The subject of water contamination by harmful plant toxins contains major knowledge gaps. It is hypothesized that toxic terrestrial plants producing compounds with certain physicochemical properties cause risk to the quality of adjacent drinking-water resources. The properties of the main groups of plant toxins are compared to emerging water contaminants of anthropogenic origin – Persistent and Mobile organic Compounds (PMOCs). Based on the properties of water solubility and aquatic persistency, intensively toxic compounds produced by common higher plants have been selected for further investigation: carcinogenic terpene glycosides from bracken fern (Pteridium sp.) and carcinogenic/hepatotoxic pyrrolizidine alkaloids from butterbur (Petasites sp.) Altogether, the presented work compiles four manuscripts (one published, one accepted for publication and two in preparation for submission) that are divided into two cases according to the subject matter.
Case 1 is centred on the terpene glycosides from Pteridium sp. The carcinogenic terpene glycoside ptaquiloside (PTA) can be released from Pteridium stands and contaminate nearby water resources. Moreover, the genus produce additional structurally nearly identical but more water soluble compounds – ptesculentoside (PTE) and caudatoside (CAU). Besides PTA, the compounds have been largely underinvestigated in regards to chemistry, toxicology, transfer to animals, humans and the environment. The research presented in this thesis created analytical standards of the compounds and of their hydrolysis products, and analytical methods needed to quantify them in plants and in water (Manuscripts 1 and 3). The methods applied to plant samples from 6 continents and to plants systematically sampled in Northern Europe demonstrated that the average combined contents of CAU and PTE in Pteridium nearly comprised the average contents of PTA (Manuscripts 1 and 2). Statistically significant geographical trends have been observed in geographic distribution between CAU and PTA contents in Pteridium in Northern Europe.
The study for the first time reported CAU, PTE and their hydrolysis products in surface waters (up to 5.3 μg/l of PTE, CAU and PTA in total) (Manuscript 3). Research performed simultaneously with the methods developed in this project for the first time demonstrated in vivo toxicity of CAU. Assuming that tolerable levels of CAU and PTE in drinking-water should be the same as for PTA (2x10-3 μg/l estimated in previous research), the terpene glycosides reported in surface water in this thesis would violate the tolerable level of carcinogens in drinking water by up to 2650 times. Stability studies revealed that PTE, CAU and PTA in natural waters degrade fast under slightly basic pH (7.4) and high temperature (15 ˚C) but are significantly more stable under moderately acidic to neutral pH (5.2-6.5) and at low temperature (5 ˚C). Exposed risk of the three compounds to drinking-water resources should be regarded of similar importance, as they were comparably stable under 16 different combinations of studied conditions.
Case 2 of this study focuses on pyrrolizidine alkaloids (PAs) from Petasites sp. By analysing water resources adjacent to the plants, up to 0.53 μg/l of these compounds were detected in a stream and up to 0.23 μg/l in seepage water in shallow groundwater wells (2.2–3.0 m depth, Denmark). No PAs were detected in studied deep groundwater (~60 m depth) in the same area nor in control seepage waters remote from the plants. This study reports the PAs in water wells for the first time (Manuscript 4). Taking into consideration tolerable levels of the PA ingestion proposed by the health authorities (0.007 μg of daily intake per kg of bodyweight), a daily intake of 1 L of the surface water or 2 L of the well water reported in this study would violate the safe limit for an average healthy human being.
This research detected two groups plant toxins in water bodies out of two options carefully selected for investigation (terpene glycosides and pyrrolizidine alkaloids). Even though the specific waters in which the compounds were detected were not consumed for drinking, the results may have far-reaching implications due to the following reasons: 1) Pteridium is the fifth most abundant plant genus in the world. As indirect result of anthropogenic activities it is expanding, and large monocultures can be encountered in proximity to drinking-water resources; 2) Petasites is regarded invasive in the region where the PAs in water were detected. The species require moist soils and therefore proliferate very closely adjacent to freshwater resources; 3) It is estimated that 3% of all flowering plants contain at least 1 PA and many others produce toxins with chemical structures determining high water-solubility and environmental persistence (e.g. other heterocyclic alkaloids, steroids, triazoles, polyketides). The observations show that higher plants producing these toxins ought to be taken into consideration when assessing environmental risks to the quality of adjacent exploited water resources.