Effects of Polyester Microfibers on Microphytobenthos and Sediment-Dwelling Infauna

Microfibers often dominate sediment microplastic samples, but little is known about their ecological effects on benthic organisms and functions. Polyethylene terephthalate) (PET) microfibers were added to 36 sediment chambers at six concentrations (0-0.5 g kg^-1 sediment) to assess the effects on microphytobenthos (MPB), a key deposit-feeding bivalve, <i>Macomona liliana</i>, and sediment nutrient pools. MPB photosynthesis was promoted in 18 chambers through a 12 h light/dark cycle. Another 18 chambers were maintained under dark conditions to inhibit photosynthesis. After 35 days of MPB growth and stabilization, four <i>M. liliana</i> were added to each chamber for a further 40 days. MPB biomass and composition were examined alongside <i>M. liliana</i> biochemical and behavioral properties and porewater dissolved inorganic nutrient concentrations. Increasing microfibers resulted in lower MPB biomass, fewer diatom-associated fatty acids (FAs), and an increase in cyanobacteria. The changes in MPB coincided with up to 75% lower energy reserves and reduced burrowing activity in <i>M. liliana</i>. In the light, nitrate + nitrate (NO<i>_x</i>) was significantly elevated and related to <i>M. liliana</i> and MPB biochemical properties. Ammoniu (NH₄⁺) concentrations increased but were variable in both the light and the dark. Our results suggest that increasing microfiber concentrations influence the interactions between <i>M. liliana</i> and MPB and affect biogeochemical processing in coastal marine sediments.