Cell size matters : nano- and micro-plastics preferentially drive declines of large marine phytoplankton due to co-aggregation

Marine plastic pollution represents a key environmental concern. Whilst ecotoxicological data for plastic is increasingly available, its impact upon marine phytoplankton remains unclear. Owing to their predicted abundance in the marine environment and likely interactions with phytoplankton, here we focus on the smaller fraction of plastic particles (~50 nm and ~2 µm polystyrene spheres). Exposure of natural phytoplankton communities and laboratory cultures revealed that plastic exposure does not follow traditional trends in ecotoxicological research, since large phytoplankton appear particularly susceptible towards plastics exposure despite their lower surface-to-volume ratios. Cell declines appear driven by hetero-aggregation and co-sedimentation of cells with plastic particles, recorded visually and demonstrated using confocal microscopy. As a consequence, plastic exposure also caused disruption to photosynthetic functioning, as determined by both photosynthetic efficiency and high throughput proteomics. Negative effects upon phytoplankton are recorded at concentrations orders of magnitude above those estimated in the environment. Hence, it is likely that impacts of NPs and MPs are exacerbated at the high concentrations typically used in ecotoxicological research (i.e., mg L-1)

Microbial pioneers of plastic colonisation in coastal seawaters

Plastics, when entering the environment, are immediately colonised by microorganisms. This modifies their physico-chemical properties as well as their transport and fate in natural ecosystems, but whom pioneers this colonisation in marine ecosystems? Previous studies have focused on microbial communities that develop on plastics after relatively long incubation periods (i.e., days to months), but very little data is available regarding the earliest stages of colonisation on buoyant plastics in marine waters (i.e., minutes or hours). We conducted a preliminary study where the earliest hours of microbial colonisation on buoyant plastics in marine coastal waters were investigated by field incubations and amplicon sequencing of the prokaryotic and eukaryotic communities. Our results show that members of the Bacteroidetes group pioneer microbial attachment to plastics but, over time, their presence is masked by other groups - Gammaproteobacteria at first and later by Alphaproteobacteria. Interestingly, the eukaryotic community on plastics exposed to sunlight became dominated by phototrophic organisms from the phylum Ochrophyta, diatoms at the start and brown algae towards the end of the three-day incubations. This study defines the pioneering microbial community that colonises plastics immediately when entering coastal marine environments and that may set the seeding Plastisphere of plastics in the oceans. [Abstract copyright: Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.

Environmentally relevant concentrations of titanium dioxide nanoparticles pose negligible risk to marine microbes.

Nano-sized titanium dioxide (nTiO ) represents the highest produced nanomaterial by mass worldwide and, due to its prevalent industrial and commercial use, it inevitably reaches the natural environment. Previous work has revealed a negative impact of nTiO upon marine phytoplankton growth, however, studies are typically carried out at concentrations far exceeding those measured and predicted to occur in the environment currently. Here, a series of experiments were carried out to assess the effects of both research-grade nTiO and nTiO extracted from consumer products upon the marine dominant cyanobacterium, , and natural marine communities at environmentally relevant and supra-environmental concentrations ( , 1 μg L to 100 mg L ). Cell declines observed in cultures were associated with the extensive aggregation behaviour of nTiO in saline media and the subsequent entrapment of microbial cells. Hence, higher concentrations of nTiO particles exerted a stronger decline of cyanobacterial populations. However, within natural oligotrophic seawater, cultures were able to recover over time as the nanoparticles aggregated out of solution after 72 h. Subsequent shotgun proteomic analysis of cultures exposed to environmentally relevant concentrations confirmed minimal molecular features of toxicity, suggesting that direct physical effects are responsible for short-term microbial population decline. In an additional experiment, the diversity and structure of natural marine microbial communities showed negligible variations when exposed to environmentally relevant nTiO concentrations ( , 25 μg L ). As such, the environmental risk of nTiO towards marine microbial species appears low, however the potential for adverse effects in hotspots of contamination exists. In future, research must be extended to consider any effect of other components of nano-enabled product formulations upon nanomaterial fate and impact within the natural environment

Mechanisms of silver nanoparticle toxicity on the marine cyanobacterium Prochlorococcus under environmentally-relevant conditions

Global demand for silver nanoparticles (AgNPs), and their inevitable release into the environment, is rapidly increasing. AgNPs display antimicrobial properties and have previously been recorded to exert adverse effects upon marine phytoplankton. However, ecotoxicological research is often compromised by the use of non-ecologically relevant conditions, and the mechanisms of AgNP toxicity under environmental conditions remains unclear. To examine the impact of AgNPs on natural marine communities, a natural assemblage was exposed to citrate-stabilised AgNPs. Here, investigation confirmed that the marine dominant cyanobacteria Prochlorococcus is particularly sensitive to AgNP exposure. Whilst Prochlorococcus represents the most abundant photosynthetic organism on Earth and contributes significantly to global primary productivity, little ecotoxicological research has been carried out on this cyanobacterium. To address this, Prochlorococcus was exposed to citrate-stabilised AgNPs, as well as silver in its ionic form (Ag2SO4), under simulated natural conditions. Both AgNPs and ionic silver were observed to reduce Prochlorococcus populations by over 90% at concentrations ≥10 μg L−1, representing the upper limit of AgNP concentrations predicted in the environment (10 μg L−1). Longer-term assessment revealed this to be a perturbation which was irreversible. Through use of quenching agents for superoxide and hydrogen peroxide, alongside incubations with ionic silver, it was revealed that AgNP toxicity likely arises from synergistic effects of toxic superoxide species generation and leaching of ionic silver. The extent of toxicity was strongly dependent on cell density, and completely mitigated in more cell-dense cultures. Hence, the calculation and reporting of the particle-to-cell ratio reveals that this parameter is effective for standardisation of experimental work, and allows for direct comparison between studies where cell density may vary. Given the key role that marine cyanobacteria play in global primary production and biogeochemical cycling, their higher susceptibility to AgNP exposure is a concern in hotspots of pollution.CJD was supported by the NERC CENTA DTP studentship NE/L002493/1. JAC-O was funded by NERC Independent Research Fellowship NE/K009044/1, Ramón y Cajal contract RYC-2017-22452 (funded by the Ministry of Science, Innovation and Universities, the National Agency of Research, and the European Social Fund) and MINECO project PID2019-109509RB-I00 (FEDER co-funding)