Abstract
Cable bacteria induce long-distance electron transport in the seafloor and can exert a powerful control on the elemental cycling in marine sediments by creating extreme excursions in porewater pH. Yet, the natural distribution of cable bacteria is still largely unknown, and so their role in coastal biogeochemical cycling remains poorly quantified. Here we show that cable bacteria can be abundant in the sediments of intertidal bivalve reefs, where they strongly influence the pore water geochemistry, resulting in a potentially beneficial interaction between the sulfur oxidizing microbes and biodepositing fauna. Cable bacteria occurred in sediments accumulating within mussel and oyster reefs in the Wadden Sea (The Netherlands), at cumulative filament densities up to 1038 m cm-2. Additionally, cable bacteria were found at moderately high cumulative filament densities (up to 56 m cm-2) in a heavily bioturbated sandy sediment adjacent to the muddy reefs. Microsensor profiling revealed strong sulfide removal and intense acid generation associated with the electrogenic sulfide oxidation metabolism of the cable bacteria. Strongly elevated concentrations of dissolved calcium (up to 35 mM), manganese (up to 250 µM), and iron (up to 700 µM) were observed in the pore waters, consistent with acidity-driven dissolution of calcium carbonates and iron sulfides. This field study provides substantive evidence that cable bacteria exert a decisive control on the cycling of sulfur and carbonate minerals in cohesive coastal sediments, and identifies that the distribution and influence of cable bacteria covers a greater range of natural habitats than previously believed.
Highlights
Intertidal epibenthic suspension-feeding bivalves are considered model ecosystem engineers because they strongly shape and modify their surrounding environment, as witnessed by the formation of elevated mussel and oyster reefs (Dame, 2012)
This study reveals that cable bacteria can be highly successful in bivalve reef sediments, demonstrating that these microbes are capable of thriving in a dynamic environment that exhibits intense and episodic sediment accumulation
This study shows that cable bacteria are widespread and abundant in muddy bivalve reefs sediments, which has important repercussions for the ecosystem functioning in these environments
Summary
Intertidal epibenthic suspension-feeding bivalves are considered model ecosystem engineers because they strongly shape and modify their surrounding environment, as witnessed by the formation of elevated mussel and oyster reefs (Dame, 2012). By increasing bed roughness, reducing local current velocity, and by depositing large quantities of fine particles as pseudofeces and feces, bivalves strongly enhance sediment accumulation in their local surroundings (Bergfeld, 1999; van der Zee et al, 2012). Sulfate reduction is the dominant pathway of organic matter mineralization in such organic rich sediments, leading to the accumulation of a large pool of solid metal sulfides and/or high levels of free sulfide in the pore waters (Dahlback and Gunnarsson, 1981; Jørgensen, 1982) Through their influence on sedimentation, reef-building bivalves are capable of creating hotspots of sulfur cycling in the coastal ocean
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