Effects of graphene oxide on algal cellular stress response: Evaluating metabolic characters of carbon fixation and nutrient removal

Low-frequency ultrasound assisted contact-electro-catalysis for efficient inactivation of Microcystis aeruginosa

Frequent cyanobacterial blooms pose a serious threat to the aquatic ecosystem and human health, so developing an efficient algae removal method is a long-term goal for bloom management. Current technologies for algal bloom control need urgent improvement in terms of algicide recovery, eco-friendliness and cost. Here we propose a contact-electro-catalytic method, using polytetrafluoroethylene (PTFE) film as a reusable catalyst. This contact-electro-catalytic approach involves the generation of reactive oxygen species (e.g., O2•-, HO•, 1O2 and H2O2) through water-PTFE contact electrification under the low-frequency ultrasonic waves, facilitating the inactivation of algae. The removal rate of the cyanobacterium Microcystis aeruginosa (M. aeruginosa) exposured to the water-PTFE contact-electro-catalytic system is almost five times greater than that of ultrasound alone after 5 h. A mechanistic investigation revealed that the contact-electro-catalytic system damaged the photosynthetic activity, antioxidant system and membrane integrity of the cells. Additionally, LC-MS metabolomic analysis indicated that this system caused substantial significant disruptions in the TCA cycle, amino acid metabolism, purine metabolism and phospholipid metabolism. Three-dimensional fluorescence spectroscopy suggested contact-electro-catalysis could further availably degrade the organic matter. We anticipate that this method can provide an eco-friendly, highly efficient and economic approach for effective control of harmful algal blooms.

The impact of nTiO2 and GO (graphene oxide), and their combinations, on freshwater Chlorella sp.: a comparative study in lake water and BG-11 media

Titanium dioxide nanoparticles (nTiO2) and graphene oxide (GO) are extensively used nanomaterials in various products and applications. Freshwater ecosystems are a crucial sink for these pollutants, posing severe threats to aquatic organisms. Although multiple studies have investigated the pristine toxicity of nTiO2 and GO in freshwater organisms, the combined toxicity of these materials remains unexplored. Interaction media is a crucial factor in evaluating toxicity nanomaterial toxicity towards algae. In this study, we have investigated the comparative effect of sterilized and filtered freshwater and BG-11 medium on the pristine and combined toxicity of nTiO2 and GO on freshwater algae Chlorella sp. Results indicated that the combination of nTiO2 and GO showed more toxicity when compared to their respective pristine forms. This could be due to the additive effect exhibited by nTiO2 and GO on Chlorella sp. The enhanced growth inhibition for the combined toxicity was in the order of 1 mg L-1 nTiO2 + 1 mg L-1 GO > 1 mg L-1 nTiO2 + 0.1 mg L-1 GO > 0.1 mg L-1 nTiO2 + 1 mg L-1 GO > 0.1 mg L-1 nTiO2 + 0.1 mg L-1 GO. All test groups that interacted in BG-11 media exhibited less toxicity when compared to corresponding groups in the lake water medium. This could be attributed to the cushioning effect of BG-11 medium, providing supplementary nutrition to the algal cells. This signifies that the environmentally relevant conditions could be more detrimental than the laboratory conditions. This study elucidates valuable insights into the potential detrimental effects associated with the combination of nTiO2 and GO on freshwater algae. Furthermore, we have evaluated the growth inhibition, oxidative stress, and photosynthetic activity of Chlorella sp. in both environmentally relevant interaction medium and well-defined culture medium.

Exploring cell aggregation as a defense strategy against perchlorate stress in Chlamydomonas reinhardtii through multi-omics analysis

Perchlorate (ClO4-) is a type of novel, widely distributed, and persistent inorganic pollutant. However, the impacts of perchlorate on freshwater algae remain unclear. In this study, the response and defense mechanisms of microalgae (Chlamydomonas reinhardtii) under perchlorate stress were investigated by integrating physiological and biochemical monitoring, transcriptomics, and metabolomics. Weighted gene co-expression network analysis (WGCNA) of transcriptome data was used to analyze the relationship between genes and phenotype and screen the key pathways. C. reinhardtii exhibited aggregate behavior when exposed to 100- and 200-mM perchlorate but was restored to its unicellular lifestyle when transferred to fresh medium. WGCNA results found that the "carbohydrate metabolism" and "lipid metabolism" pathways were closely related to cell aggregation phenotype. The differential expression genes (DEGs) and differentially accumulated metabolites (DAMs) of these pathways were upregulated, indicating that the lipid and carbohydrate metabolisms were enhanced in aggregated cells. Additionally, most genes and metabolites related to phytohormone abscisic acid (ABA) biosynthesis and the mitogen-activated protein kinase (MAPK) signaling pathway were significantly upregulated, indicating their crucial roles in the signal transmission of aggregated cells. Meanwhile, in aggregated cells, extracellular polymeric substances (EPS) and lipid contents increased, photosynthesis activity decreased, and the antioxidant system was activated. These characteristics contributed to C. reinhardtii's improved resistance to perchlorate stress. Above results demonstrated that cell aggregation behavior was the principal defense strategy of C. reinhardtii against perchlorate. Overall, this study sheds new light on the impact mechanisms of perchlorate to aquatic microalgae and provides multi-omics insights into the research of multicellular-like aggregation as an adaptation strategy to abiotic stress. These results are beneficial for assessing the risk of perchlorate in aquatic environments.

Mineralization of Few-Layer Graphene Made It Bioavailable in Chlamydomonas reinhardtii

Numerous studies have emphasized the toxicity of graphene-based nanomaterials to algae, however, the fundamental behavior and processes of graphene in biological hosts, including its transportation, metabolization, and bioavailability, are still not well understood. As photosynthetic organisms, algae are key contributors to carbon fixation and may play an important role in the fate of graphene. This study investigated the biological fate of 14C-labeled few-layer graphene (14C-FLG) in Chlamydomonas reinhardtii (C. reinhardtii). The results showed that 14C-FLG was taken up by C. reinhardtii and then translocated into its chloroplast. Metabolomic analysis revealed that 14C-FLG altered the metabolic profiles (including sugar metabolism, fatty acid, and tricarboxylic acid cycle) of C. reinhardtii, which promoted the photosynthesis of C. reinhardtii and then enhanced their growth. More importantly, the internalized 14C-FLG was metabolized into 14CO2, which was then used to participate in the metabolic processes required for life. Approximately 61.63%, 25.31%, and 13.06% of the total radioactivity (from 14CO2) was detected in carbohydrates, lipids, and proteins of algae, respectively. Overall, these results reveal the role of algae in the fate of graphene and highlight the potential of available graphene in bringing biological effects to algae, which helps to better assess the environmental risks of graphene.

Toxicity mechanisms of graphene oxide and cadmium in Microcystis aeruginosa: evaluation of photosynthetic and oxidative responses

The potential ecotoxicological hazard of gaphene oxide (GO) is not fully clarified for photoautotrophic organisms, especially when the interactions of GO with other environmental toxicants are considered. The objective of the current study was to better understand the mechanisms of toxicity of GO in the cyanobacteria Microcystis aeruginosa, and to identify its interactions with cadmium (Cd). The individual and combined contribution of both pollutants in cyanobacteria were evaluated after 96 hours of exposure to GO and/or Cd, using photosynthetic pigments, photosynthetic parameters, cellular indicators of peroxidative damage, viability, and intracellular ROS formation as indicators of toxicity. Interactions between GO and Cd were evaluated using Toxic Units based on the EC50 of each parameter evaluated. The results of this study indicate that single concentrations ≥ 5 µg mL-1 of GO and ≥ 0.1 µg mL-1 of Cd induced a decrease in cell biomass and a change in the photosynthetic parameters associated with primary productivity in M. aeruginosa. In the combined experiments, higher GO ratios (≥ 9.1 µg mL-1) in terms of Toxic Units decreased photochemical processes and cellular metabolism, increased oxidative stress, and ultimately affected the size of M. aeruginosa. Finally, the relationship between GO concentration, Cd concentration, and the adsorption capacity of GO with respect to the co-pollutant must be taken into account when assessing the environmental risk of GO in aquatic environments.

New insight into the biocompatibility/toxicity of graphene oxides and their reduced forms on Chlamydomonas reinhardtii

Graphene oxides (GOs) and their reduced forms are often discussed both positively and negatively due to the lack of information about their chemistry and structure. This study utilized GOs with two sheet sizes that were further reduced by two reducing agents (sodium borohydride and hydrazine) to obtain two different degrees of reduction. The synthesized nanomaterials were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), elemental analysis (EA), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy (RA) to understand their chemistry and structure. The second focus of our investigation included in vitro testing of the biocompatibility/toxicity of these materials on a model organism, the freshwater microalga Chlamydomonas reinhardtii. The effects were studied on the basis of biological endpoints complemented by biomass investigation (FTIR spectroscopy, EA, and atomic absorption spectrometry (AAS)). The results showed that the biocompatibility/toxicity of GOs is dependent on their chemistry and structure and that it is impossible to generalize the toxicity of graphene-based nanomaterials.

Deciphering the toxicity mechanism of haloquinolines on Chlorella pyrenoidosa using QSAR and metabolomics approaches

The hazardous potential of haloquinolines (HQLs) is becoming an issue of great concern due to its wide and long-term usage in many personal care products. We examined the growth inhibition, structure-activity relationship, and toxicity mechanism of 33 HQLs on Chlorella pyrenoidosa using the 72-h algal growth inhibition assay, three-dimensional quantitative structure-activity relationship (3D-QSAR), and metabolomics. We found that the IC₅₀ (half maximal inhibitory concentration) values for 33 compounds ranged from 4.52 to > 150 mg·L^-1, most tested compounds were toxic (1 mg·L^-1 < IC₅₀ < 10 mg·L^-1) or harmful (10 mg·L^-1 < IC₅₀ < 100 mg·L^-1) for the aquatic ecosystem. Hydrophobic properties of HQLs dominate their toxicity. Halogen atoms with large volume appear at the 2, 3, 4, 5, 6, and 7-positions of the quinoline ring to significantly increase the toxicity. In algal cells, HQLs can block diverse carbohydrates, lipids, and amino acid metabolism pathways, thereby resulting in energy usage, osmotic pressure regulation, membrane integrity, oxidative stress disorder, thus fatally damaging algal cells. Therefore, our results provide insight into the toxicity mechanism and ecological risk of HQLs.

The distinct resistance mechanisms of cyanobacteria and green algae to sulfamethoxazole and its implications for environmental risk assessment

Cyanobacteria and green algae are the OECD recommended test organisms for environmental toxicity assessments of chemicals. Whether the differences in these two species' responses to the identical chemical affect the assessment outcomes is a question worth investigating. Firstly, we investigated the distinct resistance mechanisms of Synechococcus sp. (cyanobacteria) and R. subcapitata (green algae) to sulfamethoxazole (SMX). The antioxidant system analysis demonstrated that R. subcapitata mainly relies on enhancing the activity of first line defense antioxidants, including superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx), which is the most powerful and efficient response to get rid of ROS, whereas Synechococcus sp. depends upon increasing the activity of glutathione-S-transferase (GST) and GPx to resist oxidative stress. Besides, a total 7 transformation products (TPs) of SMX were identified in R. subcapitata culture medium. The analysis of conjectural transformation pathways and the predicted toxicity indicates that R. subcapitata could relieve SMX toxicity by degrading it to low eco-toxic TPs. Additionally, we summarized numerous exposure data and assessed the environmental risk of various antibiotics, revealing an inconsistent result for the same type of antibiotic by using cyanobacteria and green algae, which is most likely due to the different resistance mechanisms. In the future, modified indicators or comprehensive assessment methods should be considered to improve the rationality of environmental toxicity assessments.

Elucidating the mechanisms underlying the cytotoxic effects of nano-/micro-sized graphene oxide on the microalgae by comparing the physiological and morphological changes in different trophic modes

Understanding the cytotoxic mechanisms of environmental contaminants is important to estimate their environmental impacts and prepare guidelines for pollution control. Many studies have assessed the cytotoxic mechanisms of graphene oxide (GO), an emerging aquatic contaminant. However, in many cases, the effect of GO size and putative trophic modes of microalgae on cytotoxicity has been neglected, hindering complete understanding of the cytotoxic mechanisms of GO. In this study, the microalga Euglena gracilis cultivated under light (phototrophic) or dark (heterotrophic) conditions was exposed to two sizes of GO [nano-sized (N) and micro-sized (M)] for assessing the effect of GO on microalgal growth. The cytotoxic effect of GO was higher under phototrophic conditions than under heterotrophic conditions, suggesting that a major cytotoxic mechanism of GO is related to photosynthetic activity inhibition. Moreover, N-GO showed higher toxicity than M-GO. The morphological and physiological changes in N-GO- and M-GO-exposed E. gracilis were assessed to further elucidate the cytotoxic mechanisms. N-GO internalized the cells via endocytic activity/piercing, whereas M-GO partially attached to the cell surface and did not enter the cells. Moreover, N-GO and M-GO negatively affected the cells by inducing oxidative stress; the oxidative stress parameters were higher in N-GO-exposed cells than in M-GO-exposed cells.

Efficient inhibition of cyanobacteria M. aeruginosa growth using commercial food-grade fumaric acid

The control of cyanobacteria blooms is a global challenge. Here, we reported the efficient inhibition of M. aeruginosa by fumaric acid (FA), an intermediate metabolite of the tricarboxylic acid cycle. FA showed strong algicidal activity with an inhibition rate of 90.5% on the 8th day at a dose of 40 mg/L. The presence of FA caused severe membrane damage, as suggested by the fluorescence flow cytometry and morphology analysis. FA inhibited the formation of chlorophyll a, interrupting the photosynthesis system. It also induced oxidative stress in cells. Principal component analysis of the indicators suggested that the FA-treated sample had a significantly different inhibitory pattern than the acid-treated sample. Thus, the inhibitory effect was not solely caused by the pH effect. Untargeted metabolomic analysis revealed that 31 metabolites were differentially expressed in response to FA stress, which were mainly involved in the metabolite processes and the membranes. A commercial food-grade FA was able to inhibit the growth of M. aeruginosa similar to the analytical-grade FA. Our results suggest that FA can be potentially an efficient and low-risk chemical for inhibiting M. aeruginosa growth, which may find future applications in cyanobacteria bloom control.

Humic acids alleviate the toxicity of reduced graphene oxide modified by nanosized palladium in microalgae

The use of graphene-family materials modified by nanosized palladium (Pd/GFMs) has intensified rapidly in various fields; however, the effects of environmental factors (e.g., natural organic matter (NOM)) on the transformation and ecotoxicity of Pd/GFMs remain largely unknown. In this study, reduced graphene oxide modified by nanosized Pd (Pd/rGO) was incubated with humic acid (HA) under light irradiation for 56 d to explore the effects of NOM on the physicochemical transformations (e.g., defects, surface charges and dispersity) and biological toxicity (e.g., growth inhibition, oxidative stress and ultrastructural damage on algae cells) of Pd/GFMs. The results revealed that HA increased the defects and dispersity of Pd/rGO. Growth inhibition, damage to cellular ultrastructures, and oxidative stress in microalgae cells were induced by Pd/rGO, and corresponding defense responses (e.g., superoxide dismutase, peroxidase and glutathione) were activated. HA diminished the above defense responses in microalgae triggered by Pd/rGO by regulating GSH metabolism and the alanine biosynthesis pathway. In the presence of HA, cell wall damage (i.e., hole formation) caused by exposure to Pd/rGO was restored, and the plasmolysis area was reduced by 28.6 %. In addition, growth inhibition, lipid peroxidation, loss of mitochondrial membrane potential and ROS formation induced by 1.0 mg/L MoS₂NPs were decreased by 1.4-65.6 %, 13.9-26.1 %, 21.8-58.3 % and 9.6-16.1 %, respectively. These findings highlight the need to consider the effects of HA on the environmental transformation and biological toxicity of Pd/GFMs, which presents significant implications for the management of Pd/GFMs.

Increasing oxytetracycline and enrofloxacin concentrations on the algal growth and sewage purification performance of an algal-bacterial consortia system

Oxytetracycline (OTC) and enrofloxacin (EFX) pollution in surface water are very common. Using the algal-bacterial consortia system to remove antibiotics remains to be further studied. In this study, the algal growth and sewage purification performance were studied in an algal-bacterial consortia system with different concentrations of antibiotics. The enzyme activity, malondialdehyde content, chlorophyll-a content, extracellular polysaccharide, and protein content of algae were also tested. It was found that the algal growth was promoted by low-dose antibiotics, 21.83% and 22.11% promotion at 0.1 mg L^-1 OTC and EFX, respectively. The nutrients and antibiotics removals of the low-dose groups (OTC <5 mg L^-1, EFX <1 mg L^-1) were not affected significantly. More than 70% of total organic carbon and total phosphorus, and 97.84-99.76% OTC, 42.68-42.90% EFX were removed in the low-dose groups. However, the algal growth was inhibited, and the nutrients removals performance also declined in the high-concentration groups (10 mg L^-1 OTC, 5 mg L^-1 EFX). The superoxide dismutase and catalase activity, and malondialdehyde content increased significantly (P < 0.05), indicating the increased activity of reactive oxygen species. In addition, the decreased chlorophyll-a content, thylakoid membrane deformation, starch granules accumulation, and plasmolysis showed that the algal physiological functions were affected. These results showed that the algal-bacterial consortia system was more suitable to treat low-concentration antibiotics and provided basic parameters for the consortia application.

Performance and responses of aerobic granular sludge at different concentrations of graphene oxide after a single administered dose

To investigate the impact of graphene oxide (GO) under different concentrations (0, 50, 100, 150, and 200 mg/L) on aerobic granular sludge (AGS) after a single administered dose, the performance of nitrogen removal, microbial enzymatic activity, extracellular polymeric substances (EPSs), and microbial community structure was analyzed in batch tests. The results showed that the impact of GO concentrations on AGS was dose- and time-dependent. Short-term GO exposure could accelerate the nitrification process of AGS, while relatively concentrations (≥100 mg/L) inhibited the process when present for extended periods of time. The microbial enzymatic activity showed similar tendency. The production of lactate dehydrogenase release (LDH) in 200 mg/L group was increased 48.04% and EPS contents decreased 30.06% compared to the control group at 30th day and showed that high concentrations of GO have toxic effects on AGS. The microbial bacteria responded differently to the stimulation of different concentrations of GO. PRACTITIONER POINTS: GO affected AGS system performance in concentration- and time-dependent manners. The nitrification rate of AGS increased in the short term and reversed over time. Long-term exposure to high GO concentrations caused toxicity to AGS. Different microorganisms had diverse responses to GO concentrations.

Beverages containing Lactobacillus paracasei LC-37 improved functional dyspepsia through regulation of the intestinal microbiota and their metabolites

Functional dyspepsia (FD) is a common disease of the digestive system and probiotics have been reported to be effective in the treatment of functional gastrointestinal diseases. The aim of this study was to determine the effect of the beverage containing Lactobacillus paracasei LC-37 (LC-37) and its ability to relieve symptoms of FD by a randomized clinical trial. Due to the mechanistic complexity underlying FD, intestinal microbiota and stool metabolomes were also evaluated. The results showed that FD was relieved in participants after treatment with the beverage containing LC-37 for 14 and 28 d. The clinical symptom scores were significantly decreased after these times (abdominal pain and belching were significantly decreased after 14 d and almost absent after 28 d of treatment). Probiotics, such as those containing the Lactobacillus, Lactococcus, and Weissella, significantly increased, and the abundance of harmful bacteria such as Lachnocliostridium significantly decreased. Furthermore, relevant beneficial intestinal metabolites such as pelargonic acid, benzoic acid, and short-chain fatty acids increased, and harmful intestinal metabolites such as hippuric acid decreased. Taken together, these findings suggested that the beverages containing LC-37 can increase the abundance of probiotics and decrease pathogenic bacteria, and thereby improve beneficial intestinal metabolites. Therefore, the beverages containing LC-37 may provide a natural alternative to combat FD.