Veil architecture in a sulphide-oxidizing bacterium enhances countercurrent flux

Abstract

Solute uptake by microorganisms is limited by molecular diffusion through a boundary layer surrounding the cells, and the uptake is not enhanced (or only insignificantly) by convective water transport or by swimming. It is generally assumed that sediment uptake of oxygen is diffusion-limited, so the steepness of the concentration gradient within the 0.5-1-mm-thick diffusive boundary layer is a measure of diffusional flux into the sediment. Here we show that veils, which are formed on sediments by the marine sulphide-oxidizing bacterium Thiovulum majus, generate convective oxygen transport through the 0.5-mm-thick water layers above the veil at rates that are about 40 times higher than molecular diffusion. Chemosensory behaviour of the cells, combined with their generation of water currents, leads to characteristic, aggregated distribution patterns: areas with high cell densities draw oxygenated water downwards through the veil, whereas areas without cells serve for the upward-directed return flow of deoxygenated water. The microbial community structure thus overcomes the limitations of diffusion and thereby enhances the rates of respiration and sulphide oxidation.