Biodegradation mechanisms of p-xylene by the oceanic microalga Rhodomonas sp. JZB-2 through transcriptomics and...

As a common hazardous chemical in maritime accidental spills, p-xylene (PX) has received widespread attention due to major environmental risks. Biodegradation of PX by marine microalgae represents a safe and efficient bioremediation strategy. It has been found that, using marine microalga Rhodomonas sp. JZB-2, PX (10 mg/L) in seawater can be efficiently removed, achieving a degradation efficiency of 100% within 4 d. However, the molecular mechanisms underlying PX biodegradation by microalgae remain unclear. In this study, the transcriptomics and metabolomics were conjointly analyzed to reveal the differentially expressed genes (DEGs) and the differentially accumulated metabolites (DAMs), respectively. During PX degradation by Rhodomonas sp. JZB-2, two intermediates, p-toluylic acid and p-tolylmethanol were identified using gas chromatography-mass spectrometry (GC-MS), both of which exhibited significantly lower toxicity than the parent compound. In algal cells exposed to PX, thirteen enzymes involved in the biodegradation of aromatic compounds, such as salicylic acid hydroxylase (SAH) and extradiol ring cleavage dioxygenase (EDO), were encoded by DEGs. Through metabolomics, p-toluenesulfonic acid, p-hydroquinone, p-hydroxybenzaldehyde, and protocatechuic acid were believed to be the metabolites of PX by the microalga. KEGG function analysis showed that DAMs were mainly enriched in intracellular aminoacyl-tRNA biosynthesis pathway. According to the significant upregulation of genes encoding enzymes (SAH and EDO) related to the degradation of main intermediates (p-methylsalicylic acid and 4-methylcatechol), the 2-hydroxypentadienoic acid pathway was identified as the main PX degradation pathway in microalgae. These findings will provide a theoretical basis for understanding the degradation of PX by marine microalgae and accelerating PX removal.