Standard inocula preparations reduce the bacterial diversity and reliability of regulatory biodegradation tests

Biodegradation assessment tests of biopolymers in standardised water: different sources of variability

This study assessed the ultimate biodegradation degree of two resins, polyhydroxybutyrate and polylactic acid (PHB and PLA), and three commercial biobased bags (BMAT, BGREEN, and BBEIGE) through the measurement of oxygen consumption in closed respirometers. Activated sludge from a wastewater treatment plant (WWTP) was used as the inoculum, cellulose was used as the reference material, and five trials were conducted with two different devices under identical conditions, with a 28-day incubation period. The results revealed statistically significant differences in the biochemical oxygen demand (BOD) measurements for cellulose, PHB, and PLA between the two devices and within the same devices across different trials. The degree of biodegradation (Dt), calculated as the percentage of theoretical oxygen demand (ThOD), varied depending on the device and trial. For cellulose, Dt ranged from 61 to 93%; for PLA, the maximum Dt was 6%; and for PHB, Dt oscillated between 16 and 72%. These findings highlight the critical importance of carefully selecting the testing equipment, as it significantly influences biodegradation results, in addition to the already known interlaboratory variability caused by the inoculum.

Assessing the aquatic biodegradation potential of polymeric excipients for pharmaceutical formulation

Polymeric excipients (PEx) are essential in drug formulation but raise environmental concerns upon wastewater release post-administration due to their potential detrimental effects to life-histories of freshwater vertebrates and invertebrates. Ten pharmaceutical polymeric compounds were assessed in a stepwise environmental biodegradation assessment according to standard OECD 301 guidelines to thoroughly evaluate biodegradability of these compounds. Polyvinyl alcohol (PVA), polyethylene glycol (PEG), chitosan, maize starch, and sodium starch glycolate (SSG) were found to be 'readily biodegradable,' although PVA and PEG showed variation across employed test systems. PEG and PVA did not degrade in OECD 301D tests having low microbial density and diversity. In contrast, in the OECD 301F tests i.e., higher microbial density and diversity, PEG exhibited 73.0 ± 3.3 % biodegradation, while PVA showed 91.2 ± 8.0 % biodegradation with secondary effluent and activated sludge, respectively. Polyvinyl pyrrolidone (PVP), Copovidone, Kollidon CL, and Eudragit derivatives EPO and L100-55 were categorized as 'non-biodegradable' (< 10 % biodegradation). No increase in degradation was observed after 42 days. This indicates their environmental persistence. This study lays the groundwork for a comprehensive understanding of the biodegradation potential of pharmaceutical polymers. It considers the influence of test conditions, inoculum sources, and compound characteristics. The environmental persistence of certain PEx underlines the urgent need to use more environmentally biodegradable alternatives in drug formulation.

Microbiological toxicity tests using standardized ISO/OECD methods-current state and outlook

Microbial toxicity tests play an important role in various scientific and technical fields including the risk assessment of chemical compounds in the environment. There is a large battery of normalized tests available that have been standardized by ISO (International Organization for Standardization) and OECD (Organization for Economic Co-operation and Development) and which are worldwide accepted and applied. The focus of this review is to provide information on microbial toxicity tests, which are used to elucidate effects in other laboratory tests such as biodegradation tests, and for the prediction of effects in natural and technical aqueous compartments in the environment. The various standardized tests as well as not normalized methods are described and their advantages and disadvantages are discussed. In addition, the sensitivity and usefulness of such tests including a short comparison with other ecotoxicological tests is presented. Moreover, the far-reaching influence of microbial toxicity tests on biodegradation tests is also demonstrated. A new concept of the physiological potential of an inoculum (PPI) consisting of microbial toxicity tests whose results are expressed as a chemical resistance potential (CRP) and the biodegradation adaptation potential (BAP) of an inoculum is described that may be helpful to characterize inocula used for biodegradation tests. KEY POINTS: • Microbial toxicity tests standardized by ISO and OECD have large differences in sensitivity and applicability. • Standardized microbial toxicity tests in combination with biodegradability tests open a new way to characterize inocula for biodegradation tests. • Standardized microbial toxicity tests together with ecotoxicity tests can form a very effective toolbox for the characterization of toxic effects of chemicals.

Biodegradation of Water-Soluble Polymers by Wastewater Microorganisms: Challenging Laboratory Testing Protocols

For water-soluble polymers (WSPs) that enter environmental systems at their end-of-life, biodegradability is a key functionality. For the development and regulation of biodegradable WSPs, testing methods that are both scientifically validated and economically practicable are needed. Here, we used respirometric laboratory tests to study the biodegradation of poly(amino acids), poly(ethylene glycol), and poly(vinyl alcohol), together with appropriate low-molecular-weight reference substrates. We varied key protocol steps of commonly used testing methods, which were originally established for small molecules and tested for effects on WSP biodegradation. We found that avoiding aeration of the wastewater inoculate prior to WSP addition, incubating WSP with filter-sterilized wastewater prior to biodegradation testing, and lowering the WSP concentration can increase biodegradation rates of WSPs. Combining the above-mentioned protocol variations substantially affected the results of the biodegradation testing for the two poly(amino acids) tested herein (i.e., poly(lysine) and poly(aspartic acid)). Our findings were consistent between microbial inocula derived from two municipal wastewater treatment plants. Our study presents promising biodegradation dynamics for poly(amino acids) and highlights the importance, strengths, and limitations of respirometric laboratory methods for WSP biodegradation testing.

Analysis of environmental biodegradability of cellulose-based pharmaceutical excipients in aqueous media

Pharmaceutical cellulosic polymers will inevitably reach natural water systems if they are not removed after entering wastewater. Biodegradation of organic chemicals in sewage or in the aquatic environment is an important removal mechanism. In this study, we investigated the environmental biodegradation of 14 cellulose derivatives commonly utilized as pharmaceutical excipients using three different test systems that are based on the closed bottle test (OECD 301D) and the manometric respirometry test (OECD 301F). For the different cellulose derivatives tested, we observed varying degrees of biodegradation ranging from 0 to 20.4 % chemical oxygen demand (COD). However, none met the criteria for classification as 'readily biodegradable'. In addition, 10 out of 14 cellulose derivatives and/or their possible transformation products formed during the experiments, may exhibit possible toxic inhibitory effects on the inoculum. This includes one or several derivatives of hydroxy propyl methyl cellulose, hydroxy propyl cellulose, methyl cellulose, ethyl cellulose, and hydroxy ethyl cellulose. Based on the results obtained, we have developed a graded classification score ('traffic light system') for excipient biodegradation. This could help streamline the assessment and classification of cellulose derivatives concerning risk of persistence and potential adverse environmental effects, thereby assisting in the prioritization of more favorable compounds. In the long term, however, excipients should be designed from the very beginning to be biodegradable and mineralizable in the environment ('benign by design').

Multi-laboratory evaluation of the reproducibility of polymer biodegradation assessments applying standardized and modified respirometry methods

This research evaluated the intra- and interlaboratory variability when applying OECD 301F and OECD 301B Ready Biodegradation respirometric test methods to quantify polymer biodegradation as well as the impact of method modifications including test duration, inoculum level and test substance concentration on results. This assessment synthesizes results of mineralization studies on 5 polymers of varying structural components, molecular weight, charge, and solubility, evaluated at 8 different laboratories in 4 different countries, providing significant geographic variation in inoculum source as well as lab to lab variations in test setup. Across all laboratories, intralaboratory variability was low (≤18 % absolute difference) indicating the reproducibility of results between replicates and uniformity of test setup in each laboratory. Interlaboratory variation was also low for all 5 polymers with extent of mineralization being comparable in all OECD 301F and 301B studies even when test methods were modified. Across all studies mean mineralization was 89 ± 5.5 % for polyethylene glycol 35,000, 85 ± 7.4 % for polyvinyl alcohol 18-88, 44 ± 13 % for carboxymethyl cellulose (DS 0.6), 48 ± 4.1 % for a modified guar gum, and 88 ± 6.2 % for microcrystalline cellulose (MCC) at study completion. Due to the lack of polymeric reference materials, MCC was evaluated and found to be a suitable reference material for polymers that biodegrade rapidly in screening studies. An additional respirometric study was conducted quantifying mineralization of the 5 polymers in river water to evaluate the relationship with OECD 301 results using activated sludge as the inoculum. A similar extent of mineralization was observed for all 5 polymers in the OECD 301 and river water studies but time to reach the maximum extent of mineralization was longer using river water as the inoculum source likely due to the lower microbial counts (106 CFU/L) in the test system.

Toward the future of OECD/ISO biodegradability testing-new approaches and developments

In the past decades, industrial and scientific communities have developed a complex standardized system (e.g., OECD, ISO, CEN) to evaluate the biodegradability of chemical substances. This system includes for OECD three levels of testing (ready and inherent biodegradability tests, simulation tests). It was adopted by many countries and is completely integrated into European legislation (registration, evaluation, authorization, and restriction of chemicals, REACH). Nevertheless, the different tests have certain deficiencies, and the question arises of how accurately these tests display the situation in the real environment and how the results can be used for predictions. This review will focus on the technical advantages and weaknesses of current tests concerning the technical setup, the inoculum characterization, and its biodegradation potential as well as the use of adequate reference compounds. A special focus of the article will be on combined test systems offering enhanced possibilities to predict biodegradation. The properties of microbial inocula are critically discussed, and a new concept concerning the biodegradation adaptation potential (BAP) of inocula is proposed. Furthermore, a probability model and different in silico QSAR (quantitative structure-activity relationships) models to predict biodegradation from chemical structures are reviewed. Another focus lies on the biodegradation of difficult single compounds and mixtures of chemicals like UVCBs (unknown or variable composition, complex reaction products, or biological materials) which will be an important challenge for the forthcoming decades. KEY POINTS: • There are many technical points to be improved in OECD/ISO biodegradation tests • The proper characterization of inocula is a crucial point in biodegradation tests • Combined biodegradation test systems offer extended possibilities for biodegradation tests.

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).

Reliability of Biodegradation Measurements for Inhibitive Industrial Wastewaters

Industrial wastewaters may contain toxic or highly inhibitive compounds, which makes the measurement of biological oxygen demand (BOD) challenging. Due to the high concentration of organic compounds within them, industrial wastewater samples must be diluted to perform BOD measurements. This study focused on determining the reliability of wastewater BOD measurement using two different types of industrial wastewater, namely pharmaceutical wastewater containing a total organic carbon (TOC) value of 34,000 mg(C)/L and industrial paper manufacturing wastewater containing a corresponding TOC value of 30 mg(C)/L. Both manometric respirometry and the closed-bottle method were used in the study, and the results were compared. It was found that the dilution wastewaters containing inhibitive compounds affected BOD values, which increased due to the decreased inhibiting effect of wastewater pollutants. Therefore, the correct BOD for effluents should be measured from undiluted samples, while the diluted value is appropriate for determining the maximum value for biodegradable organic material in the effluent. The accuracy of the results from the blank samples was also examined, and it was found that the readings of these were different to those from the samples. Therefore, the blank value that must be subtracted may differ depending on the sample.

Biotransformation of Chemicals at the Water–Sediment Interface─Toward a Robust Simulation Study Setup

Studying aquatic biotransformation of chemicals in laboratory experiments, i.e., OECD 308 and OECD 309 studies, is required by international regulatory frameworks to prevent the release of persistent chemicals into natural water bodies. Here, we aimed to address several previously described shortcomings of OECD 308/309 studies regarding their variable outcomes and questionable environmental relevance by broadly testing and characterizing a modified biotransformation test system in which an aerated water column covers a thin sediment layer. Compared to standard OECD 308/309 studies, the modified system showed little inter-replicate variability, improved observability of biotransformation, and consistency with first-order biotransformation kinetics for the majority of 43 test compounds, including pharmaceuticals, pesticides, and artificial sweeteners. To elucidate the factors underlying the decreased inter-replicate variability compared to OECD 309 outcomes, we used multidimensional flow cytometry data and a machine learning-based cell type assignment pipeline to study cell densities and cell type diversities in the sediment and water compartments. Our here presented data on cell type composition in both water and sediment allows, for the first time, to study the behavior of microbial test communities throughout different biotransformation simulation studies. We found that sediment-associated microbial communities were generally more stable throughout the experiments and exhibited higher cell type diversity than the water column-associated communities. Consistently, our data indicate that aquatic biotransformation of chemicals can be most robustly studied in test systems providing a sufficient amount of sediment-borne biomass. While these findings favor OECD 308-type systems over OECD 309-type systems to study biotransformation at the water-sediment interface, our results suggest that the former should be modified toward lower sediment-water ratios to improve observability and interpretability of biotransformation.

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).

Are biodegradable plastics an environmental rip off?

While the use of biodegradable polymers is recognized as a global strategy to minimize plastic pollution, the technical standards (TS) used to attest their biodegradability may not be in compliance with most environmental parameters observed aquatic ecosystems. Indeed, through a careful assessment of the TS currently in use, this study evidenced that these guidelines cover only a fraction of the biogeochemical parameters seen in nature and largely disregard those that occur in the deep-sea. Thus, these TS may not be able to ensure the degradation of such polymers in natural environments, where microbial activity, pH, temperature, salinity, UV radiation and pressure are highly variable. This raises environmental concern, since relevant parcel of plastic ends up in the oceans reaching deep zones. Therefore, there is an urgent need to revise these TS, which must consider the actual fate of most plastic debris and include assessments under the challenging conditions found at these types of environment, alongside microplastic formation and ecotoxicology effects. Moreover, the next generation of biodegradability tests must be designed to enable a cost-effective implementation and incorporate accurate analytical techniques to assess polymer transformation. Furthermore, certification should provide information on time scale and degradation rates and, preferably, be globally harmonized.

Biodegradability assessment of complex, hydrophobic substances: Insights from gas-to-liquid (GTL) fuel and solvent testing

The assessment of substances of Unknown or Variable composition, Complex reaction products or Biological materials (UVCBs) presents significant challenges when determining biodegradation potential and environmental persistence for regulatory purposes. An example of UVCBs is the gas-to-liquid (GTL) products, which are synthetic hydrocarbons produced from natural gas using a catalytic process known as the Fischer-Tropsch process. These synthetic hydrocarbons are fractionated into a wide array of products equivalent in function to their petroleum-derived analogues. Here we summarise the results of an extensive testing program to assess the biodegradability of several GTL products. This program highlights the challenges associated with UVCBs and provides a case study for the assessment of such substances that are also poorly soluble and volatile. When tested with the appropriate methods, all the GTL products assessed in this study were found to be readily biodegradable indicating they are not likely to be persistent in the environment.

Multi-laboratory Validation of a New Marine Biodegradation Screening Test for Chemical Persistence Assessment

Current biodegradation screening tests are not specifically designed for persistence assessment of chemicals, often show high inter- and intra-test variability, and often give false negative biodegradation results. Based on previous studies and recommendations, an international ring test involving 13 laboratories validated a new test method for marine biodegradation with a focus on improving the reliability of screening to determine the environmental degradation potential of chemicals. The new method incorporated increased bacterial cell concentrations to better represent the microbial diversity; a chemical is likely to be exposed in the sampled environments and ran beyond 60 days, which is the half-life threshold for chemical persistence in the marine environment. The new test provided a more reliable and less variable characterization of the biodegradation behavior of five reference chemicals (sodium benzoate, triethanolamine, 4-nitrophenol, anionic polyacrylamide, and pentachlorophenol), with respect to REACH and OSPAR persistence thresholds, than the current OECD 306 test. The proposed new method provides a cost-effective screening test for non-persistence that could streamline chemical regulation and reduce the cost and animal welfare implications of further higher tier testing.

Application of seven different clay types in sorbent-modified biodegradability studies with cationic biocides

The cationic surfactants cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC) can exert inhibitory effects on micro-organisms responsible for their biodegradation. However, under environmentally relevant exposure scenarios the presence of and sorption to organic and inorganic matter can lead to significant reduction of inhibitory effects. In our studies we investigated silica gel and seven clays as inert sorbents to mitigate these inhibitory effects in a 28 day manometric respirometry biodegradation test. CTAB was not inhibitory to the used inoculum, but we did observe that seven out of eight sorbents increased maximum attainable biodegradation, and four out of eight decreased the lag phase. The strongly inhibitory effect of CPC was successfully mitigated by most sorbents, with five out of eight allowing >50% biodegradation within 28 days. Results further indicate that bioaccessibility of the sorbed fractions in the stirred manometric test systems was higher than in calmly shaken headspace test systems. Bioaccessibility might also be limited depending on characteristics of test chemical and sorbent type, with montmorillonite and bentonite apparently providing the lowest level of bioaccessibility with CPC. Clay sorbents can thus be used as environmentally relevant sorbents to mitigate potential inhibitory effects of test chemicals, but factors that impede bioaccessibility should be considered. In addition to apparently increased bioaccessibility due to stirring, the automated manometric respirometry test systems give valuable and highly cost-effective insights into lag phase and biodegradation kinetics; information that is especially relevant for test chemicals of gradual biodegradability.

Increased cell numbers improve marine biodegradation tests for persistence assessment

Currently available OECD biodegradation screening tests (BSTs) are not particularly suited for persistence screening. Their duration can be much less than international half-life thresholds for persistence and they are variable and stringent, therefore prone to false negatives. The present study extended test durations beyond 28 days and increased biomass concentrations for marine BSTs to better represent the microbial diversity inherent in the sampled environment. For this so-called environmentally relevant BST (erBST) marine cell concentrations were nominally increased 100-fold by tangential flow filtration. The marine erBST was validated against a standard BST using five ¹⁴C labeled reference compounds with a range of biodegradation potentials (aniline, 4-fluorophenol, 4-nitrophenol, 4-chloroaniline and pentachlorophenol) in a modified OECD 301B test. A full mass balance was collated to follow chemical fate in the tests. The erBST was more accurate and less variable than the comparator BST in assigning the reference compounds to their expected biodegradation classifications (non-persistent or potentially persistent). According to the REACH non-persistence criterion of ≥60% biodegradation over 60 days, the erBST correctly classified 60% of chemical replicates according to their expected biodegradation classification and had a coefficient of variation of 21% between replicates. In contrast, the BST correctly assessed 40% of reference chemicals in regards to their expected biodegradation classification with a coefficient of variation of 36%. All non-persistent chemicals showed increased degradation in the erBST, except for 4-chloroaniline, which did not degrade in either BST or erBST. Both tests showed no false positive results, correctly classifying the negative control pentachlorophenol as potentially persistent. Next, it is recommended to further validate the marine erBST in an inter-laboratory study incorporating different seawater sources to fully assess its variability and reliability.

Biotransformation of Acetaminophen by intact cells and crude enzymes of bacteria: A comparative study and modelling

Acetaminophen (APAP), which is an active ingredient of many analgesic drugs, is one of the contaminants of emerging concern in the environment. Although APAP is biodegradable, it is frequently detected in treatment plant effluents, surface water and soil suggesting that there are factors affecting the fate of APAP in the environment. In this study, four strains of bacteria that can degrade APAP were isolated from soil. Those strains belonged to Rhodococcus, Pseudomonas, Flavobacterium, and Sphingobium genera of Bacteria. A series of kinetic experiments were performed on the isolates in shake-flasks to determine biodegradation rate constant as well as the effect of temperature, APAP concentration and cell density on the biodegradation rates. APAP biodegradation follows the first order reaction kinetics which is coupled with cell growth. The specific APAP biodegradation rate constant (k) for all strains was similar and equal to 0.19 ± 0.01 h^-1. The temperature, at which APAP biodegradation rate was maximum, was 35 °C. APAP biodegradation rate was linearly correlated with both the initial APAP concentration and the cell density. Initial step of the APAP biodegradation was hydrolysis of the amide bond which resulted in formation and accumulation of p-aminophenol suggesting that aryl acylamidase enzyme is responsible for the biotransformation. In addition, free and immobilized crude enzymes of the isolates transformed APAP at similar rates, comparable to the intact cells. This study showed that APAP biodegradation is achieved by a diverse group of bacteria having a similar enzyme operating at a constant kinetics which is very slow at environmentally relevant APAP concentrations. Natural removal of APAP in the environment is limited by kinetics, therefore APAP-bearing waste streams should be treated in adsorption enhanced biological systems before discharged into the environment.

A quantitative structure-biodegradation relationship (QSBR) approach to predict biodegradation rates of aromatic chemicals

The objective of this work was to develop a QSBR model for the prioritization of organic pollutants based on biodegradation rates from a database containing globally harmonized biodegradation tests using relevant molecular descriptors. To do this, we first categorized the chemicals into three groups (Group 1: simple aromatic chemicals with a single ring, Group 2: aromatic chemicals with multiple rings and Group3: Group 1 plus Group 2) based on molecular descriptors, estimated the first order biodegradation rate of the chemicals using rating values derived from the BIOWIN3 model, and finally developed, validated and defined the applicability domain of models for each group using a multiple linear regression approach. All the developed QSBR models complied with OECD principles for QSAR validation. The biodegradation rate in the models for the two groups (Group 2 and 3 chemicals) are associated with abstract molecular descriptors that provide little relevant practical information towards understanding the relationship between chemical structure and biodegradation rates. However, molecular descriptors associated with the QSBR model for Group 1 chemicals (R² = 0.89, Q²_loo = 0.87) provided information on properties that can readily be scrutinised and interpreted in relation to biodegradation processes. In combination, these results lead to the conclusion that QSBRs can be an alternative tool to estimate the persistence of chemicals, some of which can provide further insights into those factors affecting biodegradation.

Improving the biodegradability in seawater test (OECD 306)

Growth and extensive urbanisation of the human population has been accompanied by increased manufacture and use of chemical compounds. To classify the fate and behaviour of these compounds in the environment, a series of international standardised biodegradation screening tests (BSTs) were developed over 30 years ago. In recent years, regulatory emphasis (e.g. REACH) has shifted from measuring biodegradation towards prioritisations based on chemical persistence. In their current guise, BSTs are ineffective as screens for persistence. The marine BST OECD 306 in particular is prone to high levels of variation and produces a large number of fails, many of which can be considered false negatives. An ECETOC funded two-day workshop of academia, industry and regulatory bodies was held in 2015 to discuss improvements to the marine BSTs based on previous research findings from the Cefic LRI ECO11 project and other foregoing studies. During this workshop, methodological improvements to the OECD 306 test were discussed, in addition to clarifying guidance on testing and interpretation of results obtained from marine BSTs (such as pass criteria, lag phases, freshwater read across and complex substances). Methodologically: (i) increasing bacterial cell concentrations to better represent the bacterial diversity inherent in the sampled environments; and (ii) increasing test durations to investigate extended lag phases observed in marine assessments, were recommended to be validated in a multi-institutional ring test.

Toxicity mitigation and bioaccessibility of the cationic surfactant cetyltrimethylammonium bromide in a sorbent-modified biodegradation study

Biodegradation potential of cationic surfactants may be hampered by inhibition of inoculum at concentrations required to accurately measure inorganic carbon. At >0.3 mg/L cetyltrimethylammonium bromide (CTAB) negatively impacted degradation of the reference compound aniline. We used silicon dioxide (SiO₂) and illite as inorganic sorbents to mitigate toxicity of CTAB by lowering freely dissolved concentrations. In an OECD Headspace Test we tested whether 16.8 mg/L CTAB was readily biodegradable in presence of two concentrations of SiO₂ and illite. SiO₂ adsorbed 85% and 98% CTAB, resulting in concentrations of 2.5 and 0.34 mg/L, mineralized to CO₂ >60% within 16 and 23 d, respectively. With 89% and 99% sorbed to illite, 60% mineralization was reached within 9 and 23 d, respectively. However, higher sorbent concentrations increased time needed to reach >60% mineralization. Thus, desorption kinetics likely decreased bioaccessibility. It is therefore essential to determine appropriate concentrations of mitigating sorbents to render a Headspace Test based on carbon analysis suitable to determine ready biodegradability of compounds which might inhibit inoculum. This would avoid use of expensive radiolabeled compounds. However, high sorbent concentrations can reduce bioaccessibility and limit degradation kinetics, particularly for relatively toxic substances that require strong mitigation.

Improving the ecological relevance of aquatic bacterial communities in biodegradability screening assessments

Concentrating cells from aqueous samples is a common requirement for the enumeration of biomass, investigations of microbial diversity and detection of relatively rare organisms in the environment. Accurately representing the initial sampled environments in the concentrated cells is of particular importance when the subsequent analyses have tangible environmental, economic and societal consequences, as is the case with environmental exposure and risk assessment of chemicals. This study investigated the potential use of four different cell concentration methods: centrifugation, membrane filtration, tangential flow filtration and column colonisation. These methods were assessed against a series of scientific and practical criteria, including: similarity of concentrated community to initial environmental sample; cell concentration achieved; biodegradation test outcome; sample throughput; and capital and maintenance costs. All methods increased cell concentration by as little as 10-fold to as much as 1000-fold. DGGE and 454 pyrosequencing analysis showed concentrated communities to have >60% similarity to each other, and the initial sample. There was a general trend for a more reliable assessment of 4-nitrophenol biodegradation in 96-well plate biodegradation assays, with increasing cell concentration. Based on the selection criteria, it is recommended that there is not one concentration method fit for all purposes, rather, the appropriate method should be selected on a case-by-case basis. Membrane filtration would be the most suitable method for low sample volumes; the increased throughput capacity of tangential flow filtration renders it most suitable for large volumes; and centrifugation is most suitable for samples with high initial biomass concentrations. The poor similarity in microbial community composition of the column colonised samples compared to the initial samples, suggested a concentration basis; this combined with its low sample throughput precluded this approach for future concentration studies of planktonic bacterial samples.

Biodegradability standards for carrier bags and plastic films in aquatic environments: a critical review

Plastic litter is encountered in aquatic ecosystems across the globe, including polar environments and the deep sea. To mitigate the adverse societal and ecological impacts of this waste, there has been debate on whether 'biodegradable' materials should be granted exemptions from plastic bag bans and levies. However, great care must be exercised when attempting to define this term, due to the broad and complex range of physical and chemical conditions encountered within natural ecosystems. Here, we review existing international industry standards and regional test methods for evaluating the biodegradability of plastics within aquatic environments (wastewater, unmanaged freshwater and marine habitats). We argue that current standards and test methods are insufficient in their ability to realistically predict the biodegradability of carrier bags in these environments, due to several shortcomings in experimental procedures and a paucity of information in the scientific literature. Moreover, existing biodegradability standards and test methods for aquatic environments do not involve toxicity testing or account for the potentially adverse ecological impacts of carrier bags, plastic additives, polymer degradation products or small (microscopic) plastic particles that can arise via fragmentation. Successfully addressing these knowledge gaps is a key requirement for developing new biodegradability standard(s) for lightweight carrier bags.

Biofouling in membrane bioreactors: nexus between polyacrylonitrile surface charge and community composition

The influence of membrane surface charge on biofouling community composition during activated sludge filtration in a membrane bioreactor was investigated in this study using polyacrylonitrile-based membranes. Membranes with different surface properties were synthesized by phase inversion followed by a layer-by-layer modification. Various characterization results showed that the membranes differed only in their surface chemical composition and charge, ie two of them were negative, one neutral and one positive. Membrane fouling experiments were performed for 40 days and the biofouling communities were analyzed. PCR-DGGE fingerprinting indicated selective enrichment of bacterial populations from the sludge suspension within the biofilms at any time point. The biofilm community composition seemed to change with time. However, no difference was observed between the biofilm community of differently charged membranes at specific time points. It could be concluded that membrane charges do not play a decisive role in the long-term selection of the key bacterial foulants.

High Throughput Biodegradation-Screening Test To Prioritize and Evaluate Chemical Biodegradability

Comprehensive assessment of environmental biodegradability of pollutants is limited by the use of low throughput systems. These are epitomized by the Organisation for Economic Cooperation and Development (OECD) Ready Biodegradability Tests (RBTs), where one sample from an environment may be used to assess a chemical's ability to readily biodegrade or persist universally in that environment. This neglects the considerable spatial and temporal microbial variation inherent in any environment. Inaccurate designations of biodegradability or persistence can occur as a result. RBTs are central in assessing the biodegradation fate of chemicals and inferring exposure concentrations in environmental risk assessments. We developed a colorimetric assay for the reliable quantification of suitable aromatic compounds in a high throughput biodegradation screening test (HT-BST). The HT-BST accurately differentiated and prioritized a range of structurally diverse aromatic compounds on the basis of their assigned relative biodegradabilities and quantitative structure-activity relationship (QSAR) model outputs. Approximately 20 000 individual biodegradation tests were performed, returning analogous results to conventional RBTs. The effect of substituent group structure and position on biodegradation potential demonstrated a significant correlation (P < 0.05) with Hammett's constant for substituents on position 3 of the phenol ring. The HT-BST may facilitate the rapid screening of 100 000 chemicals reportedly manufactured in Europe and reduce the need for higher-tier fate and effects tests.

Assessing the biodegradability of microparticles disposed down the drain

Microparticles made from naturally occurring materials or biodegradable plastics such as poly(3-hydroxy butyrate)-co-(3-hydroxy valerate), PHBV, are being evaluated as alternatives to microplastics in personal care product applications but limited data is available on their ultimate biodegradability (mineralization) in down the drain environmental compartments. An OECD 301B Ready Biodegradation Test was used to quantify ultimate biodegradability of microparticles made of PHBV foam, jojoba wax, beeswax, rice bran wax, stearyl stearate, blueberry seeds and walnut shells. PHBV polymer was ready biodegradable reaching 65.4 ± 4.1% evolved CO₂ in 5 d and 90.5 ± 3.1% evolved CO₂ in 80 d. PHBV foam microparticles (125-500 μm) were mineralized extensively with >66% CO₂ evolution in 28 d and >82% CO₂ evolution in 80 d. PHBV foam microparticles were mineralized at a similar rate and extent as microparticles made of jojoba wax, beeswax, rice bran wax, and stearyl stearate which reached 84.8  ± 4.8, 84.9  ± 2.2, 82.7  ± 4.7, and 86.4 ± 3.2% CO₂ evolution respectively in 80 d. Blueberry seeds and walnut shells mineralized more slowly only reaching 39.3  ± 6.9 and 5.1 ± 2.8% CO₂ evolution in 80 d respectively.

Environmentally Relevant Inoculum Concentrations Improve the Reliability of Persistent Assessments in Biodegradation Screening Tests

Standard OECD biodegradation screening tests (BSTs) have not evolved at the same rate as regulatory concerns, which now place an increased emphasis on environmental persistence. Consequently, many chemicals are falsely assigned as being potentially persistent based on results from BSTs. The present study increased test duration and increased inoculum concentrations to more environmentally relevant levels to assess their impact on biodegradation outcome and intratest replicate variability for chemicals with known environmental persistence. Chemicals were assigned to potential persistence categories based on existing degradation data. These more environmentally relevant BSTs (erBSTs) improved the reliability of persistence assignment by reducing the high variability associated with these tests and the occurrence of failures at low inoculum concentrations due to the exclusion of specific degraders. Environmental fate was determined using a reference set of ¹⁴C-labeled compounds with a range of potential environmental persistences, and full mass balance data were collated. The erBST correctly assigned five reference chemicals of known biodegradabilities to their appropriate persistence category in contrast to a standard OECD Ready Biodegradation Test (RBTs, P < 0.05). The erBST was significantly more reproducible than an OECD RBT (ANOVA, P < 0.05), with more consistent rates and extent of biodegradation observed in the erBST.

Spatial and temporal variability in the potential of river water biofilms to degrade p-nitrophenol

In order to predict the fate of chemicals in the environment, a range of regulatory tests are performed with microbial inocula collected from environmental compartments to investigate the potential for biodegradation. The abundance and distribution of microbes in the environment is affected by a range of variables, hence diversity and biomass of inocula used in biodegradation tests can be highly variable in space and time. The use of artificial or natural biofilms in regulatory tests could enable more consistent microbial communities be used as inocula, in order to increase test consistency. We investigated spatial and temporal variation in composition, biomass and chemical biodegradation potential of bacterial biofilms formed in river water. Sampling time and sampling location impacted the capacity of biofilms to degrade p-nitrophenol (PNP). Biofilm bacterial community structure varied across sampling times, but was not affected by sampling location. Degradation of PNP was associated with increased relative abundance of Pseudomonas syringae. Partitioning of the bacterial metacommunity into core and satellite taxa revealed that the P. syringae could be either a satellite or core member of the community across sampling times, but this had no impact on PNP degradation. Quantitative PCR analysis of the pnpA gene showed that it was present in all samples irrespective of their ability to degrade PNP. River biofilms showed seasonal variation in biomass, microbial community composition and PNP biodegradation potential, which resulted in inconsistent biodegradation test results. We discuss the results in the context of the mechanisms underlying variation in regulatory chemical degradation tests.

Evaluating the ready biodegradability of two poorly water-soluble substances: comparative approach of bioavailability improvement methods (BIMs)

Difficulties encountered in estimating the biodegradation of poorly water-soluble substances are often linked to their limited bioavailability to microorganisms. Many original bioavailability improvement methods (BIMs) have been described, but no global approach was proposed for a standardized comparison of these. The latter would be a valuable tool as part of a wider strategy for evaluating poorly water-soluble substances. The purpose of this study was to define an evaluation strategy following the assessment of different BIMs adapted to poorly water-soluble substances with ready biodegradability tests. The study was performed with two poorly water-soluble chemicals-a solid, anthraquinone, and a liquid, isodecyl neopentanoate-and five BIMs were compared to the direct addition method (reference method), i.e., (i) ultrasonic dispersion, (ii) adsorption onto silica gel, (iii) dispersion using an emulsifier, (iv) dispersion with silicone oil, and (v) dispersion with emulsifier and silicone oil. A two-phase evaluation strategy of solid and liquid chemicals was developed involving the selection of the most relevant BIMs for enhancing the biodegradability of tested substances. A description is given of a BIM classification ratio (R BIM), which enables a comparison to be made between the different test chemical sample preparation methods used in the various tests. Thereby, using this comparison, the BIMs giving rise to the greatest biodegradability were ultrasonic dispersion and dispersion with silicone oil or with silicone oil and emulsifier for the tested solid chemical, adsorption onto silica gel, and ultrasonic dispersion for the liquid one.

From laboratory to environmental conditions: a new approach for chemical's biodegradability assessment

With thousands of organic chemicals released every day into our environment, Europe and other continents are confronted with increased risk of health and environmental problems. Even if a strict regulation such as REgistration, Authorization and restriction of CHemicals (REACH) is imposed and followed by industry to ensure that they prove the harmlessness of their substances, not all testing procedures are designed to cope with the complexity of the environment. This is especially true for the evaluation of persistence through biodegradability assessment guidelines. Our new approach has been to adapt "in the lab" biodegradability assessment to the environmental conditions and model the probability for a biodegradation test to be positive in the form of a logistic function of both the temperature and the viable cell density. Here, a proof of this new concept is proposed with the establishment of tri-dimensional biodegradability profiles of six chemicals (sodium benzoate, 4-nitrophenol, diethylene glycol, 2,4,5-trichlorophenol, atrazine, and glyphosate) between 4 to 30 °C and 10(4) to 10(8) cells ml(-1) as can be found in environmental compartments in time and space. The results show a significant increase of the predictive power of existing screening lab-scale tests designed for soluble substances. This strategy can be complementary to those current testing strategies with the creation of new indicators to quantify environmental persistence using lab-scale tests.

The Effects of Molecular Properties on Ready Biodegradation of Aromatic Compounds in the OECD 301B CO2 Evolution Test

Ready biodegradation is the primary biodegradability of a compound, which is used for discriminating whether a compound could be rapidly and readily biodegraded in the natural ecosystems in a short period and has been applied extensively in the environmental risk assessment of many chemicals. In this study, the effects of 24 molecular properties (including 2 physicochemical parameters, 10 geometrical parameters, 6 topological parameters, and 6 electronic parameters) on the ready biodegradation of 24 kinds of synthetic aromatic compounds were investigated using the OECD 301B CO2 Evolution test. The relationship between molecular properties and ready biodegradation of these aromatic compounds varied with molecular properties. A significant inverse correlation was found for the topological parameter TD, five geometrical parameters (Rad, CAA, CMA, CSEV, and N c), and the physicochemical parameter K ow, and a positive correlation for two topological parameters TC and TVC, whereas no significant correlation was observed for any of the electronic parameters. Based on the correlations between molecular properties and ready biodegradation of these aromatic compounds, the importance of molecular properties was demonstrated as follows: geometrical properties > topological properties > physicochemical properties > electronic properties. Our study first demonstrated the effects of molecular properties on ready biodegradation by a number of experiment data under the same experimental conditions, which should be taken into account to better guide the ready biodegradation tests and understand the mechanisms of the ready biodegradation of aromatic compounds.

Comprehensive review of several surfactants in marine environments: Fate and ecotoxicity

Surfactants are a commercially important group of chemicals widely used on a global scale. Despite high removal efficiencies during wastewater treatment, their high consumption volumes mean that a certain fraction will always enter aquatic ecosystems, with marine environments being the ultimate sites of deposition. Consequently, surfactants have been detected within marine waters and sediments. However, aquatic environmental studies have mostly focused on the freshwater environment, and marine studies are considerably underrepresented by comparison. The present review aims to provide a summary of current marine environmental fate (monitoring, biodegradation, and bioconcentration) and effects data of 5 key surfactant groups: linear alkylbenzene sulfonates, alcohol ethoxysulfates, alkyl sulfates, alcohol ethoxylates, and ditallow dimethyl ammonium chloride. Monitoring data are currently limited, especially for alcohol ethoxysulfates and alkyl sulfates. Biodegradation was shown to be considerably slower under marine conditions, whereas ecotoxicity studies suggest that marine species are approximately equally as sensitive to these surfactants as freshwater species. Marine bioconcentration studies are almost nonexistent. Current gaps within the literature are presented, thereby highlighting research areas where additional marine studies should focus.

Development and application of screening tools for biodegradation in water-sediment systems and soil

Two new screening-test systems for biodegradation in water-sediment systems (WSST; Water-Sediment Screening Tool) and soil (SST; Soil Screening Tool) were developed in analogy with the water-only test system OECD 301C (MITI-test). The test systems could be applied successfully to determine reproducible experimental mineralization rates and kinetics on the screening-test level for fifteen organic chemicals in water (MITI), water-sediment (WSST) and soil (SST). Substance-specific differences were observed for mineralization compared among the three test systems. Based on mineralization rate and mineralization half-life, the fifteen compounds could be grouped into four biodegradation categories: substances with high mineralization and a half-life <28 days in (1) all three test systems, (2) only in the MITI test and in the WSST, (3) only in the SST, and (4) none of the test systems. The observed differences between the MITI results and the WSST and SST biodegradation rates of the compounds do not reflect their (reversible) sorption into organic matter in terms of experimental K(oc) values and log D values for the relevant pH range. Regarding mineralization kinetics we recommend to determine the lag-phase, mineralization half-life and mineralization rate using a 5-parameter logistic regression for degradation curves with and without lag-phase. Experimental data obtained with the WSST and the SST could be verified by showing good agreement with biodegradation data from databases and literature for the majority of compounds tested. Thus, these new screening-tools for water-sediment and soil are considered suitable to determine sound and reliable quantitative mineralization data including mineralization kinetics in addition to the water-only ready biodegradability tests according to OECD 301.

Characterization of para-Nitrophenol-Degrading Bacterial Communities in River Water by Using Functional Markers and Stable Isotope Probing

Microbial degradation is a major determinant of the fate of pollutants in the environment. para-Nitrophenol (PNP) is an EPA-listed priority pollutant with a wide environmental distribution, but little is known about the microorganisms that degrade it in the environment. We studied the diversity of active PNP-degrading bacterial populations in river water using a novel functional marker approach coupled with [(13)C6]PNP stable isotope probing (SIP). Culturing together with culture-independent terminal restriction fragment length polymorphism analysis of 16S rRNA gene amplicons identified Pseudomonas syringae to be the major driver of PNP degradation in river water microcosms. This was confirmed by SIP-pyrosequencing of amplified 16S rRNA. Similarly, functional gene analysis showed that degradation followed the Gram-negative bacterial pathway and involved pnpA from Pseudomonas spp. However, analysis of maleylacetate reductase (encoded by mar), an enzyme common to late stages of both Gram-negative and Gram-positive bacterial PNP degradation pathways, identified a diverse assemblage of bacteria associated with PNP degradation, suggesting that mar has limited use as a specific marker of PNP biodegradation. Both the pnpA and mar genes were detected in a PNP-degrading isolate, P. syringae AKHD2, which was isolated from river water. Our results suggest that PNP-degrading cultures of Pseudomonas spp. are representative of environmental PNP-degrading populations.

Refinement of biodegradation tests methodologies and the proposed utility of new microbial ecology techniques

Society's reliance upon chemicals over the last few decades has led to their increased production, application and release into the environment. Determination of chemical persistence is crucial for risk assessment and management of chemicals. Current established OECD biodegradation guidelines enable testing of chemicals under laboratory conditions but with an incomplete consideration of factors that can impact on chemical persistence in the environment. The suite of OECD biodegradation tests do not characterise microbial inoculum and often provide little insight into pathways of degradation. The present review considers limitations with the current OECD biodegradation tests and highlights novel scientific approaches to chemical fate studies. We demonstrate how the incorporation of molecular microbial ecology methods (i.e., 'omics') may improve the underlying mechanistic understanding of biodegradation processes, and enable better extrapolation of data from laboratory based test systems to the relevant environment, which would potentially improve chemical risk assessment and decision making. We outline future challenges for relevant stakeholders to modernise OECD biodegradation tests and put the 'bio' back into biodegradation.

The colonization dynamics of the gut microbiota in tilapia larvae

The gut microbiota of fish larvae evolves fast towards a complex community. Both host and environment affect the development of the gut microbiota; however, the relative importance of both is poorly understood. Determining specific changes in gut microbial populations in response to a change in an environmental factor is very complicated. Interactions between factors are difficult to separate and any response could be masked due to high inter-individual variation even for individuals that share a common environment. In this study we characterized and quantified the spatio-temporal variation in the gut microbiota of tilapia larvae, reared in recirculating aquaculture systems (RAS) or active suspension tanks (AS). Our results showed that variation in gut microbiota between replicate tanks was not significantly higher than within tank variation, suggesting that there is no tank effect on water and gut microbiota. However, when individuals were reared in replicate RAS, gut microbiota differed significantly. The highest variation was observed between individuals reared in different types of system (RAS vs. AS). Our data suggest that under experimental conditions in which the roles of deterministic and stochastic factors have not been precisely determined, compositional replication of the microbial communities of an ecosystem is not predictable.