Séminaire

Redox gradients ranging from millimeters to hundreds of meters are prevalent in the marine environment. They play a key role in recycling of organic carbon, nutrients and metals in both pelagic (anoxic basins, OMZ regions) and benthic ecosystems (seeps, vents). Due to technological constraints, our knowledge of the benthic feedbacks is considerably less than pelagic processes. Our recent work in the Baltic Sea (Gotland Basin, basically a small-scale Black Sea) revealed the presence of H2S-oxidizing bacterial mats (tentatively assigned to genus Beggiatoa) covering extensive seafloor area that is intersected by a pelagic redoxcline. Integrated in situ voltammetric microsensor and benthic chamber flux measurements showed that these bacterial ‘carpets’ act as a ‘benthic sulfide filter’ in the Baltic Sea and they mitigate the input of sulfide from sediments to bottom waters, with benefits to an already degraded Baltic Sea ecosystem. We found that while oxidizing sulfide the bacteria reduced NO3- to NH4+, thereby leading to the retention of fixed nitrogen which would otherwise be lost through denitrification. The iron-sulfur redox cycling plays an even more dramatic role in seafloor hydrothermal vent ecosystems. Here entire communities of exotic invertebrates are based on a symbiosis with sulfide-oxidizing chemosynthetic bacteria. A recently emerging consensus is that these ecosystems are not isolated incidents at the seafloor but contribute significantly to ocean carbon cycle. Using in situ voltammetry, we found that the iron and sulfur cycles are closely coupled in vents and metal sulfidation controls the amount of free sulfide available to the vent microbes. Besides the chemosynthetic carbon production near vents, we found that the flux of slowly oxidizing nanoparticle iron (pyrite-FeS2) from vents to the deep-sea is significant and this previously unrecognized iron source could eventually fertilize iron-limited portions of the sunlit ocean. These recent findings reveal that the seafloor biogeochemical processes have an underestimated capacity to influence Earth System-wide phenomena in timescales much less than previously assumed. Improving our knowledge base on these aspects and a closer cooperation between observers and numerical modelers will enable a more accurate representation of ocean interior metal and sulfur fluxes in global models and therefore decrease uncertainty in predicting ocean biogeochemical response to global warming.

ruizlod@locean-ipsl.upmc.fr