Transformation mechanism of methylphosphonate to methane by Burkholderia sp: Insight from multi-labeled water isotope...

Methylphosphonate (MPn), has been identified as a likely source of methane in aerobic ocean and may be responsible for the "ocean methane paradox", that is oversaturation of dissolved methane in oxic sea waters. However, the mechanism underlying the cleavage of C-P bonds during microbial degradation is not well understood. Using multi-labeled water isotope probing (MLWIP) and transcriptome analysis, we investigated the phosphate oxygen isotope systematics and mechanisms of microbial-mediated degradation of MPn in this study. In the aerobic culture containing MPn as the only phosphorus source, there was a significant release of inorganic phosphate (149.4 μmol/L) and free methane (268.3 mg/L). The oxygen isotopic composition of inorganic phosphorus (δ¹⁸O_P) of accumulated released phosphate was 4.50‰, 23.96‰, and 40.88‰, respectively, in the corresponding ¹⁸O-labeled waters of -10.3‰, 9.9‰, and 30.6‰, and the slope obtained in plots of δ¹⁸O_P versus the oxygen isotopic composition of water (δ¹⁸O_W) was 0.89. Consequently, 89% of the oxygen atoms (Os) in phosphate (PO₄) were exchanged with ¹⁸O-labeled waters in the medium, while the rest were exchanged with intracellular metabolic water. It has been confirmed that the C-P bond cleavage of MPn occurs in the cell with both ambient and metabolic water participation. Moreover, phn gene clusters play significant roles to cleave the C-P bond of MPn for Burkholderia sp. HQL1813, in which phnJ, phnM and phnI genes are significantly up-regulated during MPn decomposition to methane. In conclusion, the aerobic biotransformation of MPn to free methane by Burkholderia sp. HQL1813 has been elucidated, providing new insights into the mechanism that bio-cleaves C-P bonds to produce methane aerobically in aqueous environments for representative phosphonates.