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AbstractBiosynthetic toxic compounds from plants and cyanobacteria constitute a chemically diverse family of at least 20,000 compounds. Recent work with natural toxin databases and toxin characterization shows that the majority of natural toxins are polar and mobile, with toxicity ranging from low to very high, while persistence is highly variable. Natural toxins may be produced in high quantities—some exceeding 10 g/m2/year—resulting in high environmental loads. Recent phytotoxin monitoring indicates that one or more natural toxin is always present in a surface water sample, but that concentrations are highly variable often with pulses during rain events. Phytotoxins belong to many classes, but often with flavonoids and alkaloids dominating. Likewise, advanced monitoring discovers a wide spectrum of cyanobacterial metabolites that are released directly into surface waters during water blooms. Except of the few known cyanobacterial toxins, we have very limited info regarding their environmental fate and toxicity.The 16 papers in this article collection present examples of natural toxin occurrence, properties, fate and toxicity. The overarching conclusion is that natural toxins should be monitored and characterized regarding their risk potential, and that natural toxins of greatest expected risk should be evaluated as thoroughly as industrial xenobiotics. Cyanotoxins are well known water contaminants that should be removed for producing drinking water, while for phytotoxins the current knowledge base is very limited. We advocate to intensify research on natural toxins, and to address the evident knowledge gaps on natural toxin analysis/monitoring, physical–chemical properties and degradation/pathways, transport modelling, and toxicity. The complex and dynamic interplays between biotic and site conditions such as vegetation, toxic plant densities, climate, soil types, nutrients and radiation, play decisive roles for both biotoxin formation and fate. Environmental and toxicological research in biosynthesized compounds extends beyond natural toxins, with important perspectives for risk assessment of biopesticides, growth regulators and biomedicine (or biologicals collectively) produced by plants and microorganisms.

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Abstract
Background
Phytotoxins of various classes and origin are often found in their cationic form in the soil environment and thus, their overall soil behavior may be strongly affected by all geosorbents presenting cation exchange capacity (CEC). In addition to soil organic carbon (SOC), clays may exhibit great potential as sorbents for cationic organic chemicals. Therefore, 52 compounds of the major phytotoxin classes alkaloids, terpenoids and steroids were investigated with regard to their sorption behavior to the clay minerals kaolinite (low CEC) and montmorillonite (high CEC) by means of continuous flow column sorption experiments as a high-throughput alternative to traditional batch sorption experiments.

Results
In total, sorption coefficients log Dclay [L kg−1] were quantifiable for 26 phytotoxins on kaolinite (log Dclay > 0.1) and 33 on montmorillonite (log Dclay > 0.5). They ranged from 0.14 ± 0.09 for the pyrrolizidine alkaloid senkirkine on kaolinite to 3.05 ± 0.03 for the indole alkaloid brucine on montmorillonite. Although maximum sorbed concentrations lay well below the CEC for both clay minerals, sorption non-linearity was observed in some cases where as little as 0.1% of all cation exchange sites were occupied. Contrary to the expectations, sorption non-linearity could not be wholly explained by saturation of available sorption sites; for protonated tertiary amines with aromatic moieties, cooperative sorption seemingly took place and the results indicated a significant increase in sorption affinities within a very limited concentration range. Comparing montmorillonite and SOC, notable differences in preferences of cationic sorbates were observed between phytotoxins with and without aromatic moieties (e.g., isoquinoline versus pyrrolizidine alkaloids) as well as between N-heterocycles and N-heteroaromatics in particular (e.g., strychnine versus gramine; both indole alkaloids).

Conclusions
Overall, clay sorption seems a result of the interplay of charge location on the sorbent and various structural features of the sorbates. To confirm observed tendencies towards cooperative sorption for certain cationic phytotoxins, further studies with higher concentrations are needed. Nevertheless, obtained sorption coefficients indicate that a high proportion of phytotoxin sorption in soils may be attributed to clay minerals. Thus, clay minerals possess the ability to decrease total cationic phytotoxin environmental mobility.

Assessing the bioaccessibility of organic pollutants in contaminated soils is considered a complement to measurements of total concentrations in risk assessment and legislation. Consequently, methods for its quantification require validation with historically contaminated soils. In this study, 35 such soils were obtained from various locations in Switzerland and Cuba. They were exposed to different pollution sources (e.g., pyrogenic and petrogenic) at various distance (i.e., urban to rural) and were subject to different land use (e.g., urban gardening and forest). Passive equilibrium sampling with polyoxymethylene was used to determine freely dissolved concentrations (C free ) of polycyclic aromatic hydrocarbons (PAHs), while sorptive bioaccessibility extraction (SBE) with silicone rods was used to determine the bioaccessible PAH concentrations (C bioacc ) of these soils. The organic carbon partition coefficients of the soils were highest for skeet soils, followed by traffic, urban garden and rural soils. Lowest values were obtained from soil exposed to petrogenic sources. Applicability of SBE to quantify C bioacc was restricted by silicone rod sorption capacity, as expressed quantitatively by the Sorption Capacity Ratio (SCR); particularly for soils with very high K D . The source of contamination determined bioaccessible fractions (f bioacc ). The smallest f bioacc were obtained with skeet soils (15%), followed by the pyrogenically influenced soils, rural soils, and finally, the petrogenically contaminated soil (71%). In conclusion, we present the potential and limitations of the SBE method to quantify bioaccessibility in real soils. These results can be used for additional development of this and similar bioaccessibility methods to guarantee sufficient sorption capacity to obtain reliable results. Freely dissolved concentrations and bioaccessible fractions of PAHs in soils depended on pollutant source and site characteristics, while the sink capacity was limiting during bioaccessibility extractions of soils ...

Abstract
Background
Substantial efforts have been made to monitor potentially hazardous anthropogenic contaminants in surface waters while for plant secondary metabolites (PSMs) almost no data on occurrence in the water cycle are available. These metabolites enter river waters through various pathways such as leaching, surface run-off and rain sewers or input of litter from vegetation and might add to the biological activity of the chemical mixture. To reduce this data gap, we conducted a LC–HRMS target screening in river waters from two different catchments for 150 plant metabolites which were selected from a larger database considering their expected abundance in the vegetation, their potential mobility, persistence and toxicity in the water cycle and commercial availability of standards.

Results
The screening revealed the presence of 12 out of 150 possibly toxic PSMs including coumarins (bergapten, scopoletin, fraxidin, esculetin and psoralen), a flavonoid (formononetin) and alkaloids (lycorine and narciclasine). The compounds narciclasine and lycorine were detected at concentrations up to 3ʵg/L while esculetin and fraxidin occurred at concentrations above 1ʵg/L. Nine compounds occurred at concentrations above 0.1ʵg/L, the Threshold for Toxicological Concern (TTC) for non-genotoxic and non-endocrine disrupting chemicals in drinking water.

Conclusions
Our study provides an overview of potentially biologically active PSMs in surface waters and recommends their consideration in monitoring and risk assessment of water resources. This is currently hampered by a lack of effect data including toxicity to aquatic organisms, endocrine disruption and genotoxicity and demands for involvement of these compounds in biotesting.

Pyrogenic carbon (PyC), produced naturally (wildfire charcoal) and anthropogenically (biochar), is extensively studied due to its importance in several disciplines, including global climate dynamics, agronomy and paleosciences. Charcoal and biochar are commonly used as analogues for each other to infer respective carbon sequestration potentials, production conditions, and environmental roles and fates. The direct comparability of corresponding natural and anthropogenic PyC, however, has never been tested. Here we compared key physicochemical properties (elemental composition, δ13C and PAHs signatures, chemical recalcitrance, density and porosity) and carbon sequestration potentials of PyC materials formed from two identical feedstocks (pine forest floor and wood) under wildfire charring- and slow-pyrolysis conditions. Wildfire charcoals were formed under higher maximum temperatures and oxygen availabilities, but much shorter heating durations than slow-pyrolysis biochars, resulting in differing physicochemical properties. These differences are particularly relevant regarding their respective roles as carbon sinks, as even the wildfire charcoals formed at the highest temperatures had lower carbon sequestration potentials than most slow-pyrolysis biochars. Our results challenge the common notion that natural charcoal and biochar are well suited as proxies for each other, and suggest that biochar's environmental residence time may be underestimated when based on natural charcoal as a proxy, and vice versa. ; C.S. and S.H.D. thank the funding support by the 'Bridging the Gaps' EPRSC program and the Leverhulme Trust grant (RPG-2014-095). C.S. was also supported by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 663830 and S.H.D. by a Leverhulme Fellowship (RF-2016-456\2). We thank N. Loader for the δ13C analysis. ; SI

Natural estrogens such as 17 alpha-estradiol (E2 alpha), 17 beta-estradiol (E2 beta), estrone (E1), and estriol (E3), released to surface waters from both urban and agricultural sources, are endocrine disrupting for fish. Here, we assess the prevalence of livestock farming derived natural estrogens in tributaries and ponds in the agriculturally dominated catchment of Lake Baldegg, Switzerland. Passive samplers were deployed in the main tributary and daily time-proportional water samples were collected in five tributaries for 30 days at the beginning of the vegetation period. Furthermore, we took grab samples of 12 ponds in the catchment. Aqueous samples were liquid-liquid extracted, derivatized, and analysed with LC-MS/MS and stream water samples additionally with ER alpha-CALUX, a bioassay for assessing total estrogenic activity. Natural estrogens were regularly detected, with mean concentrations ranging from below the limit of detection to 0.55 ng L-1 for E2 beta and E1, respectively, and passive sampling and bioassay results largely confirmed these findings. Monte Carlo simulated mean natural estrogen concentrations underestimated measured ones by a factor of three to 11. An agricultural area's hydrological contribution and connectivity to surface waters seemed to be more important for the development of estrogen concentrations in streams than livestock densities in a catchment or the actual loads of slurry applied. Pond water occasionally contained natural estrogens in concentrations up to 8.6 ng L-1 for E2 alpha. The environmental quality standards of the European Union (0.4 ng L-1 for E2 beta and 3.6 ng L-1 for E1) were never exceeded for longer than a day in tributaries, but E1 reached critical concentrations for aquatic organisms in ponds. ; ISSN:2050-7887 ; ISSN:2050-7895