Evidence for suboxic nitrification in recent marine sediments

The classical scheme of biogeochemical zones (BGZ) is known to be an oversimplification of the microbial processes that occur in organic-rich marine sediments. Results from a coupled deployment of pore-water gel probes in Loch Duich, Scotland, provide direct evidence for rapid recycling within the iron reduction (FeR) and sulphate reduction (SR) zones. High resolution porewater profiles obtained using diffusive equilibrium in thin films (DET) gel probes found a nitrate peak at the boundary between the FeR and SR zones. This non-steady state feature is consistent with recycling of reduced N occurring throughout the FeR zone. Both conventional pore-water iron profiles and results from diffusive gradient in thin films (DGT) probes indicate that iron is solubilised and precipitated in rapid Fe/S recycling reactions throughout the SR zone. The presence of such complex recycling reactions confirms the oversimplification of the classical BGZ scheme.