Identification of pterins as characteristic humic-like fluorophores released from cyanobacteria and their behavior and fate in natural and engineered water systems

[Display omitted] •Pterins were identified as recurring algae-derived humic-like fluorophores.•Redox transformation from H4Bip to biopterin occurred during oxygen aeration.•Sunlight induced algal biodegradation and photochemical oxidation of biopterin.•Pterins contributed to the absorbance and fluorescence of algae-laden lake water.•Once released during pre-oxidation, biopterin cannot be removed by coagulation. In this study, we have identified 6-L-biopterins and their glucosides as candidate structures for the consistently occurring algae-derived humic-like fluorophores (Em 440–460 nm) using a combination of fluorescence excitation-emission matrixes, size exclusion chromatography with fluorescence detector and ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry. In intact algal cells, biopterins and their glucosides existed mainly as their reduced tetrahydrobiopterin (H4Bip) forms, and redox transformation from H4Bip to biopterin occurred with the exposure of intracellular algal organic matter to oxygen. During the degradation experiment under simulated natural conditions, light irradiation facilitated the utilization of biopterin by Microcystic aeruginosa and induced the photochemical degradation of 6-L-biopterin to 6-carboxypterin. The concentrations of 6-L-biopterin (without counting any other derivatives) ranged from 0.20 μg/L to 2.78 μg/L in water samples from 5 lakes in China. For an investigated sample from Lake Tai surface water, biopterin and its derivatives contributed to 24.0 ± 1.1% of UV absorbance at 350 nm and 55.5 ± 1.7% of fluorescence at Ex350/Em450 nm of total chromophoric/fluorescent dissolved organic matter (CDOM/FDOM). During water treatment, pre-chlorination of algae-laden water induced the releases of biopterin and its derivatives into water, and the variation of biopterin fluorescence intensity can be used as a surrogate indicator for predicting algal membrane damage. Once released into water, biopterin could not be effectively removed by the conventional coagulation treatment irrespective of coagulant type, dose and water pH. The identification of ubiquitous pterins as candidate structures responsible for “humic-like” fluorophores is conducive to the understanding of the generation, property, behavior and fate of algae-derived CDOM/FDOM in natural and engineered water systems.