Sulfide production and consumption in degrading wood in the marine environment.

Woody debris is known to be transported to the seas and accumulate on the seafloor, however, little is known on the consequences of its degradation in the marine environment. In this study we monitored the degradation product sulfide with Au/Hg voltammetric microelectrodes on the surface and interior of an experimentally immersed wood for 200 d. After 5 weeks of immersion, the interior became sulfidic, and steady-state conditions were established after 13 weeks with sulfide concentration reaching about 300 µM. Although sulfide was briefly detected at the surface of wood, its concentration remained lower than 20 µM, indicating that this compound was effectively oxidized within the substrate. Fitting these data to a kinetic model lead to an estimated microbial sulfide production rate in the range of 19-28 µM d(-1) at steady state. As much as 24 µM d(-1) nitrate could be consumed by this process in the steady-state period. Before the establishment of the steady state conditions, steep fluctuations in sulfide concentration (between 1mM and several µM) were observed in the wood interior. This study is the first to document the temporal dynamics of this unsteady process, characterized by fast sulfide fluctuation and consumption. Our results point to the complex mechanisms driving the dynamics of wood biogeochemical transformations, and reveal the capacity of woody debris to generate sulfidic conditions and act as a possible sink for oxygen and nitrate in the marine environment.