel carbon substrate, likely selecting for bacteria capable of degrading polyester. Extracellular alkaline phosphatase and phenol oxidase activities, gross primary production, maximum electron transport rate of photosystem II, and nutrient cycling were unaltered. Our study highlights several implications that the novel contaminant poses to the structure and function of stream biofilms.