Biodegradability of hydrothermally altered deep-sea dissolved organic matter

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 degrees 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 degrees C, - 21.0% at 200 degrees C) that predominantly affected the solid-phase extractable (SPE-DOC) fraction (-18.2% at 100 degrees C,- 51.4% at 200 degrees C). Fourier-transform ion cyclotron resonance mass spectrometric (FT-ICR-MS) analysis confirmed a temperature-related reduction of average molecular mass, 0/C ratios, double bond equivalents (DBE) and a relative increase in aromaticity (AI(mod)). This thermally altered DOM was added (25 mu mol L-1 DOC) to deep-water samples from the South West Pacific (Kermadec Arc, RV Sonne / SO253, 32 degrees 37.706' S vertical bar 179 degrees 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 x for DOM100 degrees C and 27 x for DOM200 degrees 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 temperaturedriven 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.