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.