Bioaccumulation of Perfluoroalkyl Substances by <i>Daphnia magna</i> in Water with Different Types and Concentrations of Protein

Perfluoroalkyl substances (PFASs) are sometimes regarded as proteinophilic compounds, however, there is no research report about the effect of environmental protein on the bioaccumulation of PFASs in waters. In the present study we investigated influences of protein on the bioaccumulation of six kinds of PFASs by <i>Daphnia magna</i> in water; it included perfluorooctane sulfonate, perfluorooctanoic acid, perfluorononanoic acid, perfluorodecanoic acid, perfluoroundecanoic acid, and perfluorododecanoic acid. Two types of protein including bovine albumin from animal and soy peptone from plant were compared and the effects of protein concentration were investigated. Both types of protein at high concentrations (10 and 20 mg L^–1) suppressed the bioaccumulation of PFASs. When protein concentration increased from 0 to 20 mg L^–1, the decreasing ratios of the PFAS body burden (35.3–52.9%) in <i>Daphnia magna</i> induced by bovine albumin were significantly higher than those (22.0–36.6%) by soy peptone. The dialysis bag experiment results showed that the binding of PFASs to protein followed the Freundlich isotherm, suggesting it is not a linear partitioning process but an adsorption-like process. The partition coefficients of PFASs between bovine albumin and water were higher compared to soy peptone; this resulted in higher reducing rates of freely dissolved concentrations of PFASs with increasing bovine albumin concentration, leading to a stronger suppression of PFAS bioaccumulation. However, the presence of both types of protein with a low concentration (1 mg L^–1) enhanced the bioaccumulation of PFASs. Furthermore, the water-based bioaccumulation factor based on the freely dissolved concentrations of PFASs even increased with and the depuration rate constants of PFASs from <i>Daphnia magna</i> decreased with protein concentration, suggesting that protein would not only reduce the bioavailable concentrations and uptake rates of PFASs but also lower the elimination rates of PFASs in <i>Daphnia magna</i>. Because these two opposite effects would change with different protein concentrations in water, the net effect of protein on PFAS bioaccumulation would also vary with protein concentration

Modification of Fatty Acids in Membranes of Bacteria: Implication for an Adaptive Mechanism to the Toxicity of Carbon Nanotubes

We explored whether bacteria could respond adaptively to the presence of carbon nanotubes (CNTs) by investigating the influence of CNTs on the viability, composition of fatty acids, and cytoplasmic membrane fluidity of bacteria in aqueous medium for 24 h exposure. The CNTs included long single-walled carbon nanotubes (L-SWCNTs), short single-walled carbon nanotubes (S-SWCNTs), short carboxyl single-walled carbon nanotubes (S-SWCNT-COOH), and aligned multiwalled carbon nanotubes (A-MWCNTs). The bacteria included three common model bacteria, <i>Staphyloccocus aureus</i> (Gram-positive), <i>Bacillus subtilis</i> (Gram-positive), and <i>Escherichia coli</i> (Gram-negative), and one polybrominated diphenyl ether degrading strain, <i>Ochrobactrum</i> sp. (Gram-negative). Generally, L-SWCNTs were the most toxic to bacteria, whereas S-SWCNT-COOH showed the mildest bacterial toxicity. <i>Ochrobactrum</i> sp. was more susceptible to the toxic effect of CNTs than <i>E. coli</i>. Compared to the control in the absence of CNTs, the viability of <i>Ochrobactrum</i> sp. decreased from 71.6−81.4% to 41.8–70.2%, and <i>E. coli</i> from 93.7−104.0% to 67.7–91.0% when CNT concentration increased from 10 to 50 mg L^–1. The cytoplasmic membrane fluidity of bacteria increased with CNT concentration, and a significant negative correlation existed between the bacterial viabilities and membrane fluidity for <i>E. coli</i> and <i>Ochrobactrum</i> sp. (<i>p</i> < 0.05), indicating that the increase in membrane fluidity induced by CNTs was an important factor causing the inactivation of bacteria. In the presence of CNTs, <i>E. coli</i> and <i>Ochrobactrum</i> sp. showed elevation in the level of saturated fatty acids accompanied with reduction in unsaturated fatty acids, compensating for the fluidizing effect of CNTs. This demonstrated that bacteria could modify their composition of fatty acids to adapt to the toxicity of CNTs. In contrast, <i>S. aureus</i> and <i>B. subtilis</i> exposed to CNTs increased the proportion of branched-chain fatty acids and decreased the level of straight-chain fatty acids, which was also favorable to counteract the toxic effect of CNTs. This study suggests that the bacterial tolerances to CNTs are associated with both the adaptive modification of fatty acids in the membrane and the physicochemical properties of CNTs. This is the first report about the physiologically adaptive response of bacteria to CNTs, and may help to further understand the ecotoxicological effects of CNTs

How Does Predation Affect the Bioaccumulation of Hydrophobic Organic Compounds in Aquatic Organisms?

It is well-known that the body burden of hydrophobic organic compounds (HOCs) increases with the trophic level of aquatic organisms. However, the mechanism of HOC biomagnification is not fully understood. To fill this gap, this study investigated the effect of predation on the bioaccumulation of polycyclic aromatic hydrocarbons (PAHs), one type of HOC, in low-to-high aquatic trophic levels under constant freely dissolved PAH concentrations (1, 5, or 10 μg L^–1) maintained by passive dosing systems. The tested PAHs included phenanthrene, anthracene, fluoranthene, and pyrene. The test organisms included zebrafish, which prey on <i>Daphnia magna</i>, and cichlids, which prey on zebrafish. The results revealed that for both zebrafish and cichlids, predation elevated the uptake and elimination rates of PAHs. The increase of uptake rate constant ranged from 20.8% to 39.4% in zebrafish with the amount of predation of 5 daphnids per fish per day, and the PAH uptake rate constant increased with the amount of predation. However, predation did not change the final bioaccumulation equilibrium; the equilibrium concentrations of PAHs in fish only depended on the freely dissolved concentration in water. Furthermore, the lipid-normalized water-based bioaccumulation factor of each PAH was constant for fish at different trophic levels. These findings infer that the final bioaccumulation equilibrium of PAHs is related to a partition between water and lipids in aquatic organisms, and predation between trophic levels does not change bioaccumulation equilibrium but bioaccumulation kinetics at stable freely dissolved PAH concentrations. This study suggests that if HOCs have not reached bioaccumulation equilibrium, biomagnification occurs due to enhanced uptake rates caused by predation in addition to higher lipid contents in higher trophic organisms. Otherwise, it is only due to the higher lipid contents in higher trophic organisms

Coupled Nitrification-Denitrification Caused by Suspended Sediment (SPS) in Rivers: Importance of SPS Size and Composition

Suspended sediment (SPS) is ubiquitous in rivers, and SPS with different particle sizes and compositions may affect coupled nitrification-denitrification (CND) occurring on SPS significantly. However, there is no related research report. In this work, ¹⁵N isotope tracer technique was adopted to explore the CND in systems containing SPS (8 g L^–1 and 1 g L^–1) collected from the Yellow River with various particle sizes, including <2, 2–20, 20–50, 50–100, and 100–200 μm. The results showed that the CND occurred on SPS and the CND rate was negatively related to particle size; both nitrification and denitrification rate constants increased with decreasing SPS particle size. For instance, SPS (8 g L^–1) with a particle size below 2 μm had the highest ¹⁵N₂ emission rate of 1.15 mg-N/(m³·d), which was 2.9 times that of 100–200 μm. This is because SPS with a smaller particle size had a larger specific surface area and a higher organic carbon content, which is beneficial for bacteria growth. Both the nitrifying and denitrifying bacteria population were positively correlated with CND rate (<i>p</i> < 0.05). Different from the ¹⁵N₂ production, ¹⁵N₂O emission rate did not decrease with increasing SPS particle size. For the system containing 8 g L^–1 SPS, ¹⁵N₂O emission rate reached the highest of 1.05 μg-N/(m³·d) in the 50–100 μm SPS system, which was 17.5 times that of 100–200 μm. Similar results could be found from the system with 1 g L^–1 SPS. This is due to the fact that the oxygen concentration at the SPS-water interface increased with SPS particle size, and the oxygen conditions might be most suitable for the production of N₂O in the 50–100 μm system. This study suggests that SPS size and composition play an important role in nitrogen cycle of river systems, especially for the production of N₂O

Bioavailability of Pyrene Associated with Suspended Sediment of Different Grain Sizes to <i>Daphnia magna</i> as Investigated by Passive Dosing Devices

Suspended sediment (SPS) is widely present in rivers around the world. However, the bioavailability of hydrophobic organic compounds (HOCs) associated with SPS is not well understood. In this work, the influence of SPS grain size on the bioavailability of SPS-associated pyrene to Daphnia magna was studied using a passive dosing device, which maintained a constant freely dissolved pyrene concentration (<i>C</i>_free) in the exposure systems. The immobilization and protein as well as enzymatic activities of Daphnia magna were investigated to study the bioavailability of SPS-associated pyrene. With <i>C</i>_free of pyrene ranging from 20.0 to 60.0 μg L^–1, the immobilization of Daphnia magna in the presence of 1 g L^–1 SPS was 1.11–2.89 times that in the absence of SPS. The immobilization caused by pyrene associated with different grain size SPS was on the order of 50–100 μm > 0–50 μm > 100–150 μm. When pyrene <i>C</i>_free was 20.0 μg L^–1, the immobilization caused by pyrene associated with 50–100 μm SPS was 1.42 and 2.43 times that with 0–50 and 100–150 μm SPS, respectively. The protein and enzymatic activities of Daphnia magna also varied with the SPS grain size. The effect of SPS grain size on the bioavailability of SPS-associated pyrene was mainly due to the difference in SPS ingestion by Daphnia magna and SPS composition, especially the organic carbon type, among the three size fractions. This study suggests that not only the concentration but also the size distribution of SPS should be considered for the development of a biological effect database and establishment of water quality criteria for HOCs in natural waters

Acceleration of Denitrification in Turbid Rivers Due to Denitrification Occurring on Suspended Sediment in Oxic Waters

High suspended sediment (SPS) concentration exists in many rivers of the world. In the present study, the effects of SPS concentration on denitrification were investigated in airtight chambers with sediment samples collected from the Yellow River which is the largest turbid river in the world. Results from the nitrogen stable (¹⁵N) isotopic tracer experiments showed that denitrification could occur on SPS in oxic waters and the denitrification rate increased with SPS concentration; this was probably caused by the presence of low-oxygen microsites in SPS. For the water systems with both bed-sediment and SPS, the denitrification kinetics fit well to Logistic model, and the denitrification rate constant increased linearly with SPS concentration (<i>p</i> < 0.01). The denitrification caused by the presence of SPS accounted for 22%, 38%, 53%, and 67% of the total denitrification in systems with 2.5, 8, 15, and 20 g L^–1 SPS, respectively. The activity of denitrifying bacteria in SPS was approximately twice that in bed-sediment, and the denitrifying bacteria population showed an increasing trend with SPS concentration in both SPS and bed-sediment, leading to the increase of denitrification rate with SPS concentration. Furthermore, the denitrification in bed-sediment was accelerated by increased diffusion of nitrate from overlying water to bed-sediment under agitation conditions, which accompanied with the presence of SPS. When with 8 g L^–1 SPS, approximately 66% of the increased denitrification compared to that without SPS was attributed to denitrification on SPS and 34% to agitation conditions. This is the first report of the occurrence of denitrification on SPS in oxic waters. The results suggest that SPS plays an important role in denitrification in turbid rivers; its effect on nitrogen cycle should be considered in future study

Using ¹⁵N, ¹⁷O, and ¹⁸O To Determine Nitrate Sources in the Yellow River, China

Many previous studies have used δ¹⁵N and δ¹⁸O of nitrate (δ¹⁵N_NO3 and δ¹⁸O_NO3) to determine the nitrate sources in rivers but were subject to substantial uncertainties and limitations, especially associated with evaluating the atmospheric contribution. The Δ¹⁷O of nitrate (Δ¹⁷O_NO3) has been suggested as an unambiguous tracer of atmospheric NO₃^– and may serve as an additional nitrate source constraint. In the present study, triple nitrate isotopes (δ¹⁵N_NO3, Δ¹⁷O_NO3, and δ¹⁸O_NO3) were used for the first time to assess the sources and sinks of nitrate in the Yellow River (YR) basin, which is the second longest river in China. Results showed that the Δ¹⁷O_NO3 of the water from the YR ranged from 0‰ to 1.6‰ during two normal-water seasons. This suggested that unprocessed atmospheric nitrate accounted for 0–7% of the total nitrate in the YR. The corrected δ¹⁵N_NO3 and δ¹⁸O_NO3 values with atmospheric imprints being removed indicated that the main terrestrial sources of nitrate were sewage/manure effluents in the upstream of the YR and manure/sewage effluents and ammonium/urea-containing fertilizer in the middle and lower reaches which made comparable contributions to the nitrate. In addition, there was a significant positive relationship between δ¹⁵N_NO3 and δ¹⁸O_NO3 values of river water (<i>p</i> < 0.01) which may signal the presence of denitrification. This study indicates that the triple nitrate isotope method is useful for assessing the nitrate sources in rivers, especially for the measurements of atmospheric nitrate contribution

Single-Cell Real-Time Visualization and Quantification of Perylene Bioaccumulation in Microorganisms

Bioaccumulation of perylene in <i>Escherichia coli</i> and <i>Staphylococcus aureus</i> was visualized and quantified in real time with high sensitivity at high temporal resolution. For the first time, single-molecule fluorescence microscopy (SMFM) with a microfluidic flow chamber and temperature control has enabled us to record the dynamic process of perylene bioaccumulation in single bacterial cells and examine the cell-to-cell heterogeneity. Although with identical genomes, individual <i>E. coli</i> cells exhibited a high degree of heterogeneity in perylene accumulation dynamics, as shown by the high coefficient of variation (C.V = 1.40). This remarkable heterogeneity was exhibited only in live <i>E. coli</i> cells. However, the bioaccumulation of perylene in live and dead <i>S. aureus</i> cells showed similar patterns with a low degree of heterogeneity (C.V = 0.36). We found that the efflux systems associated with Tol C played an essential role in perylene bioaccumulation in <i>E. coli</i>, which caused a significantly lower accumulation and a high cell-to-cell heterogeneity. In comparison with <i>E. coli</i>, the Gram-positive bacteria <i>S. aureus</i> lacked an efficient efflux system against perylene. Therefore, perylene bioaccumulation in <i>S. aureus</i> was simply a passive diffusion process across the cell membrane

Effects of Chloride Ions on Dissolution, ROS Generation, and Toxicity of Silver Nanoparticles under UV Irradiation

This work investigates the effect of chloride ion (Cl^–) on dissolution, reactive oxygen species (ROS) generation, and toxicity of citrate-coated silver nanoparticles (AgNPs) under UV irradiation. The dissolution rate was decreased by 0.01 M Cl^– due to AgCl passivation on the AgNP surface. By contrast, high concentrations of Cl^– (0.1 or 0.5 M) promoted dissolution due to the formation of soluble Ag–Cl complexes (AgCl_<i>x</i>^1–<i>x</i>). The generation of O₂^•– in the AgNPs/Cl^–/UV system was promoted by 0.01 M Cl^–, whereas it was retarded by 0.1 or 0.5 M Cl^–, which was probably because the aggregation of AgNPs at high ionic strength reduced the nanoparticles’ surface areas for radical formation. Additionally, Cl^– contributed to •OH generation in the AgNPs/Cl^–/UV system, in which the produced •OH concentrations increased with increasing Cl^– concentrations. The reduction reaction between silver ions and O₂^•– resulted in lower dissolution rates of AgNPs/Cl^– mixtures under UV irradiation than those in the dark. The phototoxicity of AgNPs toward <i>E. coli</i> with different concentrations of Cl^– followed the order of 0.5 M > 0 M > 0.1 M > 0.01 M. Both ROS and dissolved Ag played significant role in the phototoxicity of AgNPs. This work demonstrates the potential importance of anions in the fate and biological impact of AgNPs

Differentially Charged Nanoplastics Induce Distinct Effects on the Growth and Gut of Benthic Insects (Chironomus kiinensis) via Charge-Specific Accumulation and Perturbation of the Gut Microbiota

Nanoplastics (NPs), as an emerging contaminant, have usually been found charged in the environment, posing threats to aquatic animals. However, the underlying mechanisms governing the gut toxicity of differentially charged NPs to benthic insects are not well understood. In this study, the gut toxicity in larvae of Chironomus kiinensis exposed to negatively charged NPs (PS-COOH, 50 nm) and positively charged NPs (PS-NH2, 50 nm) at 0.1 and 1 g/kg was investigated through fluorescence imaging, histopathology, biochemical approaches, and 16S rRNA sequencing. The results showed that PS-NH2 caused more adverse effect on the larval growth performance and induced more severe oxidative stress, epithelial damage, and inflammatory responses in the gut than PS-COOH. The stronger impact caused by PS-NH2 was because the gut accumulated PS-NH2 more readily than PS-COOH for its negatively charged cell membrane. In addition, PS-NH2 were less agglomerated compared with PS-COOH, leading to an increased interaction with gut cell membranes and microbiota. Furthermore, alpha diversity and relative abundance of the keystone microbiota related to gut barrier and nutrient absorption were markedly lower exposed to PS-NH2 than PS-COOH, indirectly exacerbating stronger gut and growth damage. This study provides novel insights into the effect mechanisms underlying differentially charged NPs on benthic insects