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 μmol 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.

亚热带南太平洋环流（SPG）涵盖了全球海洋中最大的贫营养区。在这些偏远的水中，尽管构成了维持微生物生命的潜在养分和能量来源，但溶解的有机物（DOM）仍在地表水中积聚。在整个SPG上的地带剖面上，我们对固溶提取的DOC（SPE-DOC）的超高分辨率质谱（UHR-MS）进行了定量分析，并定量了大块溶解有机碳（DOC）并评估了DOM的组成，以阐明分子水平的原因。在SPG中普遍存在对DOM的明显抵制。我们在吸收和幅度模式下对UHR-MS数据的两种单独的公式分配方法进行了比较，得出了一致的结果。

DOC浓度超过100μmolCL ^-1中央回旋的温暖和盐水中的水比邻近的南太平洋西部的地表水要高。沿着样带，SPE-DOC的浓度变化不如散装DOC。尽管如此，分子水平的调查显示，在中央SPG中积累的DOM的组成通常符合海洋表层DOM的特征，但是所有评估的特性都更为明显。我们发现了大量潜在不稳定的不饱和脂肪族分子式，而对于可能的海洋微生物来源的DOM却计算出较低的降解指数。中央回旋管中的摩​​尔N / C比值明显降低，表明优先利用微生物对含氮DOM的利用。在光层中，DOM的低芳香性表现出广泛光化学返工的独特烙印。在整个水柱中，DOM的老化通过SPE-DOC对表观氧气利用率以及分子水平的小而显着贡献而显而易见。我们的发现表明，SPE-DOC捕获与季节的时间尺度相关的碳分数，该时间尺度涵盖海洋环流，并且稳定回旋系统中的生物地球化学过程被印在DOM分子组成中。