Bioavailability and molecular composition of dissolved organic matter from a diffuse hydrothermal system

Fluids from the diffusive hydrothermal vent Woody located at the Menez Gwen hydrothermal field were collected directly from a fissure using a Kiel pumping system (KIPS). The fluids were incubated in a biodegradation experiment with the resident microbial community at in situ temperature (55 °C). In addition to the initial fluid (T0), sub-samples were collected at three time points: 12.5 h (T1), 38 h (T2) and 180 h (Tfinal). Whole water samples were characterized by spectrophotometry and for dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations. Solid-phase extracted dissolved organic matter (SPE-DOM) was molecularly characterized using a 15 Tesla Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Based on Mg2 + concentrations, vent fluids were diluted ca. 1:99% with seawater. SPE-DOC concentrations remained constantly low (30 μM) over the course of incubation, while DOC and TDN (1180 μM and 16 μM, respectively, at T0) decreased sharply (to 80 μM and 9 μM, respectively, at Tfinal). This biolabile DOC is likely composed of volatiles and short-chain hydrocarbons, and the reason behind their extraordinarily high concentration remains unknown. Molecular characterization of SPE-DOM in fluids and seawater identified over 3000 molecular formulae, which were separated into different groups according to their reactivity. The half-life of the more reactive groups ranged between 6-7 h, 18-21 h and 5-106 days, and represented 7.3, 2.0 and 7.5% of the total formulae present in T0, while the majority (83%) did not exhibit any significant trend during the experiment. Despite the small molecular changes in SPE-DOM, a decrease in the number of formulae and average molecular mass was observed, the latter consistent with an increase in the spectral slope over the wavelength range 275-295 nm (S275-295) during the experiment. An increase in aromatic molecular formulae during the incubation was also corroborated by a concurrent increase in the absorption coefficient at 254 nm normalized to DOC concentrations (SUVA254), an optical indicator of aromatic content. Thus both FT-ICR-MS and optical properties over the course of the incubation corroborated a decrease in molecular mass and increase in aromaticity. This study is the first to report the optical and molecular transformations of bio-labile DOM from diffusive hydrothermal systems.