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

Automated summary

The subtropical South Pacific Gyre contains the largest oligotrophic region in the global ocean, where dissolved organic matter accumulates in the surface waters. A molecular-level study revealed distinct characteristics of the DOM in this region, including high abundances of potentially labile unsaturated aliphatic molecular formulas and a low degradation index. Additionally, microbial utilization of nitrogen-containing DOM and extensive photochemical reworking were evident, reflecting carbon fractions relevant on different timescales.