Silicon Limitation Impairs the Tolerance of Marine Diatoms to Pristine Microplastics

The interaction mechanisms of algal organic matter (AOM) and various types and aging degrees of microplastics

Algal blooms can produce substantial amounts of algal organic matter (AOM). Microplastics (MPs) in aquatic environments inevitably interact with AOM. Meanwhile, the aging and type of MPs may increase the uncertainty surrounding interaction. This study focused on polyethylene (PE) and polylactic acid (PLA) to investigate their interaction with AOM before and after aging. The results shw that PLA has a stronger adsorption capacity for AOM than PE. Meanwhile, aging enhanced and weakened the adsorption of PE and PLA for AOM. Compared to unaged PE (UPE) and aged PLA (APLA), aged PE (APE) and unaged PLA (UPLA) more significantly promote the humification of AOM and alter its functional groups. 2D-IR-COS analysis reveals that the sequence of functional group changes in AOM interacting with MPs is influenced by the type and aging of MPs. After interacting with AOM, surface roughness increased for all MPs. FTIR and XPS analyses show that the addition of AOM accelerated the oxidation of MPs surfaces, especially for UPE and APLA, with oxygen content increasing by 9.32 % and 1 %. Aging enhances the interaction between PE and AOM, while weakening the interaction between PLA and AOM. These findings provide new insights into understanding the interplay between AOM and MPs.

Nanoplastics impair growth and nitrogen fixation of marine nitrogen-fixing cyanobacteria

Nanoplastics pollution is a growing environmental problem worldwide. Recent research has demonstrated the toxic effects of nanoplastics on various marine organisms. However, the influences of nanoplastics on marine nitrogen-fixing cyanobacteria, a critical nitrogen source in the ocean, remained unknown. Here, we report that nanoplastics exposure significantly reduced growth, photosynthetic, and nitrogen fixation rates of Crocosphaera watsonii (a major marine nitrogen-fixing cyanobacterium). Transcriptomic analysis revealed that nanoplastics might harm C. watsonii via downregulation of photosynthetic pathways and DNA damage repair genes, while genes for respiration, cell damage, nitrogen limitation, and iron (and phosphorus) scavenging were upregulated. The number and size of starch grains and electron-dense vacuoles increased significantly after nanoplastics exposure, suggesting that C. watsonii allocated more resources to storage instead of growth under stress. We propose that nanoplastics can damage the cell (e.g., DNA, cell membrane, and membrane-bound transporters), inhibit nitrogen and carbon fixation, and hence lead to nutrient limitation and impaired growth. Our findings suggest the possibility that nanoplastics pollution could reduce the new nitrogen input and hence affect the productivity in the ocean. The impact of nanoplastics on marine nitrogen fixation and productivity should be considered when predicting the ecosystem response and biogeochemical cycling in the changing ocean.

Optimization of microplastic removal based on the complementarity of constructed wetland and microalgal-based system

As one of the emblematic emerging contaminants, microplastics (MPs) have aroused great public concern. Nevertheless, the global community still insufficiently acknowledges the ecological health risks and resolution strategies of MP pollution. As the nature-based biotechnologies, the constructed wetland (CW) and microalgal-based system (MBS) have been applied in exploring the removal of MPs recently. This review separately presents the removal research (mechanism, interactions, implications, and technical defects) of MPs by a single method of CWs or MBS. But one thing with certitude is that the exclusive usage of these techniques to combat MPs has non-negligible and formidable challenges. The negative impacts of MP accumulation on CWs involve toxicity to macrophytes, substrates blocking, and nitrogen-removing performance inhibition. While MPs restrict MBS practical application by making troubles for separation difficulties of microalgal-based aggregations from effluent. Hence the combined strategy of microalgal-assisted CWs is proposed based on the complementarity of biotechnologies, in an attempt to expand the removing size range of MPs, create more biodegradable conditions and improve the effluent quality. Our work evaluates and forecasts the potential of integrating combination for strengthening micro-polluted wastewater treatment, completing the synergistic removal of MP-based co-pollutants and achieving long-term stability and sustainability, which is expected to provide new insights into MP pollution regulation and control.

Impact of facemask debris on marine diatoms: Physiology, surface properties, sinking rate, and copepod ingestion

Discarded surgical masks have become a new source of plastic waste in seawater capable of releasing numerous micro and nano plastic fragments. However, little information is available about how this waste impacts the ecological state of marine phytoplankton. Here, we exposed two model marine diatoms (Phaeodactylum tricornutum and Thalassiosira weissflogii) to mask-released debris (MD) that is characterized by various differently-charged functional groups. Although MD could only bind loosely to diatoms, it still inhibited their growth and significantly altered cell surface physicochemical properties. At the nanoscale, MD-exposed cell walls showed enhanced roughness and modulus, besides declined electrical potential, adhesion, and proportion of oxygen-containing compounds. As a result, diatom ingestion by copepods was reduced, and the sinking rate of the carbon pool consisting of MD plus diatoms decreased as well. Our study indicated that MD effects on diatoms have the potential to slow down carbon export from surface seawater to the deep sea. Since oxidation and generation of functional groups are common during the aging process of microplastics (MPs) in nature, the interactions between the diatom cell surface and MD have important environmental significance.

Tracking Nano- and Microplastics Accumulation and Egestion in a Marine Copepod by Novel Fluorescent AIEgens: Kinetic Modeling of the Rhythm Behavior

Nano- and microplastics (NMPs) are now prevalent in the marine environment. This study quantified the uptake and depuration kinetics of spherical polystyrene NMPs of different particle sizes (200 nm/30 μm) and functional groups (-NH2/-COOH) in a temperate calanoid copepod Calanus sinicus (C. sinicus), which exhibited rhythmic feeding patterns in natural environments. Aggregated-induced emission (AIE) fluorescent probes were employed to track and quantify the kinetics of NMPs with excellent photostability and biocompatibility. The results showed that C. sinicus consumed all NMPs types, with preference of NMPs to small size and amino group. Increased diatom concentrations also inhibited the bioaccumulation of NMPs. Influenced by rhythmic behavior, the bioaccumulation of NMPs by C. sinicus was nonstationary during the 6 h uptake phase. After 1-3 h of rapid uptake, the body burden peaked and then slowly declined. During the 3 h depuration phase, C. sinicus rapidly and efficiently removed NMPs with a mean half-life of only 0.23 h. To further quantify the body burden of C. sinicus under the influence of rhythmic feeding behavior, a biokinetic model was established, and the Markov chain Monte Carlo method was used to estimate the parameter distribution. Our results highlighted that copepods exhibited unique rhythmic feeding behavior under environmentally relevant concentrations of NMPs exposure, which may influence the bioaccumulation, trophic transfer, and environmental fate of NMPs.

Microplastics Reshape the Fate of Aqueous Carbon by Inducing Dynamic Changes in Biodiversity and Chemodiversity

The interactions among dissolved organic matter (DOM), microplastics (MPs) and microbes influence the fate of aqueous carbon and greenhouse gas emissions. However, the related processes and mechanisms remain unclear. Here, we found that MPs determined the fate of aqueous carbon by influencing biodiversity and chemodiversity. MPs release chemical additives such as diethylhexyl phthalate (DEHP) and bisphenol A (BPA) into the aqueous phase. The microbial community, especially autotrophic bacteria such as Cyanobacteria, showed a negative correlation with the additives released from MPs. The inhibition of autotrophs promoted CO₂ emissions. Meanwhile, MPs stimulated microbial metabolic pathways such as the tricarboxylic acid (TCA) cycle to accelerate the DOM biodegradation process, and then the transformed DOM presented low bioavailability, high stability, and aromaticity. Our findings highlight an urgent need for chemodiversity and biodiversity surveys to assess ecological risks from MP pollution and the impact of MPs on the carbon cycle.

From promoting aggregation to enhancing obstruction: A negative feedback regulatory mechanism of alleviation of trivalent chromium toxicity by silicon in rice

Trivalent chromium [Cr(III)] is a threat to the environment and crop production. Silicon (Si) has been shown to be effective in mitigating Cr(III) toxicity in rice. However, the mechanisms by which Si reduces Cr(III) uptake in rice are unclear. Herein, we hypothesized that the ability of Si to obstruct Cr(III) diffusion via apoplastic bypass is related to silicic acid polymerization, which may be affected by Cr(III) in rice roots. To test this hypothesis, we employed hydroponics experiments on rice (Oryza sativa L.) and utilized apoplastic bypass tracer techniques, as well as model simulations, to investigate 1) the effect of Si on Cr(III) toxicity and its obstruction capacity via apoplastic bypass, 2) the effect of Cr(III) on silicic acid polymerization, and 3) the relationship between the degree of silicic acid polymerization and its Cr(III) obstruction capacity. We found that Si reversed the damage caused by Cr(III) stress in rice. Si exerted an obstruction effect in the apoplast, significantly decreasing the share of Cr(III) uptake via the apoplastic bypass from 18% to 11%. Moreover, Cr(III) reduced silica particles' radii and increased Si concentration in roots. Modeling revealed that a 5-fold reduction in their radii decreased the diffusion of Cr(III) in apoplast by approximately 17%. We revealed that Cr(III) promoted silicic acid polymerization, resulting in the formation of a higher number of Si particles with a smaller radius in roots, which in turn increased the ability of Si to obstruct Cr(III) diffusion. This negative feedback regulatory mechanism is novel and crucially important for maintaining homeostasis in rice, unveiling the unique role of Si under Cr(III) ion stress and providing a theoretical basis for promoting the use of Si fertilizer in the field.