Microcystin-LR bioaccumulation and depuration kinetics in lettuce and arugula: Human health risk assessment

Highlights

• MC-LR kinetics in arugula and lettuce were investigated

• Estimated Daily Intake of MC-LR by humans was higher than the tolerable WHO limit

• The lack of bioaccumulation of MC-LR in arugula requires further investigations

• Kinetics results showed that ca. 30 days are required for MC-LR saturation in lettuce

• It was possible to recover MC-LR contaminated lettuce during depuration of the toxin

Abstract

Microcystin-LR (MC-LR) is one of the most toxic and common microcystins (MCs) variant found in aquatic ecosystems. Little is known about the possibility of recovering microcystins contaminated agricultural crops. The objectives of this study were to determine the bioaccumulation and depuration kinetics of MC-LR in leaf tissues of lettuce and arugula, and estimate the total daily intake (ToDI) of MC-LR via contaminated vegetables by humans. Arugula and lettuce were irrigated with contaminated water having 5 and 10 μg L^− 1 of MC-LR for 7 days (bioaccumulation), and subsequently, with uncontaminated water for 7 days (depuration). Quantification of MC-LR was performed by LC-MS/MS. The one-compartment biokinetic model was employed for MC-LR bioaccumulation and depuration data analysis. MC-LR was only accumulated in lettuce. After 7 days of irrigation with uncontaminated water, over 25% of accumulated MC-LR was still retained in leaf tissues of plants treated with 10 μg L^− 1 MC-LR. Total daily toxin intake by adult consumers (60 kg-bw) exceeded the 0.04 μg MC-LR kg^− 1 limit recommended by WHO. Bioaccumulation was found to be linearly proportional to the exposure concentration of the toxin, increasing over time; and estimated to become saturated after 30 days of uninterrupted exposure. On the other hand, MC-LR depuration was less efficient at higher exposure concentrations. This is because biokinetic half-life calculations gave 2.9 and 3.7 days for 5 and 10 μg L^− 1 MC-LR treatments, which means 29–37 days are required to eliminate the toxin. For the first time, our results demonstrated the possibility of MC-LR decontamination of lettuce plants.

Introduction

Cyanobacteria are photosynthetic prokaryotes that occur in natural and man-made aquatic ecosystems (Sivonen and Jones, 1999). They produce a variety of toxins such as anatoxins, cylindrospermopsins, saxitoxins and microcystins, which are of serious environmental and public health risks (Kozdęba et al., 2014). Microcystins (MCs) are monocyclic heptapeptides produced through nonribosomal peptides synthases (Tillett et al., 2000). They are potent inhibitors of protein phosphatases 1 and 2A in plants and animals (Mackintosh et al., 1990). MCs cause oxidative stress and alter the activities of antioxidant enzymes in plants (Pflugmacher et al., 1998, Babica et al., 2006, Pflugmacher et al., 2007).

In mammals, MCs alter the cytoskeletons of hepatocytes, induce intrahepatic hemorrhage and hepatic insufficiency of liver tissues (Yoshida et al., 1997). Furthermore, chronic exposure to MCs has been linked to serious health problems such as liver and colorectal cancers (Sivonen and Jones, 1999). For example, human related deaths were reported of hemodialysis patients exposed to MCs contaminated water in Brazil (Jochimsen et al., 1998). This makes the evaluation of potential exposure routes, public health risk and cyanotoxins characterization a priority.

Water supply reservoirs used for irrigation are sometimes contaminated with toxic cyanobacteria and may contain high concentrations of MCs (Orihel et al., 2012, Singh et al., 2015). When irrigated with contaminated water, terrestrial plants are exposed to MCs; and some may accumulate the toxins (Crush et al., 2008, Mohamed and Al-Shehri, 2009, Hereman and Bittencourt-Oliveira, 2012, Bittencourt-Oliveira et al., 2016). In response to the threat posed by this exposure route (Codd et al., 1999, Hereman and Bittencourt-Oliveira, 2012, Bittencourt-Oliveira et al., 2016), the World Health Organization (WHO, 2011) recommends a human Tolerable Daily Intake (TDI) limit of 0.04 μg MC-LR kg^− 1 body mass d^− 1.

Although recent studies have shown that MCs can be retained in terrestrial plant tissues (Crush et al., 2008, Mohamed and Al-Shehri, 2009, Gutiérrez-Praena et al., 2014, Corbel et al., 2014, Corbel et al., 2016), not much is known about their accumulation and depuration in vegetables. Further investigations are required to understand the uptake and fate of MCs in food plants, and their persistence in vegetables (Corbel et al., 2014, Corbel et al., 2016). To date, bioaccumulation studies have revealed that additional information is needed on the dynamism and mechanism of toxin uptake and accumulation, translocation and metabolism in plants (Gutiérrez-Praena et al., 2014). Studies on bioaccumulation and depuration kinetics of cyanotoxins in plants enable the calculation of their persistence in vegetable tissues, as well as the time required to recover contaminated vegetables.

The leaves of vegetables such as lettuce (Lactuca sativa L.) and arugula (Eruca sativa Mill.) are consumed worldwide (McMichael, 1994). It is for this reason that the choice of their leaves for bioaccumulation and depuration studies of MCs is very important. The objectives of the present study were to (1) determine the bioaccumulation and depuration kinetics of MC-LR, and (2) estimate total daily intake (ToDI) of MC-LR based on the amount accumulated in leaf tissues of lettuce and arugula, after irrigation with contaminated water. Total daily intake information is important for evaluating public health risk associated with the consumption of MC-LR contaminated vegetables, at different bioaccumulation and depuration stages.