The Black Sea is rich on hydrogen sulfide. Googling on cable bacteria and Black Sea showed this:
Cable bacteria can limit sulfide release by promoting iron oxide formation in sediments.[1]
[...] we used laboratory incubations to assess whether cable bacteria can establish in iron (Fe) monosulphide-poor coastal Black Sea sediment and to determine the impact of their activity on the cycling of Fe, phosphorus (P) and sulphur (S). [...] we show that the enrichments in Fe oxides induced by cable bacteria are located in a thin subsurface layer of 0.3 mm. We show that similar subsurface layers enriched in Fe and P are also observed at field sites where cable bacteria were recently active and little bioturbation occurs.[2]