We described in 2017 how weathering plastic litter in the marine environment fulfils two of three criteria to impose a planetary boundary threat related to "chemical pollution and the release of novel entities": (1) planetary-scale exposure, which (2) is not readily reversible. Whether marine plastics meet the third criterion, (3) eliciting a disruptive impact on vital earth system processes, was uncertain. Since then, several important discoveries have been made to motivate a re-evaluation. A key issue is if weathering macroplastics, microplastics, nanoplastics, and their leachates have an inherently higher potential to elicit adverse effects than natural particles of the same size. We summarize novel findings related to weathering plastic in the context of the planetary boundary threat criteria that demonstrate (1) increasing exposure, (2) fate processes leading to poorly reversible pollution, and (3) (eco)toxicological hazards and their thresholds. We provide evidence that the third criterion could be fulfilled for weathering plastics in sensitive environments and therefore conclude that weathering plastics pose a planetary boundary threat. We suggest future research priorities to better understand (eco)toxicological hazards modulated by increasing exposure and continuous weathering processes, to better parametrize the planetary boundary threshold for plastic pollution.

Background: Ecological impacts of micro-and nanoplastics particles (MNP) are among the most discussed environmental concerns. In algae, MNP are commonly hypothesized to reduce growth, which is a standard ecotoxicological endpoint. However, the reported test outcomes vary, with both growth inhibition and stimulation being observed. Due to this conflict of information, a data synthesis for MNP potential to cause growth inhibition in toxicity testing is needed. Methods: We performed a meta-analysis study to assess the effect of MNP exposure on algal growth. Twenty studies published between 2010 and 2020 and representing 16 algal species and five polymer materials administered as particles in size range 0.04-3,000 µm were included in this meta-analysis. A random-effect model was used to estimate the effect size in three datasets: (1) Low concentration range (<100 mg/L), (2) High concentration range (≥100 mg/L), and (3) Full range model (0.004-1,100 mg/L), which encompassed all studies using the combination of experimental settings (test species, MNP concentration, polymer material, and particle size) yielding the highest effect size within a study. Reichelt and Gorokhova Nano/Microplastic Effects on Algal Growth particulates in the test system, and, hence, does not allow to attribute observed effects to the test polymers.

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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.

Hazard assessment of microplastic is challenging because standard toxicity testing is targeting soluble (at least partially) chemicals. Adverse effects can occur when test organisms are exposed to turbid environments in the presence of various particulate matter (PM), both natural, such as clay and cellulose, 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 comparison is possible when reference materials are included in the testing of microplastic hazard potential. Here, we evaluated growth inhibition in unicellular alga Raphidocelis subcapitata exposed to different PM (microplastic, kaolin, and cellulose; 10, 100, and 1,000 mg/L); algae without added solids were used as a control. Also, aggregate formation in the exposure systems was analyzed using particle size distribution (PSD) data. At 10–100 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 1,000 mg/L, all tested materials exerted growth inhibition, with no significant differences among the materials. Comparing PSD s 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 a similar capacity to cause growth inhibition. Linking effects in unicellular organisms to microplastic fragments remains a challenge since plastics incorporate chemicals that may leach and elicit specific effects relative to the particulates. The use of reference materials in hazard assessment of plastic litter is needed to delineate these effects.