# Sequential UV-C Irradiation and Sphingopyxis sp. m6 Biodegradation for Enhanced Degradation and Detoxification of Microcystin-LR

## TL;DR

A new method combining UV light and bacteria efficiently removes and detoxifies a harmful toxin from water.

## Contribution

A novel sequential process using UV-C irradiation and Sphingopyxis sp. m6 biodegradation for rapid and efficient MC-LR detoxification.

## Key findings

- Sequential UV-C pretreatment followed by biodegradation completely degraded MC-LR within 1 hour.
- Low-dose UV-C induced MC-LR photoisomerization, which was then mineralized by Sphingopyxis sp. m6.
- The process restored HepG2 cell proliferation and protein phosphatase 2A activity, confirming detoxification.

## Abstract

Microcystins (MCs), a group of potent hepatotoxins from cyanobacterial blooms, threaten global water security due to the resistance to conventional treatment processes and multi-organ toxicity to human. This study innovatively proposed a novel sequential process combining UV irradiation with biodegradation by Sphingopyxis sp. m6 for efficient microcystin-LR (MC-LR) removal. Results revealed that sequential UV-C pretreatment followed by Sphingopyxis sp. m6 biodegradation achieved complete degradation of 1 mg/L of MC-LR within 1 h of the biological phase, drastically reducing the treatment time compared to biodegradation alone (5 h). Mechanistic investigation revealed that low-dose UV-C (50 mJ/cm2) pretreatment induced MC-LR photoisomerization consistently with previously reported Adda geometric isomers. These photoisomers, along with residual parent MC-LR, were subsequently mineralized by Sphingopyxis sp. m6. Enzymatic pathway analysis confirmed a dual-pathway degradation, where Mlr enzymes processed both the native toxin and its isomeric forms, leading to a series of linearized peptides and Adda-derived products. Critically, the process achieved efficient detoxification, as confirmed by the restoration of HepG2 cell proliferation and protein phosphatase 2A activity. Moreover, response surface methodology optimized the key parameters (31.49 °C, pH of 7.36, 0.23 mg/L) for the highest degradation efficiency. This work provides an energy- and cost-efficient strategy for MC-LR remediation and elucidates the molecular mechanism of UV-induced photoisomerization facilitating subsequent biodegradation.