Detection of freshwater cyanotoxins and measurement of masked microcystins in tilapia from Southeast Asian aquaculture farms

Abstract

Recently, there has been a rise in freshwater harmful algal blooms (HABs) globally, as well as increasing aquaculture practices. HABs can produce cyanotoxins, many of which are hepatotoxins. An ultra-performance liquid chromatography tandem mass spectrometry method was developed and validated for nine cyanotoxins across three classes including six microcystins, nodularin, cylindrospermopsin and anatoxin-a. The method was used to analyse free cyanotoxin(s) in muscle (n = 34), liver (n = 17) and egg (n = 9) tissue samples of 34 fish sourced from aquaculture farms in Southeast Asia. Conjugated microcystin was analysed by Lemieux oxidation to ascertain the total amount of microcystin present in muscle. Some tilapia accumulated free microcystin-LR in the muscle tissue at a mean of 15.45 μg/kg dry weight (dw), with total microcystin levels detected at a mean level of 110.1 μg/kg dw, indicating that the amount of conjugated or masked microcystin present in the fish muscle accounted for 85% of the total. Higher levels of cyanotoxin were detected in the livers, with approximately 60% of those tested being positive for microcystin-LR and microcystin-LF, along with cylindrospermopsin. Two fish from one of the aquaculture farms contained cylindrospermopsin in the eggs; the first time this has been reported. The estimated daily intake for free and total microcystins in fish muscle tissue was 2 and 14 times higher, respectively, than the tolerable daily intake value. This survey presents the requirement for further monitoring of cyanotoxins, including masked microcystins, in aquaculture farming in these regions and beyond, along with the implementation of guidelines to safeguard human health. Graphical abstract ᅟ.

Keywords: Aquaculture; Bioaccumulation; Harmful algal bloom; Human health; Microcystin; UPLC-MS/MS.

Graphical abstract

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Fig. 1

Chemical structures of the a cyclic heptapeptide microcystin-LR with X and Y being the variable amino acids as indicated in the accompanying table; b cyclic pentapeptide nodularin, with only one variable amino acid (X = arginine), the most common congener; c tricyclic alkaloid cylindrospermopsin (CYN); and d the alkaloid anatoxin-A (ATX-A)

Fig. 2

Results of the free toxin levels detected in the muscle tissue, liver and eggs of the tilapia harvested from the aquaculture farms

Fig. 3

Results of the MMPB detected in fish from aquaculture farm 1–1 with the molar equivalents of MC-LR calculated