Oxygen dynamics in the rhizosphere of Cymodocea rotundata

The spatial distribution of oxygen and the dynamics of the oxic microzone around roots of Cymodocea rotundata were studied using oxygen microelectrodes under constant light conditions and during light-dark transitions. Under daylight steady state conditions, oxygen was present at concentrations up to 75% of air saturation at the root surface, and the oxic microzone around the roots was 80 pm thick. Steady state oxygen concentrations were reached within 1.5 h after light-dark shifts. Under darkness, free oxygen, at about 20% of air saturation, was still present on the root surface at steady state, but the thickness of the oxic microzone shrank to 50 μm. The oxygen present in the rhizosphere during darkness was supplied from the water column to roots via, primarily, gas-phase diffusion in leaves and rhizomes. The oxic microzone around roots comprised about 0.5‰ of the total volume of the seagrass rhizosphere, and the root-mediated oxygen supply was estimated to be insignificant for the whole sediment oxygen budget contributing about 1% of total oxygen consumption only. However, the continuous oxygen supply may ensure a persistent oxic environment for belowground tissues of C. rotundata and, hence, protect the plant from reduced phytotoxins.