Uptake, effects, and metabolism of cyanobacterial toxins in the emergent reed plant Phragmites australis (Cav.) Trin. ex steud

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Abstract
The commonly occurring cyanobacterial toxin microcystin‐LR (MC‐LR) was rapidly taken up by the emergent reed plant Phragmites australis with clear distribution in the different cormus parts of the plant. Highest uptake was detected in the stem, followed by the rhizome. Enzyme extracts of the rhizome system, the stem, and the leaf revealed the presence of soluble glutathione S‐transferases (sGST) measured with the model substrate 1‐chloro‐2,4‐dinitrobenzene. A significant elevation of sGST activity in the rhizome and stem parts of P. australis was detected after a 24‐h exposure to 0.5 μg/L MC‐LR. Rhizome, stem, and leaf tissues were also able to conjugate several microcystin toxins. However, no conjugation, either chemical nor enzymatic, was detected using the related cyanobacterial toxin nodularin as substrate. Highest glutathione S‐transferase activity for the toxin substrates was detected in the pkat/mg range in the stem of P. australis. For MC‐LR, a complete metabolism from the formation of a glutathione conjugate to the degradation of a cysteine conjugate in all cormus parts of the plant is reported. The stepwise degradation of the MC‐LR‐glutathione conjugate to a γ‐glutamylcysteine and a cysteine conjugate was demonstrated by comparison with chemically formed reference compounds and by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry. This is the first evidence for the uptake and metabolism of cyanobacterial toxins by an emergent aquatic macrophyte.