Determining Biodegradation Kinetics of Hydrocarbons at Low Concentrations: Covering 5 and 9 Orders of Magnitude of Kow and Kaw

Differences in phytoplankton population vulnerability in response to chemical activity of mixtures

Hydrophobic organic contaminants (HOCs) affect phytoplankton at cellular to population levels, ultimately impacting communities and ecosystems. Baseline toxicants, such as some HOCs, predominantly partition to biological membranes and storage lipids. Predicting their toxic effects on phytoplankton populations therefore requires consideration beyond cell uptake and diffusion. Functional traits like lipid content and profile can offer insights into the diverse responses of phytoplankton populations exposed to HOCs. Our study investigated the vulnerability of five phytoplankton species populations to varying chemical activities of a mixture of polycyclic aromatic hydrocarbons (PAHs). Population vulnerability was assessed based on intrinsic sensitivities (toxicokinetic and toxicodynamic), and demography. Despite similar chemical activities in biota within the exposed algae, effects varied significantly. According to the chemical activity causing 50% of the growth inhibition (Ea50), we found that the diatom Phaeodactylum tricornutum (Ea50 = 0.203) was the least affected by the chemical exposure and was also a species with low lipid content. In contrast, Prymnesium parvum (Ea50 = 0.072) and Rhodomonas salina (Ea50 = 0.08), both with high lipid content and high diversity of fatty acids in non-exposed samples, were more vulnerable to the chemical mixture. Moreover, the species P. parvum, P. tricornutum, and Nannochloris sp., displayed increased lipid production, evidenced as 5-10% increase in lipid fluorescence, after exposure to the chemical mixture. This lipid increase has the potential to alter the intrinsic sensitivity of the populations because storage lipids facilitate membrane repair, reconstitution and may, in the short-term, dilute contaminants within cells. Our study integrated principles of thermodynamics through the assessment of membrane saturation (i.e. chemical activity), and a lipid trait-based assessment to elucidate the differences in population vulnerability among phytoplankton species exposed to HOC mixtures.

Whole UVCB tests can yield biotic and abiotic degradation kinetics of known and unknown constituents for an enhanced UVCB degradation profile

The green transition and move towards safe and sustainable-by-design chemicals entail the need for new methods to study the biodegradability of UVCBs (substances of Unknown or Variable composition, Complex reaction products, and Biological materials). Standard simulation biodegradation tests have been developed for single substances and are generally not applicable for UVCBs. The aims of this study were (1) to combine a whole UVCB biodegradation test with a sensitive constituent-specific analytical technique, (2) to measure biotic and abiotic degradation of known and unknown UVCB constituents, and (3) to determine the impact of a wastewater treatment plant (WWTP) discharge on the constituent specific biodegradation in stream water. Lavender oil and black pepper oil are of significance in the perfume and cosmetics industries and served as model UVCBs. Stream water sampled upstream and downstream of a WWTP discharge point was characterized and used as inoculum (i.e., naturally and wastewater-adapted bacterial consortia). Tests were conducted in gastight headspace vials, and automated Arrow Solid Phase Microextraction GC-MS-scan was applied on unopened vials. Peak area ratios between biotic test systems and abiotic controls were used to determine primary biodegradation kinetics, and freshly spiked analytical references to separate biotic from abiotic degradation. Biodegradation half-times were below 20 days for all known (8-12) and unknown constituents (>78) in the essential oils. A dual-column GC-MS analysis produced a level 2 identification of 16 unknown lavender constituents. Biodegradation kinetics were similar in inoculum taken before and after the WWTP outlet, confirming that native stream microorganisms were competent degraders.

Determining Marine Biodegradation Kinetics of Chemicals Discharged from Offshore Oil Platforms─Whole Mixture Testing at High Dilutions Increases Environmental Relevance

Offshore oil platforms discharge enormous volumes of produced water that contain mixtures of petrochemicals and production chemicals. It is crucial to avoid the discharge of particularly those chemicals that are persistent in the marine environment. This study aims to (1) develop a biodegradation testing approach for discharged chemicals by native marine microorganism, (2) determine how dilution affects biodegradation, and (3) determine biodegradation kinetics for many discharged chemicals at low and noninhibitory concentrations. Produced water from an offshore oil platform was diluted in the ratio of 1:20, 1:60, and 1:200 in seawater from the same location and incubated for 60 days at 10 °C. Automated solid-phase microextraction GC-MS was used as a sensitive analytical technique, and chemical-specific primary degradation was determined based on peak area ratios between biotic test systems and abiotic controls. Biodegradation was inhibited at lower dilutions, consistent with ecotoxicity tests. Biodegradation kinetics were determined at the highest dilution for 139 chemicals (43 tentatively identified), and 6 chemicals were found persistent (half-life >60 days). Nontargeted analysis by liquid chromatography-high-resolution MS was demonstrated as a proof-of-principle for a comprehensive assessment. Biodegradation testing of chemicals in discharges provides the possibility to assess hundreds of chemicals at once and find the persistent ones.

Biodegradation of flavonoids - Influences of structural features

Flavonoids, a class of natural products with a variety of applications in nutrition, pharmacy and as biopesticides, could substitute more harmful synthetic chemicals that persist in the environment. To gain a better understanding of the biodegradability of flavonoids and the influence of structural features, firstly, the ultimate biodegradation of 19 flavonoids was investigated with the Closed Bottle Test according to the OECD guideline 301 D. Secondly, regarding the fast abiotic degradation reported for several flavonoids with severe concentration decrease within hours and its possible impacts on the processes behind the ultimate biodegradation, primary degradation of 4 selected flavonoids was compared at conditions representing biodegradation, abiotic degradation, and mixed substrates by monitoring the flavonoids' concentrations with HPLC-UV/vis. Our results showed that 17 out of the 19 tested flavonoids were readily biodegradable. Structural features like a hydroxy group at C3, the C2-C3 bond order, a methoxy group in the B ring, and the position of the B ring in regard to the chromene core did not affect biodegradation of the tested flavonoids. Only flavone without any hydroxy groups and morin with an uncommon 2',4' pattern of hydroxy groups were non-readily biodegradable. Monitoring the concentration of 4 selected flavonoids by HPLC-UV/vis revealed that biodegradation occurred faster than abiotic degradation at CBT conditions with no other carbon sources present. The presence of an alternative carbon source tends to increase lag phases and decrease biodegradation rates. At this condition, abiotic degradation contributed to the degradation of unstable flavonoids. Overall, as a first tier to assess the environmental fate, our results indicate low risks for persistence of most flavonoids. Thus, flavonoids could represent benign substitutes for persistent synthetic chemicals.

Predicting Hydrocarbon Primary Biodegradation in Soil and Sediment Systems Using System Parameterization and Machine Learning

Technical complexity associated with biodegradation testing, particularly for substances of unknown or variable composition, complex reaction products, or biological materials (UVCB), necessitates the advancement of non-testing methods such as quantitative structure-property relationships (QSPRs). Models for describing the biodegradation of petroleum hydrocarbons (HCs) have been previously developed. A critical limitation of available models is their inability to capture the variability in biodegradation rates associated with variable test systems and environmental conditions. Recently, the Hydrocarbon Biodegradation System Integrated Model (HC-BioSIM) was developed to characterize the biodegradation of HCs in aquatic systems with the inclusion of key test system variables. The present study further expands the HC-BioSIM methodology to soil and sediment systems using a database of 2195 half-life (i.e., degradation time [DT]50) entries for HCs in soil and sediment. Relevance and reliability criteria were defined based on similarity to standard testing guidelines for biodegradation testing and applied to all entries in the database. The HC-BioSIM soil and sediment models significantly outperformed the existing biodegradation HC half-life (BioHCWin) and virtual evaluation of chemical properties and toxicities (VEGA) quantitative Mario Negri Institute for Pharmacological Research (IRFMN) models in soil and sediment. Average errors in predicted DT50s were reduced by up to 6.3- and 8.7-fold for soil and sediment, respectively. No significant bias as a function of HC class, carbon number, or test system parameters was observed. Model diagnostics demonstrated low variability in performance and high consistency of parameter usage/importance and rule structure, supporting the generalizability and stability of the models for application to external data sets. The HC-BioSIM provides improved accuracy of Persistence categorization, with correct classification rates of 83.9%, and 90.6% for soil and sediment, respectively, demonstrating a significant improvement over the existing BioHCWin (70.7% and 58.6%) and VEGA (59.5% and 18.5%) models. Environ Toxicol Chem 2024;43:1352-1363. © 2024 Concawe. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Should Transformation Products Change the Way We Manage Chemicals?

When chemical pollutants enter the environment, they can undergo diverse transformation processes, forming a wide range of transformation products (TPs), some of them benign and others more harmful than their precursors. To date, the majority of TPs remain largely unrecognized and unregulated, particularly as TPs are generally not part of routine chemical risk or hazard assessment. Since many TPs formed from oxidative processes are more polar than their precursors, they may be especially relevant in the context of persistent, mobile, and toxic (PMT) and very persistent and very mobile (vPvM) substances, which are two new hazard classes that have recently been established on a European level. We highlight herein that as a result, TPs deserve more attention in research, chemicals regulation, and chemicals management. This perspective summarizes the main challenges preventing a better integration of TPs in these areas: (1) the lack of reliable high-throughput TP identification methods, (2) uncertainties in TP prediction, (3) inadequately considered TP formation during (advanced) water treatment, and (4) insufficient integration and harmonization of TPs in most regulatory frameworks. A way forward to tackle these challenges and integrate TPs into chemical management is proposed.

Assessing the effects of a mixture of hydrophobic contaminants on the algae Rhodomonas salina using the chemical activity concept

The production and release of chemicals from human activities are on the rise. Understanding how the aquatic environment is affected by the presence of an unknown number of chemicals is lacking. We employed the chemical activity concept to assess the combined effects of hydrophobic organic contaminants on the phytoplankton species Rodomonas salina. Chemical activity is additive, and refers to the relative saturation of a chemical in the studied matrix. The growth of R. salina was affected by chemical activity, following a chemical activity-response curve, resulting in an Ea50 value of 0.078, which falls within the baseline toxicity range observed in earlier studies. The chlorophyll a content exhibited both increases and decreases with rising chemical activity, with the increase possibly linked to an antioxidant mechanism. Yet, growth inhibition provided more sensitive and robust responses compared to photosynthesis-related endpoints; all measured endpoints correlated with increased chemical activity. Growth inhibition is an ecologically relevant endpoint and integrates thermodynamic principles such as membrane disruption. Our study utilized passive dosing, enabling us to control exposure and determine activities in both the medium and the algae. The concept of chemical activity and our results can be extended to other neutral chemical groups as effects of chemical activity remain independent of the mixture composition.

Closed aerobic biodegradation kinetics test with activated sludge and low concentration chemical mixtures

The activated sludge process at wastewater treatment plants is important to prevent discharge of organic pollutants to the environment. Determination of biodegradation kinetics in activated sludge is challenging for mixtures that cover a diverse range of structures. The aims of this study were to (1) design a closed aerobic biodegradation batch test with activated sludge and (2) develop a sample preparation procedure that is compatible with LC-MS and Solid Phase Microextraction (SPME) coupled to GC-MS. A headspace:sludge ratio of 4:1 was sufficient to ensure aerobic conditions in activated sludge for 7 days at co-solvent concentrations <0.01%. Ethanol was added to sub-samples (50%) to stop biodegradation, extract sorbed chemicals and allow storage at -18 °C without ice formation. The ethanol extracted the chemicals from the sludge before filtration (0.2 μm). The filtrate was diluted in ultrapure water to <12% ethanol before analysis by SPME GC-MS/MS and was suitable for direct injection on LC-MS/MS. Biodegradation was distinguished from sorption through abiotic controls using autoclaved poisoned sludge. Linalool, naphthalene, α-isomethylionone, phenanthrene, citronellol, drometrizole, 2-ethylhexyl 4-methoxycinnamate, dicyclohexyl phthalate, BP-1, BP-3, methyl-, ethyl-, propylparaben, alkyl sulfates and isethionates degraded within 48 h in activated sludge, while musk ketone, tonalide and 1,3,5-trichlorobenzene did not. A 10 times reduction of sludge density did not markedly affect the microbial diversity but slowed biodegradation kinetics (partly explained by theory). This study demonstrated a 'cold' alternative to an OECD 314b test and how biodegradation kinetics can be determined for mixtures of diverse chemicals in closed batch tests with activated sludge.

Biodegradation of chemicals tested in mixtures and individually: mixture effects on biodegradation kinetics and microbial composition

Biodegradation in the aquatic environment occurs in the presence of many chemicals, while standard simulation biodegradation tests are conducted with single chemicals. This study aimed to investigate the effect of the presence of additional chemicals on (1) biodegradation kinetics of individual chemicals and (2) the microbial composition in test systems. Parallel mixture and single substance experiments were conducted for 9 chemicals (phenethyl benzoate, oxacycloheptadec-10-en-2-one, α-ionone, methyl 2-naphthyl ether, decan-5-olide, octan-2-one, 2'-acetonaphthanone, methyl N-methylanthranilate, (+)-menthone) using inoculum from a Danish stream. Biotic and abiotic test systems were incubated at 12 °C for 1-30 days. Primary biodegradation kinetics were then determined from biotic/abiotic peak area ratios using SPME GC/MS analysis. The effect of the mixture on biodegradation varied with test chemical and was more pronounced for chemicals with lag-phases above 14 days: two chemicals degraded in the mixture but not when tested alone (i.e., positive mixture effect), and two degraded when tested alone but not in the mixture (i.e., negative mixture effect). Microbial composition (16S rRNA gene amplicon sequencing) was highly affected by 14 days incubation and the presence of the mixture (significant carbon source), but less by single chemicals (low carbon source). Growth on chemical mixtures resulted in consistent proliferation of Pseudomonas and Malikia, while specific chemicals increased the abundance of putative degraders belonging to Novosphingobium and Zoogloea. The chemical and microbiological results support (1) that simulation biodegradation kinetics should be determined in mixtures at low environmentally relevant concentrations and (2) that degradation times beyond some weeks are associated with more uncertainty.

Using weight of evidence to assess degradation potential of UVCB hydrocarbon solvents

Hydrocarbon solvents are a diverse group of petrochemical substances that are identified as unknown or variable composition, complex reaction products, or biological materials (UVCBs) and may contain tens of thousands of individual chemical constituents. As such, it is generally not possible to analytically resolve every chemical constituent in a hydrocarbon solvent. This, along with the low water solubility and/or high vapor pressure of constituents, precludes the use of many standardized tests designed to determine biodegradation in the environment (e.g., Organization for Economic Co-operation and Development [OECD] 309). A weight of evidence approach may be needed to reduce uncertainty to an acceptable level such that a determination on the biodegradation of the substance can be drawn. Based on the OECD 2019 weight of evidence guidance, we present a framework using various lines of evidence that can be used to evaluate the biodegradation of a UVCB solvent in a weight of evidence approach. The lines of evidence include whole substance testing, data on representative constituents, quantitative structure activity relationship (QSAR) models, and biological plausibility. Using these lines of evidence, "Hydrocarbon, C11-C14, normal alkane, isoalkane, cyclic, <2% aromatics" (EC# 926-141-6) was evaluated in a case study. Data from three whole substance tests, 43 constituents (representing 152 data points), three QSAR models and evidence of microbial degradation pathways were evaluated. Based on the available data, it is concluded that the solvent for the case study is not expected to persist in the environment. This framework sets out a real-world example of how the weight of evidence can be used to evaluate hydrocarbon solvents. While focused on persistence, similar approaches can be used to evaluate other endpoints such as bioaccumulation and toxicity. Integr Environ Assess Manag 2023;00:1-11. © 2023 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Technical guidance on biodegradation testing of difficult substances and mixtures in surface water

The aim of this article is to address critical challenges in the OECD 309 "Aerobic mineralization in surface water - simulation biodegradation test" for volatile chemicals, highly hydrophobic chemicals, mixtures or UVCBs (unknown or variable composition, complex reaction products or biological materials). Several modifications are presented to address technical challenges (minimize and account for losses), make testing more environmentally relevant (lower concentrations) and generate data for multiple substances (more and better aligned data):•Minimizing and accounting for test substance losses: Aqueous solutions are handled using gas tight syringes, tests are conducted in gas tight vials, and automated analysis is performed directly on unopened test vials. Abiotic losses are accounted for via concentration ratios between test systems and abiotic controls that are incubated and measured in parallel.•Testing at low environmentally relevant concentrations: Substances are tested at low concentrations to avoid toxicity and solubility artefacts and analyzed using a sensitive analytical method. Substances are added without co-solvent (using passive dosing) or with a minimum of co-solvent (using microvolume spiking).•Testing of multiple chemicals in mixtures combined with constituent specific analysis: Primary biodegradation kinetics of chemicals are determined in tests of multi-constituent mixtures or UVCBs using constituent specific analysis.

Scientific concepts and methods for moving persistence assessments into the 21st century

The evaluation of a chemical substance's persistence is key to understanding its environmental fate, exposure concentration, and, ultimately, environmental risk. Traditional biodegradation test methods were developed many years ago for soluble, nonvolatile, single-constituent test substances, which do not represent the wide range of manufactured chemical substances. In addition, the Organisation for Economic Co-operation and Development (OECD) screening and simulation test methods do not fully reflect the environmental conditions into which substances are released and, therefore, estimates of chemical degradation half-lives can be very uncertain and may misrepresent real environmental processes. In this paper, we address the challenges and limitations facing current test methods and the scientific advances that are helping to both understand and provide solutions to them. Some of these advancements include the following: (1) robust methods that provide a deeper understanding of microbial composition, diversity, and abundance to ensure consistency and/or interpret variability between tests; (2) benchmarking tools and reference substances that aid in persistence evaluations through comparison against substances with well-quantified degradation profiles; (3) analytical methods that allow quantification for parent and metabolites at environmentally relevant concentrations, and inform on test substance bioavailability, biochemical pathways, rates of primary versus overall degradation, and rates of metabolite formation and decay; (4) modeling tools that predict the likelihood of microbial biotransformation, as well as biochemical pathways; and (5) modeling approaches that allow for derivation of more generally applicable biotransformation rate constants, by accounting for physical and/or chemical processes and test system design when evaluating test data. We also identify that, while such advancements could improve the certainty and accuracy of persistence assessments, the mechanisms and processes by which they are translated into regulatory practice and development of new OECD test guidelines need improving and accelerating. Where uncertainty remains, holistic weight of evidence approaches may be required to accurately assess the persistence of chemicals. Integr Environ Assess Manag 2022;18:1454-1487. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

The Next Frontier of Environmental Unknowns: Substances of Unknown or Variable Composition, Complex Reaction Products, or Biological Materials (UVCBs)

Substances of unknown or variable composition, complex reaction products, or biological materials (UVCBs) are over 70 000 "complex" chemical mixtures produced and used at significant levels worldwide. Due to their unknown or variable composition, applying chemical assessments originally developed for individual compounds to UVCBs is challenging, which impedes sound management of these substances. Across the analytical sciences, toxicology, cheminformatics, and regulatory practice, new approaches addressing specific aspects of UVCB assessment are being developed, albeit in a fragmented manner. This review attempts to convey the "big picture" of the state of the art in dealing with UVCBs by holistically examining UVCB characterization and chemical identity representation, as well as hazard, exposure, and risk assessment. Overall, information gaps on chemical identities underpin the fundamental challenges concerning UVCBs, and better reporting and substance characterization efforts are needed to support subsequent chemical assessments. To this end, an information level scheme for improved UVCB data collection and management within databases is proposed. The development of UVCB testing shows early progress, in line with three main methods: whole substance, known constituents, and fraction profiling. For toxicity assessment, one option is a whole-mixture testing approach. If the identities of (many) constituents are known, grouping, read across, and mixture toxicity modeling represent complementary approaches to overcome data gaps in toxicity assessment. This review highlights continued needs for concerted efforts from all stakeholders to ensure proper assessment and sound management of UVCBs.

Predicting Primary Biodegradation of Petroleum Hydrocarbons in Aquatic Systems: Integrating System and Molecular Structure Parameters using a Novel Machine-Learning Framework

Quantitative structure-property relationship (QSPR) models for predicting primary biodegradation of petroleum hydrocarbons have been previously developed. These models use experimental data generated under widely varied conditions, the effects of which are not captured adequately within model formalisms. As a result, they exhibit variable predictive performance and are unable to incorporate the role of study design and test conditions on the assessment of environmental persistence. To address these limitations, a novel machine-learning System-Integrated Model (HC-BioSIM) is presented, which integrates chemical structure and test system variability, leading to improved prediction of primary disappearance time (DT50) values for petroleum hydrocarbons in fresh and marine water. An expanded, highly curated database of 728 experimental DT50 values (181 unique hydrocarbon structures compiled from 13 primary sources) was used to develop and validate a supervised model tree machine-learning model. Using relatively few parameters (6 system and 25 structural parameters), the model demonstrated significant improvement in predictive performance (root mean square error = 0.26, R²  = 0.67) over existing QSPR models. The model also demonstrated improved accuracy of persistence (P) categorization (i.e., "Not P/P/vP"), with an accuracy of 96.8%, and false-positive and -negative categorization rates of 0.4% and 2.7%, respectively. This significant improvement in DT50 prediction, and subsequent persistence categorization, validates the need for models that integrate experimental design and environmental system parameters into biodegradation and persistence assessment. Environ Toxicol Chem 2022;41:1359-1369. © 2022 ExxonMobil Biomedical Sciences, Inc. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Linking biodegradation kinetics, microbial composition and test temperature - Testing 40 petroleum hydrocarbons using inocula collected in winter and summer

Many factors affect the biodegradation kinetics of chemicals in test systems and the environment. Empirical knowledge is needed on how much test temperature, inoculum, test substances and co-substrates influence the biodegradation kinetics and microbial composition in the test. Water was sampled from the Gudenaa river in winter (2.7 °C) and summer (17 °C) (microbial inoculum) and combined with an aqueous stock solution of >40 petroleum hydrocarbons prepared by passive dosing. This resulted in low-concentration test systems that were incubated for 30 days at 2.7, 12 and 20 °C. Primary biodegradation kinetics, based on substrate depletion relative to abiotic controls, were determined with automated Solid Phase Microextraction coupled to GC/MS. Biodegradation kinetics were remarkably similar for summer and winter inocula when tested at the same temperature, except when cooling summer inoculum to 2.7 °C which delayed degradation relative to winter inoculum. Amplicon sequencing was applied to determine shifts in the microbial composition between season and during incubations: (1) the microbial composition of summer and winter inocula were remarkably similar, (2) the incubation and the incubation temperature had both a clear impact on the microbial composition and (3) the effect of adding >40 petroleum hydrocarbons at low test concentrations was limited but resulted in some proliferation of the known petroleum hydrocarbon degraders Nevskia and Sulfuritalea. Overall, biodegradation kinetics and its temperature dependency were very similar for winter and summer inoculum, whereas the microbial composition was more affected by incubation and test temperature compared to the addition of test chemicals at low concentrations.

Biodegradation Kinetics of Fragrances, Plasticizers, UV Filters, and PAHs in a Mixture─Changing Test Concentrations over 5 Orders of Magnitude

Biodegradation of organic chemicals emitted to the environment is carried out by mixed microbial communities growing on multiple natural and xenobiotic substrates at low concentrations. This study aims to (1) perform simulation type biodegradation tests at a wide range of mixture concentrations, (2) determine the concentration effect on the biodegradation kinetics of individual chemicals, and (3) link the mixture concentration and degradation to microbial community dynamics. Two hundred ninety-four parallel test systems were prepared using wastewater treatment plant effluent as inoculum and passive dosing to add a mixture of 19 chemicals at 6 initial concentration levels (ng/L to mg/L). After 1-30 days of incubation at 12 °C, abiotic and biotic test systems were analyzed using arrow solid phase microextraction and GC-MS/MS. Biodegradation kinetics at the highest test concentrations were delayed for several test substances but enhanced for the reference chemical naphthalene. Test concentration thus shifted the order in which chemicals were degraded. 16S rRNA gene amplicon sequencing indicated that the highest test concentration (17 mg C/L added) supported the growth of the genera Acidovorax, Novosphingobium, and Hydrogenophaga, whereas no such effect was observed at lower concentrations. The chemical and microbial results confirm that too high mixture concentrations should be avoided when aiming at determining environmentally relevant biodegradation data.

Improving our understanding of the environmental persistence of chemicals

Significant progress has been made in the scientific understanding of factors that influence the outcome of biodegradation tests used to assess the persistence (P) of chemicals. This needs to be evaluated to assess whether recently acquired knowledge could enhance existing regulations and environmental risk assessments. Biodegradation tests have limitations, which are accentuated for "difficult-to-test" substances, and failure to recognize these can potentially lead to inappropriate conclusions regarding a chemical's environmental persistence. Many of these limitations have been previously recognized and discussed in a series of ECETOC reports and workshops. These were subsequently used to develop a series of research projects designed to address key issues and, where possible, propose methods to mitigate the limitations of current assessments. Here, we report on the output of a Cefic LRI-Concawe Workshop held in Helsinki on September 27, 2018. The objectives of this workshop were to disseminate key findings from recent projects and assess how new scientific knowledge can potentially support and improve assessments under existing regulatory frameworks. The workshop provided a unique opportunity to initiate a process to reexamine the fundamentals of degradation and what current assessment methods can achieve by (1) providing an overview of the key elements and messages coming from recent research initiatives and (2) stimulating discussion regarding how these interrelate and how new findings can be developed to improve persistence assessments. Opportunities to try and improve understanding of factors affecting biodegradation assessments and better understanding of the persistence of chemicals (particularly UVCBs [substances of unknown or variable composition, complex reaction products, or biological materials]) were identified, and the workshop acted as a catalyst for further multistakeholder activities and engagements to take the persistence assessment of chemicals into the 21st century. Integr Environ Assess Manag 2021;17:1123-1135. © 2021 European Petroleum Refiners Association - Concawe Division. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Biodegradation of an essential oil UVCB - Whole substance testing and constituent specific analytics yield biodegradation kinetics of mixture constituents

Testing and assessing the persistency, bioaccumulative and toxic properties of UVCBs (substances of Unknown or Variable composition, Complex reaction products or Biological materials) pose major technical and analytical challenges. The main aim of this study was to combine whole substance biodegradation testing with constituent specific analytics for determining primary biodegradation kinetics of the main UVCB constituents. An additional aim was to link the primary biodegradation kinetics of the main constituents to the bioaccumulation potential and baseline toxicity potential of the UVCB. Two closed biodegradation experiments were conducted using similar test systems but different analyses. The model substance, cedarwood Virginia oil, was tested at a low concentration and wastewater treatment plant effluent served as inoculum. We used microvolume solvent spiking for a quantitative mass transfer of the UVCB, while avoiding that co-solvent degradation would lead to anaerobic conditions. The biodegradation of UVCB constituents was determined with automated solid-phase microextraction coupled to GC-MS/MS using targeted analysis for main constituents and non-targeted analysis for minor constituents and non-polar degradation products. Primary biodegradation kinetics of main constituents, accounting for 73% w/w of the mixture, were successfully determined with degradation rate constants ranging from 0.09 to 0.25 d^-1. Minor constituents were also degraded and non-polar degradation products were not observed. Finally, the bioaccumulation potential and baseline toxicity potential of the mixture at test start were calculated and both parameters decreased then substantially. The strength of the new approach is the possibility of biodegradation testing of a whole UVCB at low concentration while generating constituent specific biodegradation kinetics.

Determining the Temperature Dependency of Biodegradation Kinetics for 34 Hydrocarbons while Avoiding Chemical and Microbial Confounding Factors

Biodegradation kinetics data are keystone for evaluating the environmental persistence and risk of chemicals. Biodegradation kinetics depend highly on the prevailing temperature, which influences microbial community structures, metabolic rates, and chemical availability. There is a lack of high-quality comparative biodegradation kinetics data that are determined at different test temperatures but with the same microbial inoculum and chemical availability. The present study was designed to determine the effect of test temperature on the biodegradation kinetics of hydrocarbons while avoiding confounding factors. We used inocula from a Northern river (2.7 °C) and a Central European river (12.5 °C). Aqueous stock solutions containing 45 individual hydrocarbons were generated by passive dosing and added to river water containing the native microorganisms. Compound-specific biodegradation kinetics were then determined at 2.7, 12, and 20 °C based on substrate depletion. Main findings comprise the following: (1) Degradation half-times (DegT₅₀) of 34 test chemicals were determined at different test temperatures and were largely consistent with the Arrhenius equation (activation energy, 65.4 kJ/mol). (2) Differences in biodegradation kinetics between tested isomers were rather limited. (3) The recent lowering of standard test temperature from 20 to 12 °C results typically in a doubling of DegT₅₀ values and can lead to a stricter persistency assessment.

Passive Dosing of Petroleum and Essential Oil UVCBs-Whole Mixture Toxicity Testing at Controlled Exposure

Petroleum products and essential oils are produced and used in large amounts and are categorized as "Substances of Unknown or Variable composition, Complex reaction products or Biological materials (UVCBs)." These UVCBs are notorious difficult-to-test substances, since they are complex mixtures of hydrophobic and volatile compounds. This study introduces two passive dosing (PD) approaches for whole UVCB toxicity testing: (1) headspace PD applies the UVCB and purified lipid oil as a donor to control exposure via the headspace and (2) silicone rod PD applies UVCB-loaded silicone rods to control exposure via an aqueous test medium and headspace. Headspace gas chromatography-mass spectrometry measurements were used to cross-validate the approaches at the saturation level and to confirm exposure and maintain mixture composition at varying donor concentration levels. Both approaches were applied to whole-mixture toxicity tests of petroleum and essential oil UVCBs with daphnia and algae. Finally, the observed toxicity was linked to concentrations in the donor and in lipid membranes at equilibrium with the donors. Dose-response curves were similar across the dosing approaches and tested species for petroleum products but differed by an order of magnitude between essential oils and PD systems. All observed toxic effects were consistent with baseline toxicity, and no excess mixture toxicity was observed.

Improving the Environmental Risk Assessment of Substances of Unknown or Variable Composition, Complex Reaction Products, or Biological Materials

Substances of unknown or variable composition, complex reaction products, or biological materials (UVCBs) pose unique risk assessment challenges to regulators and to product registrants. These substances can contain many constituents, sometimes partially unknown and/or variable, depending on fluctuations in their source material and/or manufacturing process. International regulatory agencies have highlighted the difficulties in characterizing UVCBs and assessing their toxicity and environmental fate. Several industrial sectors have attempted to address these issues by developing frameworks and characterization methods. Based on the output of a 2016 workshop, this critical review examines current practices for UVCB risk assessment and reveals a need for a multipronged and transparent approach integrating whole-substance and constituent-based information. In silico tools or empirical measurements can provide information on discrete and/or blocks of UVCB constituents with similar hazard properties. Read-across and/or whole-substance toxicity and fate testing using adapted emerging methods can provide whole-substance information. Continued collaboration of stakeholders representing government, industry, and academia will facilitate the development of practical testing strategies and guidelines for addressing regulatory requirements for UVCBs. Environ Toxicol Chem 2020;39:2097-2108. © 2020 Health and Environmental Sciences Institute. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Biodegradation kinetics testing of two hydrophobic UVCBs - potential for substrate toxicity supports testing at low concentrations

The biodegradation kinetics of UVCB substances (unknown or variable composition, complex reaction products or biological materials) should be determined below the solubility limit to avoid experimental artefacts by the non-dissolved mixture. Recently, we reported delayed biodegradation kinetics of single petroleum hydrocarbons even at concentrations just below the solubility limit and attributed this to toxicity. The present study aimed to determine the concentration effect on biodegradation kinetics for constituents in two UVCBs, using surface water from a rural stream as the inoculum. Parallel biodegradation tests of diesel and lavender oil were conducted at concentrations just below the solubility limit and two orders of magnitude lower. The biodegradation kinetics of diesel oil constituents were generally similar at the two concentrations, which coincided with the stimulation of bacterial productivity (growth) at both concentrations, determined by [³H]leucine incorporation. By contrast, the biodegradation of lavender oil constituents was significantly delayed or even halted at the high test concentration. This was consistent with lavender oil stimulating bacterial growth at low concentration but inhibiting it at high concentration. The delayed biodegradation kinetics of lavender oil constituents at high concentration was best explained by mixture toxicity near the solubility limit. Consequently, biodegradation testing of hydrophobic UVCBs should be conducted at low, environmentally relevant concentrations ensuring that mixture toxicity does not affect the biodegradation kinetics.

Is the Arrhenius-correction of biodegradation rates, as recommended through REACH guidance, fit for environmentally relevant conditions? An example from petroleum biodegradation in environmental systems

Biodegradation is a major determinant of chemical persistence in the environment and an important consideration for PBT and environmental risk assessments. It is influenced by several environmental factors including temperature and microbial community structure. According to REACH guidance, a temperature correction based on the Arrhenius equation is recommended for chemical persistence data not performed at the recommended EU mean surface water temperature. Such corrections, however, can lead to overly conservative P/vP assessments. In this paper, the relevance of this temperature correction is assessed for petroleum hydrocarbons, using measured surface water (marine and freshwater) degradation half-time (DT50) and degradation half-life (HL) data compiled from relevant literature. Stringent screening criteria were used to specifically select data from biodegradation tests containing indigenous microbes and conducted at temperatures close to their ambient sampling temperature. As a result, ten independent studies were identified, with 993 data points covering 326 hydrocarbon constituents. These data were derived from tests conducted with natural seawater, or freshwater, at temperatures ranging from 5 to 21 °C. Regressions were performed on the full hydrocarbon dataset and on several individual hydrocarbons. The results were compared to the trend as predicted by the Arrhenius equation and using the activation energy (E_a) as recommend in the REACH Guidance. The comparison shows that the correction recommended in REACH Guidance over predicts the effect of temperature on hydrocarbon biodegradation. These results contrast with temperature manipulated inocula where the test temperature is different from the ambient sampling temperature. In these manipulated systems, the effect of temperature follows the Arrhenius equation more closely. In addition, a more striking effect of temperature on the lag phase was observed with longer lag phases more apparent at lower temperatures. This indicates that the effect of temperature may indeed be even lower when considering hydrocarbon biodegradation without the initial lag phase.

Accelerated Passive Dosing of Hydrophobic Complex Mixtures-Controlling the Level and Composition in Aquatic Tests

Petroleum products and essential oils are complex mixtures of hydrophobic and volatile chemicals and are categorized as substances of unknown or variable composition, complex reaction products, or biological materials (UVCBs). In aquatic testing and research of such mixtures, it is challenging to establish initial concentrations without the addition of cosolvents, to maintain constant concentrations during the test, and to keep a constant mixture composition in dilution series and throughout test duration. Passive dosing was here designed to meet these challenges by maximizing the surface area (A_donor/V_medium = 3.8 cm²/mL) and volume (V_donor/V_medium > 0.1 L/L) of the passive dosing donor in order to ensure rapid mass transfer and avoid donor depletion for all mixture constituents. Cracked gas oil, cedarwood Virginia oil, and lavender oil served as model mixtures. This study advances the field by (i) showing accelerated passive dosing kinetics for 68 cracked gas oil constituents with typical equilibration times of 5-10 min and for 21 cederwood Virginia oil constituents with typical equilibration times < 1 h, (ii) demonstrating how to control mixture concentration and composition in aquatic tests, and (iii) discussing the fundamental differences between solvent spiking, water-accommodated fractions, and passive dosing.

Comparing passive dosing and solvent spiking methods to determine the acute toxic effect of pentachlorophenol on Daphnia magna

Pentachlorophenol (PCP) is a widespread and persistent hydrophobic organic pollutant in the environment despite its restricted public use. Risk assessment of such hydrophobic organic compounds (HOCs) is challenging because sorption and volatilization issues during toxicity test often lead to inconsistent exposure concentration. Considering the hydrophobicity of the PCP, in this study, a passive dosing format was applied by adopting a silicone O-ring as a reservoir and evaluated its applicability on the determination of PCP on Daphnia magna. Results obtained with passive dosing method were compared with that of solvent spiking method. We hypothesized that the passive dosing method may provide more reliable and accurate toxicity results than conventional solvent spiking approach. As a result, the partition coefficient of PCP between methanol and a test medium (log K_MeOH:ISO) was 2.1, which enabled the maintenance of reliable exposure concentration throughout the experiment. In the acute toxicity tests, passive dosing and solvent spiking showed similar EC50 values of 576 and 485 µg/L for 24 h, and 362 and 374 µg/L for 48 h, respectively, which overlap with EC50 values of previous studies. Altogether, both methods were suitable for the acute toxicity assessment of hydrophobic PCP.

Comparison of Small Molecule Biotransformation Half-Lives between Activated Sludge and Soil: Opportunities for Read-Across?

Compartment-specific degradation half-lives are essential pieces of information in the regulatory risk assessment of synthetic chemicals. However, their measurement according to regulatory testing guidelines is laborious and costly. Despite the obvious ecological and economic benefits of knowing environmental degradability as early as possible, its consideration in the early phases of rational chemical design is therefore challenging. Here, we explore the possibility to use half-lives determined in highly time- and work-efficient biotransformation experiments with activated sludge and mixtures of chemicals to predict soil half-lives from regulatory simulation studies. We experimentally determined half-lives for 52 structurally diverse agrochemical active ingredients in batch reactors with three concentrations of the same activated sludge. We then developed bi- and multivariate models for predicting half-lives in soil by regressing the experimentally determined half-lives in activated sludge against average soil half-lives of the same chemicals extracted from regulatory data. The models differed in how we accounted for sorption-related bioavailability differences in soil and activated sludge. The best-performing models exhibited good coefficients of determination (R² of around 0.8) and low average errors (<factor of 3 in half-life predictions) and were robust in cross-validation. From a practical perspective, these results suggest that it may indeed be possible to read across from half-lives determined in highly efficient biotransformation experiments in activated sludge to soil half-lives, which are obtained from much more work- and resource-intense regulatory studies, and that these predictions are clearly superior to predictions based on the output of BIOWIN, a publicly available quantitative structure-biodegradation relationship (QSBR) model. From a theoretical perspective, these results suggest that soil and activated sludge microbial communities, although certainly different in terms of taxonomic composition, may be functionally similar with respect to the enzymatic transformation of environmentally relevant concentrations of a diverse range of chemical compounds.

Headspace Passive Dosing of Volatile Hydrophobic Organic Chemicals from a Lipid Donor-Linking Their Toxicity to Well-Defined Exposure for an Improved Risk Assessment

High hydrophobicity and volatility of chemicals often lead to substantial experimental challenges but were here utilized in headspace passive dosing (HS-PD) to establish and maintain exposure: the pure chemical served as a passive dosing donor for controlling exposure at saturation, whereas triglyceride oil containing the chemical was used to control lower exposure levels. These donor solutions were added to glass inserts placed in the closed test systems. Mass balance calculations confirmed a dominant donor capacity for all chemicals except isooctane. This HS-PD method was applied to algal growth inhibition and springtail lethality tests with terpenes, alkanes, and cyclic siloxanes. Headspace concentrations above the lipid donors were measured for three chemicals to determine their chemical activity, using saturated vapor as the analytical standard and thermodynamic reference. Toxicity was related to chemical activity and calculated concentrations in membranes at equilibrium with the lipid donor. For both tests and all chemicals, toxic effects were observed within or above the reported range for baseline toxicity, meaning that no excess toxicity was observed. The toxicity of siloxanes was markedly higher to the terrestrial springtail than the aquatic algae, which is consistent with a more efficient mass transfer of these volatile hydrophobic chemicals in air compared to water.

Time-Resolved Freely Dissolved Concentrations of Semivolatile and Hydrophobic Test Chemicals in In Vitro Assays-Measuring High Losses and Crossover by Headspace Solid-Phase Microextraction

In vitro assays are normally conducted in plastic multiwell plates open to exchange with the ambient air. The concentration of test substances freely available to cells is often not known, can change over time, and is difficult to measure in the small volumes in microplates. However, even a well-characterized toxicological response is of limited value if it cannot be linked to a well-defined exposure level. The aim of this study was to develop and apply an approach for determining time-resolved freely dissolved concentrations of semivolatile and hydrophobic organic chemicals (SVHOCs) in in vitro assays: (1) free fractions were measured by a new medium dilution method and (2) time-resolved loss curves were obtained by measurements of total concentrations in 96-well plates during incubations at 37 °C. Headspace solid-phase microextraction was used as an analytical technique for 24 model chemicals spanning 6 chemical groups and 4-5 orders of magnitude in K_ow and K_aw. Free fractions were >30% for chemicals with log K_ow < 3.5 and then decreased with increasing log K_ow. Medium concentrations declined significantly (>50%) within 24 h of incubation for all 20 chemicals having log K_ow > 4 or log K_aw > -3.5 in serum-free medium. Losses of chemicals were lower for medium containing 10% fetal bovine serum, most significantly for chemicals with log K_ow > 4. High crossover to neighboring wells also was observed below log K_ow of 4 and log K_aw of -3.5. Sealing the well plates had limited effect on the losses but clearly reduced crossover. The high losses and crossover of most tested chemicals question the suitability of multiwell plates for in vitro testing of SVHOCs and call for (1) test systems that minimize losses, (2) methods to control in vitro exposure, (3) analytical confirmation of exposure, and (4) exposure control and confirmation being included in good in vitro reporting standards.

Baseline Toxicity and Volatility Cutoff in Reporter Gene Assays Used for High-Throughput Screening

Most studies using high-throughput in vitro cell-based bioassays tested chemicals up to a certain fixed concentration. It would be more appropriate to test up to concentrations predicted to elicit baseline toxicity because this is the minimal toxicity of every chemical. Baseline toxicity is also called narcosis and refers to nonspecific intercalation of chemicals in biological membranes, leading to loss of membrane structure and impaired functioning of membrane-related processes such as mitochondrial respiration. In cells, baseline toxicity manifests as cytotoxicity, which was quantified by a robust live-cell imaging method. Inhibitory concentrations for baseline toxicity varied by orders of magnitude between chemicals and were described by a simple quantitative structure activity relationship (QSAR) with the liposome-water partition constant as a sole descriptor. The QSAR equations were remarkably similar for eight reporter gene cell lines of different cellular origin, six of which were used in Tox21. Mass-balance models indicated constant critical membrane concentrations for all cells and all chemicals with a mean of 69 mmol·kg_lip^-1(95% CI: 49-89), which is in the same range as for bacteria and aquatic organisms and consistent with the theory of critical membrane burden of narcosis. The challenge of developing baseline QSARs for cell lines is that many confirmed baseline toxicants are rather volatile. We deduced from cytotoxicity experiments with semi-volatile chemicals that only chemicals with medium-air partition constants >10,000 L/L can be tested in standard robotic setups without appreciable loss of effect. Chemicals just below that cutoff showed crossover effects in neighboring wells, whereas the effects of chemicals with lower medium-air partition constants were plainly lost. Applying the "volatility cut-off" to >8000 chemicals tested in Tox21 indicated that approximately 20% of Tox21 chemicals could have partially been lost during the experiments. We recommend applying the baseline QSARs together with volatility cut-offs for experimental planning of reporter gene assays, that is, to dose only chemicals with medium-air partition constants >10,000 at concentrations up to the baseline toxicity level.

The treatment of biodegradation in models of sub-surface oil spills: A review and sensitivity study

Biodegradation is important for the fate of oil spilled in marine environments, yet parameterization of biodegradation varies across oil spill models, which usually apply constant first-order decay rates to multiple pseudo-components describing an oil. To understand the influence of model parameterization on the fate of subsurface oil droplets, we reviewed existing algorithms and rates and conducted a model sensitivity study. Droplets were simulated from a blowout at 2000 m depth and were either treated with sub-surface dispersant injection (2% dispersant to oil ratio) or untreated. The most important factor affecting oil fate was the size of the droplets, with biodegradation contributing substantially to the fate of droplets ≤0.5 mm. Oil types, which were similar, had limited influence on simulated oil fate. Model results suggest that knowledge of droplet sizes and improved estimation of pseudo-component biodegradation rates and lag times would enhance prediction of the fate and transport of subsurface oil.

Mixture Effects on Biodegradation Kinetics of Hydrocarbons in Surface Water: Increasing Concentrations Inhibited Degradation whereas Multiple Substrates Did Not

Most biodegradation tests are conducted using single chemicals at high concentrations, although these chemicals are present in the environment as mixtures at low concentrations. A partitioning-based platform was recently developed for biodegradation testing of composed mixtures of hydrophobic chemicals at ng/L to μg/L concentrations. We used this platform to study the concentration and mixture effect on biodegradation kinetics. Biodegradation tests were conducted in 20 mL vials using environmental water samples as inocula. Passive dosing was applied (1) to vary initial test concentrations of individual test compounds and (2) to vary the number of mixture components between 1 and 16. Automated solid-phase microextraction coupled to gas chromatography-mass spectrometry was used to measure substrate depletion relative to abiotic controls. The number of mixture components had no or only a limited effect on the biodegradation half times for three compounds when tested at environmentally relevant concentrations. In contrast, longer lag phases and half lives were observed for single compounds when tested at higher concentrations that approached aqueous solubility. The obtained results support that simultaneous testing of multiple chemicals at low concentrations can accelerate the generation of biodegradation kinetic data, which are more environmentally relevant compared with data from tests conducted with single chemicals at much higher concentrations.

Biodegradation of Volatile Chemicals in Soil: Separating Volatilization and Degradation in an Improved Test Setup (OECD 307)

During environmental risk assessments of chemicals, higher-tier biodegradation tests in soil, sediment, and surface-water systems are required using OECD standards 307, 308, and 309 guidelines, respectively. These guidelines are not suitable for testing highly volatile chemicals, and a biometer closed-incubation setup is recommended for testing slightly volatile chemicals. In this setup, the degradation kinetics of highly volatile chemicals can largely be influenced by volatilization. Additionally, guidelines lack sufficient information on test-system geometry and guidance on how to measure and maintain aerobic conditions during the test. Our objectives were (1) to design a closed test setup for biodegradation tests in soil in which the maintaining and measuring of aerobic conditions was possible without the loss of volatile test chemicals and (2) to suggest data-treatment measures for evaluating the degradation kinetics of volatile test chemicals. With the new setup, full-scale OECD 307 tests were performed using the volatile 14C-labeled chemicals decane and tetralin. For both test chemicals, reproducible complete mass balances were observed, and the new setup ensured that the volatilization losses were kept below the mineralized fraction. Based on the obtained data, an extended model was developed that enabled consideration of the volatilization in the modeling of degradation kinetics.

Prediction of biopersistence of hydrocarbons using a single parameter

Aerobic biodegradation is an important attenuation process for petroleum hydrocarbons (PHCs) in the natural environment. It has also been frequently used in engineered systems to remediate PHC-contaminated sites. A model such as a quantitative structure property relationship (QSPR) that can predict the biodegradation rate of PHCs would be helpful prior to implementing any extensive environmental measurements and bioremediation strategies. Existing QSPRs either have a large number of predictor variables that may cause overfitting or are based on a small dataset of PHCs. The goal of this study is to develop a simple, portable QSPR that has only a few predicator variables but can accurately predict the biodegradation half-lives of a large group of PHCs. To this end, more than 500 molecular variables were screened, and candidate variables were refined by a feature selection method and fitted to biodegradation data of a group of structurally heterogeneous PHCs (n = 173). The model was established by means of hierarchical clustering and classification and regression tree algorithms, which was optimized by an internal validation procedure and validated by an external dataset. The optimal QSPR model, containing only one predictor variable (the number of bonds that do not contain hydrogen), was able to accurately predict biodegradation half-lives for a wide variety of PHCs. The internal validation test indicated an overall prediction accuracy of 93%, and predictions applied to an independent external set of 64 PHCs yielded 95% accuracy. The new model is transparent and easily portable from one user to another.

Headspace passive dosing of volatile hydrophobic chemicals - Aquatic toxicity testing exactly at the saturation level

It is challenging to conduct aquatic tests with highly hydrophobic and volatile chemicals while avoiding substantial sorptive and evaporative losses. A simple and versatile headspace passive dosing (HS-PD) method was thus developed for such chemicals: The pure liquid test chemical was added to a glass insert, which was then placed with the open end in the headspace of a closed test system containing aqueous test medium. The test chemical served as the dominating partitioning donor for establishing and maintaining maximum exposure levels in the headspace and aqueous solution, without direct contact between the donor and the test medium. The HS-PD method was cross validated against passive dosing with a saturated silicone elastomer, using headspace gas chromatography as analytical instrument and saturated vapors as reference. The HS-PD method was then applied to control the exposure in algal growth inhibition tests with the green algae Raphidocelis subcapitata. The model chemicals were C9-C14 n-alkanes and the cyclic volatile methyl siloxanes octamethyltetracyclosiloxane (D4) and decamethylpentacyclosiloxane (D5). Growth rate inhibition at the solubility limit was 100% for C9-C13 n-alkanes and 53 ± 31% (95% CI) for tetradecane. A moderate inhibition of 11 ± 4% (95% CI) was observed for D4, whereas no inhibition was observed for D5. The present study introduces an effective method for aquatic toxicity testing of a difficult-to-test group of chemicals and provides an improved experimental basis for investigating toxicity cut-offs.

Passive dosing yields dissolved aqueous exposures of crude oil comparable to the CROSERF (Chemical Response to Oil Spill: Ecological Effects Research Forum) water accommodated fraction method

The Chemical Response to Oil Spill: Ecological Effects Research Forum's water accommodated fraction procedure was compared with 2 alternative techniques in which crude oil was passively dosed from silicone tubing or O-rings. Fresh Macondo oil (MC252) was dosed at 30 mg/L using each approach to investigate oil dissolution kinetics, which was monitored by fluorometry as estimated oil equivalents (EOEs). Subsequent experiments with each dosing method were then conducted at multiple oil loadings. Following equilibration, test media were analytically characterized for polyaromatic hydrocarbons (PAHs) using gas chromatography (GC)-mass spectrometry and dissolved oil using biomimetic solid-phase microextraction (SPME). The results showed that equilibrium was achieved within 72 h for all methods. Measured PAH concentrations were compared with oil solubility model predictions of dissolved exposures. The concentration and composition of measured and predicted dissolved PAHs varied with oil loading and were consistent between dosing methods. Two-dimensional GC compositional data for this oil were then used to calculate dissolved toxic units for predicting MC252 oil acute toxicity across the expected range of species sensitivities. Predicted toxic units were nonlinear with loading and correlated to both EOE and biomimetic SPME. Passive dosing methods provide a practical strategy to deliver and maintain dissolved oil concentrations while avoiding the complicating role that droplets can introduce in exposure characterization and test interpretation. Environ Toxicol Chem 2018;37:2810-2819. © 2018 SETAC.