Uncovering the biological toll of microplastics on Moina macrocopa: Population responses and stress markers

Microplastic pollution has infiltrated freshwater ecosystems. This exploratory study examined population responses and biochemical stress indicators in Moina macrocopa Straus, 1820, exposed to microplastics. Two 11-day experiments used nominal concentrations of 0, 1, 5, and 10 mg/L. Kamnoetvidya Science Academy (KVIS), Thailand, tested low-density polyethylene (LDPE) with three independent beakers per concentration; the School of Science and Technology, Singapore (SST), tested polyethylene terephthalate (PET) with one beaker per concentration. Both sites assessed populations and malondialdehyde (MDA); SST additionally measured catalase endpoint absorbance. MDA measurements at both sites were interpreted descriptively. At SST, Day-11 counts decreased across increasing concentrations from 89 to 62, 51, and 36 individuals. MDA increased with concentration on Days 3 and 5, reaching 64.5 nmol/g fresh weight at 10 mg/L on Day 5 versus 5.93 nmol/g in the control. KVIS population density increased over time, without significant concentration (p = 0.415) or concentration-by-time effects (p = 0.324). KVIS MDA and SST catalase absorbance showed no consistent concentration-dependent pattern. SST results were descriptive, and protocol differences limited direct comparisons between the two polymer experiments. At SST, elevated early MDA and lower later population counts formed a pattern consistent with microplastic-associated stress. At KVIS, independent replication enabled observed population differences to be evaluated against between-beaker variability, without establishing significant concentration-related effects. Together, these findings highlight the value of combining biochemical and population measurements with independent replication when investigating potential early-warning indicators. Further replicated studies are needed to determine whether early biochemical responses reliably predict subsequent population change.