Accumulation of DOC in the South Pacific Subtropical Gyre from a molecular perspective

The subtropical South Pacific Gyre (SPG) encompasses the largest oligotrophic region of global ocean. In these remote waters dissolved organic matter (DOM) accumulates in surface waters, though constituting a potential source nutrients and energy to sustain microbial life. On zonal transect across SPG, we quantified bulk carbon (DOC) assessed DOM composition via ultrahigh resolution mass spectrometry (UHR-MS) solid-phase extracted DOC (SPE-DOC) elucidate molecular-level reasons behind apparent recalcitrance prevailing SPG. We included comparison between two individual formula assignment approaches UHR-MS data absorption magnitude mode which yielded consistent results. concentrations exceeding 100 ?mol C L?1 warm saline central gyre were higher than adjacent western Pacific. Along transect, SPE-DOC less variable DOC. Nevertheless, investigation revealed that accumulated SPG generally conformed characteristics ocean DOM, but all properties more pronounced. found high abundances potentially labile unsaturated aliphatic molecular formulas low calculated degradation index for likely marine origin. Markedly decreased molar N/C ratios indicated preferential utilization nitrogen-containing DOM. A distinct imprint extensive photochemical reworking was manifested aromaticity photic layer. Over whole water column, ageing evident through small, significant contribution oxygen as well on level. Our findings demonstrate captures fractions relevant timescales seasons covering circulation biogeochemical processes stable systems are imprinted composition.