Effects of different oxidants on the behaviour of microplastic hetero-aggregates

Microplastic hetero-aggregates are stable forms of microplastics in the aqueous environment. However, when disinfecting water containing microplastic hetero-aggregates, the response of them in water to different oxidizing agents and the effect on water quality have not been reported. Our results showed that Ca(ClO)2, K2S2O8, and sodium percarbonate (SPC) treatment could lead to the disaggregation of microplastic hetero-aggregates as well as a rise in cell membrane permeability, which caused a large amount of organic matter to be released. When the amount of oxidant dosing is insufficient, the oxidant cannot completely degrade the released organic matter, resulting in DOC, DTN, DTP and other indicators being higher than before oxidation, thus causing secondary pollution of the water body. In comparison, K2FeO4 can purify the water body stably without destroying the microplastic hetero-aggregates, but it only weakly inhibits the toxic cyanobacteria Microcystis and Pseudanabaena, which may cause cyanobacterial bloom as well as algal toxin and odorant contamination in practical application. Compared with the other oxidizers, K2S2O8 provides better inhibition of toxic cyanobacteria and has better ecological safety. Therefore, when treating microplastic-containing water bodies, we should consider both water purification and ecological safety, and select appropriate oxidant types and dosages to optimize the water treatment.

Secondary pollution of microplastic hetero-aggregates after chlorination: Released contaminants rarely re-adsorbed by the second-formed hetero-aggregates

In urban waters, microplastics (MPs) usually form hetero-aggregates through adsorption of organics and microbes. However, the effects of hetero-aggregates on water quality are rarely reported. In this study we found that the hetero-aggregates, which accumulated contaminants, were like a "time bomb". Chlorination was able to trigger the "time bomb" through destruction of hetero-aggregates, lysis of microbial cells and elevation of the concentration of low-molecular-mass organics. Thereupon previously adhered organics desorbed from MPs, intracellular metabolites were released from lysed cells, and re-formation of hetero-aggregates was limited. This process rapidly increased the concentration of organics but prevented the re-adsorption of organics, which leads to secondary pollution. Thus, to alleviate the risks of secondary pollution caused by hetero-aggregates, the choice of oxidant species and dose should be optimized based on the characteristics of existent hetero-aggregates when purifying urban waters containing MPs.

Ecological implications beyond the ecotoxicity of plastic debris on marine phytoplankton assemblage structure and functioning

Plastic pollution is a global issue posing a threat to marine biota with ecological implications on ecosystem functioning. Micro and nanoplastic impact on phytoplankton autotrophic species (e.g., cell growth inhibition, decrease in chlorophyll a and photosynthetic efficiency and hetero-aggregates formation) have been largely documented. However, the heterogeneity of data makes rather difficult a comparison based on size (i.e. micro vs nano). In addition, knowledge gaps on the ecological impact on phytoplankton assemblage structure and functioning are evident. A new virtual meta-analysis on cause-effect relationships of micro and nanoplastics on phytoplankton species revealed the significant effect posed by polymer type on reducing cell density for tested PVC, PS and PE plastics. Linked with autotrophic phytoplankton role in atmospheric CO₂ fixation, a potential impact of plastics on marine carbon pump is discussed. The understanding of the effects of microplastics and nanoplastics on the phytoplankton functioning is fundamental to raise awareness on the overall impact on the first level of marine food web. Interactions between micro and nanoplastics and phytoplankton assemblages have been quite documented by in vitro examinations; but, further studies considering natural plankton assemblages and/or large mesocosm experiments should be performed to evaluate and try predicting ecological impacts on primary producers.

Addressing challenges in providing a reliable ecotoxicology data for graphene-oxide (GO) using an algae (Raphidocelis subcapitata), and the trophic transfer consequence of GO-algae aggregates

The graphene oxide (GO) due to its exceptional structure, physicochemical and mechanical properties is a very attractive material for industry application. Even though, the unique properties of GO (e.g. structure, size, shape, etc.) make the risk assessment of this nanomaterial very challenging in comparison with conventional ecotoxicology studies required by regulators. Thus, there is a need for standardized characterization techniques and methodology to secure a high quality/reliable data on the ecotoxicology of GO, and to establish environmentally acceptable levels. Herein, authors address the crucial quality criteria when evaluating the ecotoxicology of GO using an algae (Raphidocelis subcapitata) and a shrimp (Paratya australiensis). This study provides a detail characterization and modification of the used GO, robust quantification and a suspension stability in different media for ecotoxicology studies. It was observed that under the same exposure conditions the behavior of GO and the estimated outcomes (IC₅₀ values) in modified algae media differed in comparison to the referent media. Further to that, the adverse effects of GO on the algae cell structure and the potential uptake of GO by the algae cells were examined using the TEM with different staining techniques to avoid artefacts. Shrimps which were exposed to GO-algae aggregates via the food intake did not indicate stress or accumulation of GO. Our work presents an important insight to necessity of establishing a benchmark ecotoxicology assays for GO (e.g. characterization techniques, choice of media, etc.) and providing a reliable data to be used by regulators in risk assessment of two-dimensional (2D) nanomaterials.

Marine vs freshwater microalgae exopolymers as biosolutions to microplastics pollution

Microalgae can excrete exopolymer substances (EPS) with a potential to form hetero-aggregates with microplastic particles. In this work, two freshwater (Microcystis panniformis and Scenedesmus sp.) and two marine (Tetraselmis sp. and Gloeocapsa sp.) EPS producing microalgae were exposed to different microplastics. In this study, the influence of the microplastic particles type, size and density in the production of EPS and hetero-aggregates potential was studied. Most microalgae contaminated with microplastics displayed a cell abundance decrease (of up to 42%) in the cultures. The results showed that the formed aggregates were composed of microalgae and EPS (homo-aggregates) or a combination of microalgae, EPS and microplastics (hetero-aggregates). The hetero-aggregation was dependent on the size and yield production of EPS, which was species specific. Microcystis panniformis and Scenedesmus sp. exhibited small EPS, with a higher propension to disaggregate, and consequently lower capabilities to aggregate microplastics. Tetraselmis sp. displayed a higher ability to aggregate both low and high-density microplastics, being partially limited by the size of the microplastics. Gloeocapsa sp. had an outstanding EPS production and presented excellent microplastic aggregation capabilities (adhered onto the surface and also incorporated into the EPS). The results highlight the potential of microalgae to produce EPS and flocculate microplastics, contributing to their vertical transport and consequent deposition. Thus, this work shows the potential of microalgae as biocompatible solutions to water microplastics treatment.

Microplastic interactions with freshwater microalgae: Hetero-aggregation and changes in plastic density appear strongly dependent on polymer type

In this study, the interactions between microplastics, chosen among the most widely used in industry such as polypropylene (PP) and high-density polyethylene (HDPE), and a model freshwater microalgae, Chlamydomas reinhardtii, were investigated. It was shown that the presence of high concentrations of microplastics with size >400 μm did not directly impact the growth of microalgae in the first days of contact and that the expression of three genes involved in the stress response was not modified after 78 days. In parallel, a similar colonization was observed for the two polymers. However, after 20 days of contact, in the case of PP only, hetero-aggregates constituted of microalgae, microplastics and exopolysaccharides were formed. An estimation of the hetero-aggregates composition was approximately 50% of PP fragments and 50% of microalgae, which led to a final density close to 1.2. Such hetero-aggregates appear as an important pathway for the vertical transport of PP microplastics from the water surface to sediment. Moreover, after more than 70 days of contact with microplastics, the microalgae genes involved in the sugar biosynthesis pathways were strongly over-expressed compared to control conditions. The levels of over-expression were higher in the case of HDPE than in PP condition. This work presents the first evidence that depending on their chemical nature, microplastics will follow different fates in the environment.