Biotransformation Dynamics and Products of Cyanobacterial Secondary Metabolites in Surface Waters

Cyanobacteria produce toxic and bioactive secondary metabolites, posing risks to ecosystems and human health, yet their transformation pathways in surface waters remain unclear. We assessed biotransformation for 40 cyanopeptides including microcystins, anabaenopeptins and cyanopeptolins in surface waters and in situ enriched biofilm suspensions. In surface waters, most cyanopeptides did not degrade significantly over the course of 7 days. A wide range of biodegradability across cyanopeptides was apparent in biofilm suspensions from three rivers. Increasing the biofilm density shortened the lag time and increased initial removal of cyanopeptides. Increasing the initial cyanopeptide concentration lengthened the lag time and decreased their initial removal, supporting inhibitory effects of cyanopeptides toward enzymes involved in their own transformation. Transformation kinetics and product analysis demonstrated a structure–reactivity relationship across and within cyanopeptide classes. Anabaenopeptins were hydrolyzed at the C-terminus when arginine, tyrosine and (iso)leucine were present, but not when phenylalanine or tryptophan was present. Microcystins showed tetrapeptide formation when adda linked to arginine but not when it linked to alanine, leucine, or tyrosine. Oxidation of tyrosine and deamination of arginine residues showed an interdependence across cyanopeptide classes. These novel insights into biotransformation products and pathways of a wide range of cyanopeptides facilitate assessment of exposure scenarios in surface waters and inform about kinetics and product formation in biological water treatment.