Oxic microzones and radial oxygen loss from roots of Zostera marina

Oxygen microelectrodes and planar oxygen optodes were used to map the microdistribution of oxygen and the radial oxygen loss (ROL) from roots of Zostera marina kept in natural sediment. Substantial heterogeneity in the oxygen distribution was seen along the roots, with oxygen mainly leaking out from the root tips. Maximum oxygen levels at the root surface reached 19 to 80 % of air saturation in the light and the oxygenated zone extended 1 to 2 mm away from the root tip. The oxygen concentration at the root surface decreased to 0-5 % of air saturation at positions 3 to 6 mm behind the root apex. The high oxygen levels at the root tip surface were due to an effective barrier to ROL on the older part of the roots and the presence of an effective gas-transport system in the plant, with numerous intercellular spaces extending very close to the apical meristem of the root. Radial diffusion of oxygen from the root surface created a dynamic 0 to 1 mm-wide oxic microzone around the similar to 0.3 mm wide roots of Z. marina that varied with irradiance and distance from the root tip. Rootsurface oxygen concentrations and ROL measured 2 mm behind the apex increased with increasing irradiance until ROL saturation was reached at irradiances > 400 mu mol photons m(-2) s(-1). The ROL increased from 16.2 nmol O-2 cm(-2) h(-1) in darkness to 21.6, 28.8 and 36.0 nmol O-2 cm(-2) h(-1) at incident irradiances of 25, 111 and 467 mu mol photons m(-2) s(-1), respectively. Based on measured steady-state radial oxygen profiles, the total oxygen export from one 6 cm long root of the first actively growing root bundle with a total surface area of 0.56 cm(2) was estimated to be 6.0 to 6.7 nmol O-2 h(-1) in saturating light. The total subsurface input of plant-mediated oxygen was estimated to be only in the order of 2 to 14 % of the total diffusive oxygen uptake (DOU) across the sediment-water interface. The local input of oxygen from the root tip was, however, similar to the DOU at the primary sediment-water interface, and 35 to 43 % of the total oxygen loss occurred from the outermost 0 to 3.5 mm of the root tip. The roots of Z. marina grow similar to 5 mm d(-1), and significant oxygen levels will therefore only be present at a given spot for less than 24 h during root growth. The rhizosphere of Z. marina is thus characterised by a constantly changing mosaic of ephemeral oxic microniches in the reduced sulphidic sediment, leaving behind an anoxic but oxidised zone around the more mature parts of the roots.