Interaction of microplastics and glyphosate: Effects on the freshwater green alga <i>Chlamydomonas reinhardtii</i>

Microplastics (particles 0.1 μm-5 mm) are an increasing ecological problem in aquatic environments. Effects of microplastics on microalgae and interactions with co-occurring contaminants, such as the herbicide glyphosate, are poorly understood. We examined direct effects of microplastics and interactions with glyphosate using the chlorophyte Chlamydomonas reinhardtii grown in saturating irradiance under nutrient-replete conditions. Cells were exposed to polyethylene beads (45-53 µm diameter) at low (0.1 g L-1) or high (1 g L-1) concentrations, in combination with low (44 µM or equivalent bound to beads) or high (220 µM or equivalent bound to beads) glyphosate for 14 days. We measured growth (chl a fluorescence and cells), photosynthetic quantum yield (Fv/Fm) and cell mortality (SYTOX-Green staining). Glyphosate significantly reduced fluorescence-based growth rates and Fv/Fm and increased cell mortality compared to controls, but growth rates of culture exposed to beads alone were higher than controls, despite having lower Fv/Fm. Complex antagonistic interactions between glyphosate and microplastic treatments were observed; microplastics ameliorated effects of glyphosate on growth rates and cell mortality. Comparisons with previous studies emphasize the importance of plastic polymer, microplastic concentration and experimental preparation. Future work should focus on clear experimental protocols, mixed algal species, and use of aged plastics.