Epsilon Archive for Student Projects

Abstract

In recent years the number of reports of toxic cyanobacterial blooms in meso-eutrophic surface water bodies both in Sweden and globally has increased. Frequently occuring water blooms lead to an increased risk of human exposure through drinking water, recreational activities (e.g. bathing) and food. Microcystins (Mcyst) is the most commonly produced toxin family associated with cyanobacteria and epidemiological evidence has shown that chronic exposure to microcystins might be one contributing factor to enhanced frequency of liver cancer in some areas in China. It is therefore important to quantitatively and qualitatively study different uptake routes to humans.
In this study the uptake and translocation of microcystins in lettuce and radish was quantitatively analysed with an enzyme-linked immunosorbent assay (ELISA) method. One week old plants were exposed daily to environmentally relevant levels (0.74 and 8 µg Mcyst/1) of microcystins for 12 days. The uptake routes tested were either through the roots of plants grown in hydrocultures or through the whole plant surface for plants grown on a conventional soil medium. Whole roots and radishes and lettuce leaves were pooled (from 4 individuals), freeze dried and homogenised. Some samples were rinsed in destilled water before being pooled. The microcystins were extracted in an ammonium hydrogen carbonate buffer and filtrated (0.45 µm) before adequate dilution in a phosphate buffer solution (pH 7.5) and analysed with an ELISA-test kit.
In both lettuce and radish, grown in hydrocultures (8 µg Mcyst/1) uptake through the roots and transport to the upper parts of the plants were observed. In the radish an average of 0.18 µg Mcyst/g dry weight (d.w.) and in the radish roots 0.07 µg Mcyst/g d.w. was recorded. Values for lettuce was 0.01 µg Mcyst/g d.w. in the leaves and 0.15 µg Mcyst/g d.w. in the roots. In plants grown on soil, watered with water containing 8 µg Mcyst/1 an average of 0.024 µg Mcyst/g d.w. was recorded in the radish (stem) and 0.074 µg Mcyst/g d.w. in lettuce leaves. The microcystin content was about 75% lower in rinsed lettuce leaves implying that much of the microcystin is not absorbed but remain on the leaf surface when watered from above. In plants exposed to 0.74 µg Mcyst/1 no uptake through the root or plant surface was observed. No cross reactivity of the ELISA-test with compounds in the plant tissue was observed.
The results indicate that it is possible for humans to be exposed to microcystins through 2 vegetables. It is unlikely that acute toxic effects will occur but the possibility of long term effects cannot be excluded. The tolerable daily intake (TDI), 0.04 µg Mcyst/kg body weight (WHO provisional recommendation) might be exceeded in areas where microcystin contaminated water is used for irrigation, if the microcystin levels observed in this study are representative for levels in staple crops as cereals, potatoes, rice etc.