Deep-sea dissolved organic matter (DOM) constitutes a huge carbon reservoir in the worlds' oceans that – despite its abundance – is virtually unused as a substrate by marine heterotrophs. Heating within hydrothermal systems induces major molecular modifications of deep-sea DOM. Here, we tested the hypothesis that hydrothermal heating of deep-sea DOM enhances bioavailability. Aliquots of DOM extracted from the deep North Pacific (North Equatorial Pacific Intermediate Water; NEqPIW) were re-dissolved in artificial seawater and subjected to temperatures of 100 and 200 °C (40 MPa) using Dickson-type reactors. In agreement with earlier findings we observed a temperature-related drop in dissolved organic carbon (DOC) concentration (−6.1% at 100 °C, −21.0% at 200 °C) that predominantly affected the solid-phase extractable (SPE-DOC) fraction (−18.2% at 100 °C, −51.4% at 200 °C). Fourier-transform ion cyclotron resonance mass spectrometric (FT-ICR-MS) analysis confirmed a temperature-related reduction of average molecular mass, O/C ratios, double bond equivalents (DBE) and a relative increase in aromaticity (AI_mod). This thermally altered DOM was added (25 μmol L⁻¹ DOC) to deep-water samples from the South West Pacific (Kermadec Arc, RV Sonne / SO253, 32° 37.706′ S | 179° 38.728′ W) and incubated with the prevailing natural microbial community. After 16 days at 4 °C in the dark, prokaryotic cell counts in incubations containing the full spectrum of thermally-degraded DOM (extractable and non-extractable compounds) had increased considerably (on average 21× for DOM_100°C and 27× for DOM_200°C). In contrast, prokaryotic growth in incubations to which only solid-phase extractable thermally-altered DOM was added was not enhanced compared to control incubations. The experiments demonstrate that temperature-driven degradation of deep-sea recalcitrant DOM within hydrothermal systems turns fractions of it accessible to microbes. The thermally-produced DOM compounds that stimulate microbial growth are not retained on reversed-phase resins (SPE-DOM) and are likely low-molecular mass organic acids. Despite the comprehensive compositional modifications of the solid-phase extractable (SPE-DOM) fraction through heating, it remains inaccessible to microbes at the investigated concentration levels. The microbial incubation resulted in only minor and mostly insignificant overall changes in SPE-DOM molecular composition and concentration.

深海溶解有机物（DOM）在世界海洋中构成了一个巨大的碳库，尽管其碳氢化合物丰富，但实际上几乎没有被海洋异养生物用作底物。水热系统中的加热引起深海DOM的主要分子修饰。在这里，我们测试了深海DOM的水热加热可提高生物利用度的假设。从北太平洋深部（北赤道太平洋中间水； NEqPIW）提取的DOM等分试样重新溶解在人造海水中，并使用Dickson型反应器经受100和200°C（40 MPa）的温度。与之前的发现一致，我们观察到与温度相关的溶解有机碳（DOC）浓度下降（100°C下为-6.1％，200°C下为-21.0％），这主要影响了固相可萃取物（SPE-DOC）分数（−18。在100°C时为2％，在200°C时为-51.4％）。傅里叶变换离子回旋共振质谱（FT-ICR-MS）分析证实了与温度相关的平均分子量，O / C比，双键当量（DBE）降低和芳香度（AI）相对增加_mod）。将这种热变化的DOM（25μmolL ^-1 DOC）添加到西南太平洋的深水样品中（Kermadec Arc，RV Sonne / SO253，32°37.706'S | 179°38.728'W），并与盛行的样品一起孵育天然微生物群落。在黑暗中于4°C下放置16天后，含有全光谱热降解DOM（可萃取和不可萃取化合物）的培养物中的原核细胞计数显着增加（对于DOM _100°C，平均21倍，对于_100℃，平均27倍）。 DOM _{200°摄氏度}）。相反，与对照培养相比，仅添加了固相可萃取热改变的DOM的培养中原核生物的生长并未得到增强。实验表明，温度驱动的热液系统中深海顽固性DOM的降解使微生物可以接触到其一部分。刺激微生物生长的热产生DOM化合物未保留在反相树脂（SPE-DOM）上，可能是低分子量有机酸。尽管通过加热对固相可萃取（SPE-DOM）馏分进行了全面的成分修饰，但在所研究的浓度水平下，微生物仍难以接近。微生物温育仅导致SPE-DOM分子组成和浓度的总体变化很小且几乎不明显。