Phosphate oxygen isotope evidence for methylphosphonate sources of methane and dissolved inorganic phosphate

摘要

The ocean is an important source of methane, however, the sources of oceanic methane and mechanisms of its release to the atmosphere have only recently begun to be understood. Recent studies have identified methylphosphonate (MPn) as a previously unknown and likely source of methane in the aerobic ocean (Karl et al., 2008), as well as shown the biosynthesis of methylphosphonic acid to be a widespread trait in marine microbes (Metcalf et al., 2012). The mechanisms and reaction pathways from MPn to free methane, however, have not been well studied. Here we present results of laboratory studies on the photo-degradation of MPn, a likely mechanism of methane release to the atmosphere and phosphate release to the surface oceans. Phosphonoacetic acid was also studied as an additional model compound for comparison. We used the multi-labeled water isotope probing (MLWIP) approach, involving O-18-labeled waters to probe the photolytic mechanism of C-P bond cleavage in phosphates through analysis of P released from MPn as PO4. These studies identified distinct reaction pathways involving phosphates compared with other common organophosphorus compounds (e.g., phosphoesters), as well as suggest the involvement of both ambient water and atmospheric oxygen in C-P bond cleavage. There is only a small amount of water oxygen incorporated into product PO4 after cleavage of the C-P bond in MPn, suggesting atmospheric O-2 or radicals formed from O-2 under Ultra Violet Radiation (UVR), as the primary source of O that replaces C in the C-P bond of MPn. Model calculations suggest that the delta O-18(P) signature of phosphate released via UV-degradation of phosphates is largely (75%) inherited from the original phosphate substrate. This opens up the possibility of tracing and differentiating specific phosphate sources of dissolved phosphate from other organophosphorus (Porg) sources (e.g., phosphoesters) used in primary production, as well as for tracing specific MPn sources of atmospheric methane. (C) 2018 Published by Elsevier B.V.