Oxygen dynamics in the rhizosphere of Zostera marina: A two-dimensional planar optode study

The oxygen dynamics in the rhizosphere of Zostera marina was studied by use of planar optodes. Oxygen leakage to the rhizosphere was restricted to the root tip and extended only up to ;8 mm up along the root. The oxic sediment volume around the roots increased linearly with irradiance in the interval of 0–250 mmol photons m22 s21, but the leakage rate saturated at the maximum irradiance of 500 mmol photons m22 s21. Oxygen leakage decreased by ;60% from light to darkness and Z. marina was able to maintain an oxic zone around the root tip even in darkness as long as oxygen in the overlying water was at 100% air saturation (280 mmol L21). O2 leakage from the root tips stopped at 25% air saturation (70 mmol L21) and the oxic microniche rapidly disappeared. Increasing the oxygen concentration above 100% air saturation induced oxygen leakage from zones that otherwise appeared impermeable to oxygen. The roots on average grew by 8.7 mm d21, and a series of O2 images documented the high spatial and temporal dynamics of the oxic microniches around the root tips. The estimated total oxygen release to the rhizosphere of Z. marina beds was 2.3 mmol m22 d21, which only corresponded to 12% of the diffusive oxygen uptake at the primary sediment–water interface. Rhizospheres of seagrass are thus probably of minor importance for total benthic O2 uptake rates. Seagrasses are worldwide distributed and are estimated to cover ;10% of coastal sediments (Charpy-Roubaud and Sournia 1990). The plants colonize a wide range of sediment types, and their roots are, in most cases, growing in anoxic and highly reduced environments. Being obligate aerobes, seagrasses have adapted efficient strategies to maintain oxia in their root systems via an interconnected system of gas spaces (lacunae) transporting O2 from the leaves down to the roots (Penhale and Wetzel 1983; Larkum et al. 1989). The oxygen transport is driven by a partial-pressure gradient from the photosynthetically active leaves toward the O2-consuming root system. However, radial oxygen loss (ROL) from the roots into the sediment may occur (Caffrey and Kemp 1991; Pedersen et al. 1998), and the oxygenated zones along roots have been suggested to be an adaptive feature providing an oxidative shield against harmful phytotoxins, such as Fe21, Mn21, and sulfides (Penhale and Wetzel 1983). Recent studies of Halophila ovalis and Zostera marina have shown that a barrier against ROL exists along most of the root length (Connell et al. 1999; Jensen et al. 2005). The 1 Present address: Greenland Institute of Natural Resources, Kivioq 2, Box 570, 3900 Nuuk, Greenland. 2 Corresponding author ([email protected]).