Laying Cables; Redressing Sulphide Oxidation in Seagrass Rhizospheres

Seagrasses thrive in anoxic sediments where sulphide, a by-product of essential nutrient acquisition processes, accumulates to phytotoxic levels. So how do seagrasses persist in this environment? Here, we propose that oxygen loss from active root tips protects seagrasses from sulphide intrusion not only by abiotically oxidising sulphides in the rhizosphere, but also by manipulating the abundance of sulphate reducing and sulphide oxidising bacteria. We used a novel multifarious approach combining imaging techniques (confocal fluorescence in situ hybridisation, oxygen planar optodes and sulphide diffusive gradients in thin films) with molecular sequencing to build a complete picture of the micro-environment of growing roots of Halophila ovalis and Zostera mucronata. We show that oxygen loss is highly localised to active root tips, and that roots will enhance sulphide production over sulphide oxidation on the meadow-scale. Despite the restricted nature of oxygen loss, oxygen leakage corresponded with decreased abundance of potential sulphate reducing bacteria and decreased sulphide concentrations in the surrounding rhizosphere. Roots leaking oxygen had a higher abundance of sulphide oxidising cable bacteria within the root hair zone; the first report of these important bacteria for seagrasses. Thus, oxygen leakage can enhance sulphide consumption while restrict bacterial sulphide production on the micro-scale, hence helping seagrasses to colonise and persistence in sulphide rich sediments.