Effects of heavy metals released from sediment accelerated by artificial sweeteners and humic acid on a green algae Scenedesmus obliquus

Accumulation, translocation and transformation of artificial sweeteners in plants: A critical review

Artificial sweeteners (ASs) have become an increasingly significant concern as an emerging contaminant. The widespread utilization has given rise to environmental consequences that are progressively harder to disregard. ASs infiltrate both aquatic and terrestrial ecosystems through the discharge of wastewater effluents and the application of manure and biosolids. These compounds can be absorbed and accumulated by plants from soil, water and the atmosphere, posing potential risks to ecological systems and human health. However, limited data available on plant absorption, translocation, and metabolism of ASs hinders a comprehensive understanding of their impact on ecosystem. This study aims to comprehensively summarize the global distribution of ASs, along with elucidating patterns of their uptake and accumulation within plants. Furthermore, it seeks to elucidate the pivotal factors governing ASs absorption and translocation, encompassing hydrophilicity, ionic nature, plant physiology, and environmental conditions. Notably, there remains a significant knowledge gap in understanding the biodegradation of ASs within plants, with their specific degradation pathways and mechanisms largely unexplored, thereby necessitating further investigation. Additionally, this review provides valuable insights into the ecotoxicological effects of ASs on plants. Finally, it identifies research gaps and outlines potential avenues for future research, offering a forward-looking perspective on this critical issue.

Evaluation of heavy metal accumulation and sources in surface sediments of the Pearl River Estuary (China)

The Pearl River Estuary (PRE) has experienced an influx of metals and nutrients, predominantly from the Pearl River, which has led to a potential threat to the estuarine ecosystem. In this study, sediment samples were densely collected to clarify the accumulation, and source contributions of heavy metals (namely Hg, Zn, Cu, As, Pb, Cd, and Cr) in the PRE. The spatial distributions of these metals exhibited significant differences, with higher values detected in the offshore areas and lower values further away. The metal values along the western coast tended to be significantly elevated compared to that of the eastern seaboard, which may relate to anthropogenic pollution, the discharge of industrial and domestic effluents in the region. The geological accumulation index (Igeo) was utilized to evaluate the pollution status, categorized as ranging from light to moderate pollution levels. The homology of metal elements was determined through Pearson correlation analysis and principal component analysis (PCA). A receptor model of positive matrix factorization (PMF) was developed to quantify the contributions of various sources to the accumulation of metal elements in the PRE. Industrial sources contributed the most to sediment metals (37.07%), followed by agricultural and natural sources, with transportation sources contributing the least (11.17%).

Untreated wastewater impact and environmental risk assessment of artificial sweeteners in river water and sediments of the Danube River Basin in Serbia

Artificial sweeteners are receiving increasing attention as newly recognized emerging contaminants that mainly reach the aquatic environment through the discharge of municipal wastewater containing large amount of these compounds. In this study, the impact of raw untreated wastewater discharges on the levels and the water/sediment distribution of artificial sweeteners in the Danube River and its largest tributaries in Serbia was evaluated, and a comprehensive assessment of environmental risks for freshwater and benthic organisms was performed. Acesulfame and sucralose were detected in all river water samples (100%), while saccharin (59%) and cyclamate (12%) were less frequently found, indicating long-term continuous sewage-derived pollution. Aspartame (100%) and neotame (60%) were the only artificial sweeteners recorded in the sediment samples due to their preference to sorb to particulate matter in the water/sediment system. In terms of ecotoxicological risk, a low risk for aquatic organisms was determined at the detected levels of saccharin in river water, while a high to medium risk was found for benthic biota at the concentrations of neotame and aspartame detected in sediments. The largest contribution to the pollution of the Danube River Basin with artificial sweeteners, and consequently the highest environmental risk, was determined in the two largest cities, the capital Belgrade and Novi Sad, which raises the issue of transboundary pollution.

Sediment-seawater exchange altered adverse effects of ocean acidification towards marine microalgae

Ocean acidification (OA) exhibits high threat to marine microalgae. However, the role of marine sediment in the OA-induced adverse effect towards microalgae is largely unknown. In this work, the effects of OA (pH 7.50) on the growth of individual and co-cultured microalgae (Emiliania huxleyi, Isochrysis galbana, Chlorella vulgaris, Phaeodactylum tricornutum, and Platymonas helgolandica tsingtaoensis) were systematically investigated in the sediment-seawater systems. OA inhibited E. huxleyi growth by 25.21 %, promoted P. helgolandica (tsingtaoensis) growth by 15.49 %, while did not cause any effect on the other three microalgal species in the absence of sediment. In the presence of the sediment, OA-induced growth inhibition of E. huxleyi was significantly mitigated, because the released chemicals (N, P and Fe) from seawater-sediment interface increased the photosynthesis and reduced oxidative stress. For P. tricornutum, C. vulgaris and P. helgolandica (tsingtaoensis), the growth was significantly increased in the presence of sediment in comparison with those under OA alone or normal seawater (pH 8.10). For I. galbana, the growth was inhibited when the sediment was introduced. Additionally, in the co-culturing system, C. vulgaris and P. tricornutum were the dominant species, while OA increased the proportions of dominant species and decreased the community stability as indicated by Shannon and Pielou's indexes. After the introduction of sediment, the community stability was recovered, but remained lower than that under normal condition. This work demonstrated the role of sediment in the biological responses to OA, and could be helpful for better understanding the impact of OA on marine ecosystems.

A microplate-based bioassay for toxicity testing using the large benthic algal species Closterium ehrenbergii

Pollution of water bodies by metals has long been studied but still remains a threat to healthy ecosystems. While most ecotoxicological studies on algae are performed with planktonic standard species such as Raphidocelis subcapitata, benthic algae may depict the majority of the algal flora in rivers and streams. These species encounter different exposure scenarios to pollutants as they are sedentary and not carried away by the current. This particular way of life leads to an integration of toxic effects over time. Therefore, in this study, the effects of six metals on the large unicellular benthic species Closterium ehrenbergii were examined. A miniaturized bioassay with low cell densities of 10-15 cells/mL using microplates was developed. Through chemical analysis, metal complexing properties in the culture medium were demonstrated, that could lead to an underestimation of metal toxicity. Thus, the medium was modified by excluding EDTA and TRIS. The toxicity of the six metals ranked by EC₅₀ values in descending order, was as follows: Cu (5.5 µg/L) > Ag (9.2 µg/L) > Cd (18 µg/L) > Ni (260 µg/L) > Cr (990 µg/L) > Zn (1200 µg/L). In addition, toxic effects on the cell morphology were visualized. Based on a literature review, C. ehrenbergii was shown to be partly more sensitive than R. subcapitata which suggests that it can be a useful addition to ecotoxicological risk assessment.

Heavy metals (HMs) pollution in the aquatic environment: Role of probiotics and gut microbiota in HMs remediation

The presence of heavy metals (HMs) in aquatic ecosystems is a universal concern due to their tendency to accumulate in aquatic organisms. HMs accumulation has been found to cause toxic effects in aquatic organisms. The common HMs-induced toxicities are growth inhibition, reduced survival, oxidative stress, tissue damage, respiratory problems, and gut microbial dysbiosis. The application of dietary probiotics has been evolving as a potential approach to bind and remove HMs from the gut, which is called "Gut remediation". The toxic effects of HMs in fish, mice, and humans with the potential of probiotics in removing HMs have been discussed previously. However, the toxic effects of HMs and protective strategies of probiotics on the organisms of each trophic level have not been comprehensively reviewed yet. Thus, this review summarizes the toxic effects caused by HMs in the organisms (at each trophic level) of the aquatic food chain, with a special reference to gut microbiota. The potential of bacterial probiotics in toxicity alleviation and their protective strategies to prevent toxicities caused by HMs in them are also explained. The dietary probiotics are capable of removing HMs (50-90%) primarily from the gut of the organisms. Specifically, probiotics have been reported to reduce the absorption of HMs in the intestinal tract via the enhancement of intestinal HM sequestration, detoxification of HMs, changing the expression of metal transporter proteins, and maintaining the gut barrier function. The probiotic is recommended as a novel strategy to minimize aquaculture HMs toxicity and safe human health.

Potential mechanism of humic acid attenuating toxicity of Pb2+ and Cd2+ in Vallisneria natans

Humic substances are widely present in aquatic environments. Due to the high affinity of humic substances for metals, the interactions have been particularly studied. To assess the effect of humic acid (HA) on submerged macrophytes and biofilms exposed to heavy metal stress, Vallisneria natans was exposed to solutions containing different concentrations of HA (0.5-2.0 mg·L^-1), Pb²⁺ (1 mg·L^-1) and Cd²⁺ (1 mg·L^-1). Results suggested that HA positively affected the plant growth and alleviated toxicity by complexing with metals. HA increased the accumulation of metals in plant tissues and effectively induced antioxidant responses and protein synthesis. It was also noted that the exposure of HA and metals promoted the abundance and altered the structure of microbial communities in biofilms. Moreover, the positive effects of HA were considered to be related to the expression of related genes resulting from altered DNA methylation levels, which were mainly reflected in the altered type of demethylation. These results demonstrate that HA has a protective effect against heavy metal stress in Vallisneria natans by inducing effective defense mechanisms, altering biofilms and DNA methylation patterns in aquatic ecosystems.

Sediment and residual feed from aquaculture water bodies threaten aquatic environmental ecosystem: Interactions among algae, heavy metals, and nutrients

The effect of sediment and residual fish feed on aquaculture water bodies has gained increasing attention to alleviate the eutrophication and heavy metals enrichment induced by aquaculture. Thus, this study intended to reveal the possible interactions among nutrients, heavy metals, and Chlorella vulgaris (C. vulgaris) in aquaculture water bodies containing fish feed and sediment. The analyses showed that consistent with the composition of heavy metals in fish feed, manganese (Mn) and zinc (Zn) accounted for the highest proportions (68-78%) of heavy metals in sediment. Meanwhile, sediment in the centre of aquaculture water bodies (S2) contained more heavy metals than those in the perimeter (S1), but the released concentrations and rates (R_release) of heavy metals from S1 were higher than those from S2. Moreover, the biomass, growth rate, specific growth rate, and nitrogen and carbon fixation rate of C. vulgaris increased with adding fish feed, whereas superoxide dismutase (SOD) and malondialdehyde (MDA) decreased. In addition, with C. vulgaris, influenced by the release process from sediment and the uptake by C. vulgaris, the concentration and R_release of Mn, Pb, Cu, Mn, Cr and Cd from sediments coexisting with fish feed in water first increased and then decreased in general. The C. vulgaris biomass was significantly negatively related to Mn, Pb, Cu, Ni, Cr, and Cd and PO₄^3-P (P < 0.05), which was caused by the uptake of C. vulgaris and indicated that C. vulgaris biomass is easily affected by these factors. Accordingly, the input of residual fish feed and sediment should be controlled.

Methane production from peroxymonosulfate pretreated algae biomass: Insights into microbial mechanisms, microcystin detoxification and heavy metal partitioning behavior

This study investigated the methane production potential of algal biomass by anerobic digestion with the addition of peroxymonosulfate (PMS), the removal of microcystin were analyzed and discussed. The microcystin concentration in the collected algal sludge was 1.20 μg/L in the liquid phase and 1393 μg/g in the algal sludge before anaerobic fermentation. The microcystin concentration decreased to 0.20-0.35 μg/L in the liquid phase and 4.16-11.51 μg/g in the sludge phase after 60 days of digestion. The initial PMS dose and residue microcystin concentration could be simulated with a logarithmic decay model (R² > 0.87). Anaerobic digestion could recover energy from algal source in the form of methane gas, which was not affected in the presence of microcystin, and the microcystin removal rate was >99%. Digestion decreased the total contents of Cd and Zn in the liquid phase and increased the total contents of Cr and Pb in the liquid phase. The microbial community and function prediction results indicated that the PMS0.1 system had the highest methane production, which was attributed to the high abundance of Mechanosaeta (40.52%). This study provides insights into microbial mechanisms, microcystin detoxification and the heavy metal partitioning behavior of the algal biomass during methane production.

Effect of application mode (capping and amendment) on the control of cadmium release from sediment by apatite/calcite mixture and its phosphorus release risk

In this research, the influence of application mode (capping and amendment) on the control of cadmium (Cd) liberation from sediment by apatite/calcite mixture and its phosphorus release risk were investigated. The results showed that calcite addition had a limited effect on the speciation of Cd in sediment, but apatite addition had a significant impact on the fractionation of Cd in sediment. Apatite amendment could effectively immobilize the most readily mobilized Cd by transferring the acid-soluble fraction to the reducible and residual fractions. Apatite addition also could effectively reduce the concentration of toxicity characteristic leaching procedure (TCLP)-leachable Cd in sediment, and apatite had a much higher reduction efficiency of TCLP-leachable Cd than calcite. Apatite/calcite mixture capping could reduce the risk of Cd liberation from sediment into the overlying water, and the controlling efficiency of apatite/calcite mixture capping was higher than that of apatite/calcite mixture amendment. The effect of apatite/calcite mixture addition on the concentration of reactive soluble phosphorus (SRP) in the overlying water was limited. The introduction of calcite into the apatite capping layer could lower the risk of phosphorus release from apatite to the overlying water as compared to single apatite capping. However, the apatite/calcite mixture capping layer still had a relatively high risk of phosphorus liberation into the overlying water. Results of this work suggest that apatite/calcite mixture has a high potential to be used as a capping material to control Cd release from sediment from the perspective of controlling efficiency and application convenience.

Toxic effect of nickel on microalgae Phaeodactylum tricornutum (Bacillariophyceae)

Nickel acts as an essential trace nutrient or toxicant for organisms, depending on its concentration. The increased concentrations of nickel, due to anthropogenic activity, in the aquatic environment are potential threats to aquatic organisms. However, the knowledge on toxic mechanisms of nickel to microalgae remains incompletely understood. In the present study, we investigated the toxic effects of nickel in the cosmopolitan diatom Phaeodactylum tricornutum via evaluation of physiological and transcriptome responses. The results showed that the median effective concentration-72 h (EC₅₀-72 h) and EC₅₀-96 h of nickel was 2.48 ± 0.33 and 1.85 ± 0.17 mg/L, respectively. The P. tricornutum cell abundance and photosynthesis significantly decreased by 1 mg/L of nickel. Results from photosynthetic parameters including efficiency of the oxygen evolving complex (OEC) of photosystem II (PSII) (Fv/F₀), maximum photosynthetic efficiency of PS II (Fv/Fm), electron transport rate (ETR), actual photosynthetic efficiency of PS II (Y(II)), non-photochemical quenching (NPQ), and photochemical quenching (qP) indicated that OEC of PS II might be impaired by nickel. The transcriptome data also reveal that OEC apparatus coding gene PS II oxygen-evolving enhancer protein 2 (PsbP) was regulated by nickel. Moreover, induced reactive oxygen species (ROS) production and chlorophyll a content were also detected under nickel stress. Transcriptome analysis revealed that nickel affected a variety of differentially expressed genes (DEGs) that involved in redox homeostasis, nitrogen metabolisms, fatty acids, and DNA metabolism. However, thiol-disulfide redox system might play important roles in nickel-induced oxidative stress resistance. This study improved the understanding of the toxic effect of nickel on the diatom P. tricornutum.

Cultivating Scenedesmus dimorphus in lactic acid wastewater for cost-effective biodiesel production

The combination of lactic acid production wastewater and oil-producing microalgal culture could not only achieve harmless treatment of wastewater but also provided nutrients and significant amounts of water for microalgal culture. Thus the effects of different nutrients on the biomass yield, lipid yield of Scenedesmus dimorphus with lactic acid wastewater were investigated. Although lactic acid wastewater was very suitable for the cultivation of oil-producing microalgae, some nutrients were still needed. So 0.79 g/L NaNO₃, 14 mg/L MgSO₄·7H₂O, 4 mg/L K₂HPO₄·3H₂O, and trace elements needed to be added in the microalgal culture with lactic acid wastewater. In the optimized wastewater medium, the lipid yield could reach 1.54 ± 0.04 g/L, which was 48.1% higher than the level of 1.04 ± 0.06 g/L in the BG11 medium. Microalgae cells had high absorption capacity for nitrogen and phosphorus. The nitrogen, phosphorus removal rate of wastewater reached 96.31% and 90.78%, respectively, after 10 days of culture. And the treated wastewater could be used for lactic acid production for four times. These investigations laid a foundation for reducing the pollution of lactic acid wastewater, exploring a late-model for oleaginous microalgae cleaner production.

Artificial sweeteners in end-use biosolids in Australia

Artificial sweeteners are contaminants of emerging concern that can enter the aquatic and terrestrial environments via wastewater effluent discharge and the environmental application of biosolids. The release of artificial sweeteners from the use of biosolids in Australia was assessed. The concentration of seven artificial sweeteners was quantified in biosolids samples collected from 71 wastewater treatment plants (WWTPs) across Australia during Census 2016. Sucralose, saccharin, acesulfame, aspartame and cyclamate were detected in biosolids samples at median concentrations ranging from 0.18 ng/g (dry weight) (range: <LOQ-34 ng/g) for cyclamate to 220 ng/g (range: <LOQ -3,670 ng/g) for sucralose, while neotame and neohesperidin dihydrochalcone were not detected. The relationship between the concentration of artificial sweeteners in biosolids and moisture content was assessed with the concentration of artificial sweeteners decreasing as dewatering time increased in a biosolids drying hall. The geometric means (± standard deviation) for per capita loads of individual artificial sweeteners ranged from 8.7 (1.6, 48) µg year^-1 person^-1 for cyclamate to 4,000 (1,000, 15,000) µg year^-1 person^-1 for sucralose with 223 kg of artificial sweeteners released to terrestrial environment from biosolids end-use annually in Australia. Due to the low loads of artificial sweeteners in biosolids compared with wastewater effluent, risks associated with artificial sweeteners in biosolids are likely limited.