Algal growth at environmentally relevant concentrations of suspended solids: implications for microplastic hazard assessment

Abstract Hazard assessment of microplastic is challenging because standard toxicity testing is mostly developed for soluble (at least partially) chemicals. Adverse effects can occur when test organisms are exposed to turbid environments with various particulate matter (PM), both natural, such as sediment, and anthropogenic, such as microplastic. It is, therefore, relevant to compare responses to PM exposure between the microplastic and other suspended solids present at ecologically relevant concentrations; this can be done by using reference materials when assessing hazard potential of microplastics. Here, we evaluated growth inhibition in unicellular alga Raphidocelis subcapitata exposed to different suspended solids (microplastic, kaolin, and cellulose; 10, 100 and 1000 mg/L) during 72 h; algae without added solids were used as a control. In addition, aggregate formation in the exposure systems was analyzed using particle size distribution data. At 10 and 100 mg/L, no adverse growth effects were observed in any treatments; moreover, algal growth was significantly stimulated in kaolin and cellulose treatments compared to the control. However, at 1000 mg/L, all tested materials exerted growth inhibition, with no significant differences among the treatments. The comparison among particle size distributions across the treatments showed that both PM concentration and size of the particle aggregates were significant growth predictors for all materials tested. Therefore, at high concentrations, both natural and anthropogenic materials have similar capacity to cause adverse effects in algal growth inhibition tests, which must be taken into account in hazard assessment of plastic litter.