Cable bacteria delay euxinia and modulate phosphorus release in coastal hypoxic systems.

Cable bacteria are long, filamentous bacteria with a unique metabolism involving centimetre-scale electron transport. They are widespread in the sediment of seasonally hypoxic systems and their metabolic activity stimulates the dissolution of iron sulfides (FeS), releasing large quantities of ferrous iron (Fe2+ ) into the pore water. Upon contact with oxygen, Fe2+ oxidation forms a layer of iron(oxyhydr)oxides (FeOx ), which in its turn can oxidize free sulfide (H2 S) and trap phosphorus (P) diffusing upward. The metabolism of cable bacteria could thus prevent the release of H2 S from the sediment and reduce the risk of euxinia, while at the same time modulating P release over seasonal timescales. However, experimental support for this so-called 'iron firewall hypothesis' is scarce. Here, we collected natural sediment in a seasonally hypoxic basin in three different seasons. Undisturbed sediment cores were incubated under anoxic conditions and the effluxes of H2 S, dissolved iron (dFe) and phosphate (PO4 3- ) were monitored for up to 140 days. Cores with recent cable bacterial activity revealed a high stock of sedimentary FeOx , which delayed the efflux of H2 S for up to 102 days. Our results demonstrate that the iron firewall mechanism could exert an important control on the prevalence of euxinia and regulate the P release in coastal oceans.