COS 139-1 - Microplastics impact on Freshwater Symbiosis; an evolutionary superpower during the Anthropocene?

Abstract: Every year in the US, it is estimated that between 1.13 to 2.24 metric tons of plastic waste is either mismanaged or littered, bypassing waste management practices. This pollution leads to the spread of plastic debris in the environment, which undergoes natural degradation and fragmentation processes. Furthermore, microplastic pollution in marine and freshwater systems can vary greatly, with concentrations ranging from (4137 to 12,000 items/m³). To study this issue, we conducted mesocosm-based exposure-response assays, exposing crayfish to microplastics of fibrous, microsphere, and tire wear particle morphologies at varying symbiont densities. This experiment aimed to assess the impact of microplastic pollution on freshwater ecosystems, particularly on the symbiotic relationship between crayfish and annelids. Found in the freshwater estuaries of the New River Basin, C. Appalachiensis serve as hosts to obligate ectosymbiotic annelids, known as Branchiobdellidans. Previous research, utilizing this same study-system, has demonstrated mutualistic benefits in the form of increased growth rate, reproduction, and resiliency when exposed to pH variability. With this mutualistic benefit in mind, we observed the effects of an 80-day chronic exposure on crayfish mortality, growth, and plastic particle bioaccumulation where respective symbiont densities were replenished biweekly. Based on past findings, our crayfish host demonstrated resilience to microplastic exposure, with mortality ranging from 0 to 5%, despite notable symbiont death. In fact, there were no significant differences observed in past growth rate between the control group and the three plastic morphologies (p > 0.05). The average percent change in crayfish mass and length was (0.94% ± 1.79) and (0.43% ± 0.69), respectively. Although no toxicological effects were observed, there was an inherent plastic particle bioaccumulation that could potentially explain these results. Moreover, based on this bioaccumulation, crayfish may act as a vector for introducing microplastics into the freshwater food web due to their role as an intermediate prey-species. Ultimately, this study addresses whether mutualistic symbiosis will bolster resiliency to emerging contaminants expected within the Anthropocene.