Abstract
The O2 budget of seagrasses is a complex interaction between several sources and sinks, which is strongly regulated by light availability and mass transfer over the diffusive boundary layer (DBL) surrounding the plant. Epiphyte growth on leaves may thus strongly affect the O2 availability of the seagrass plant and its capability to aerate its rhizosphere as a defence against plant toxins. We used electrochemical and fiber-optic microsensors to quantify the O2 flux, DBL and light microclimate around leaves with and without filamentous algal epiphytes. We also quantified the below-ground radial O2 loss from roots (~1 mm from the root-apex) to elucidate how this below-ground oxic microzone was affected by the presence of epiphytes. Epiphyte-cover on seagrass leaves (~21% areal cover) resulted in reduced light quality and quantity for photosynthesis, thus leading to reduced plant fitness. A ~4 times thicker diffusive boundary layer around leaves with epiphyte-cover impeded gas (and nutrient) exchange with the surrounding water-column and thus the amount of O2 passively diffusing into the leaves in darkness. During light exposure of the leaves, radial oxygen loss from the below-ground tissue was ~2 times higher from plants without epiphyte-cover. In contrast, no O2 was detectable at the surface of the root-cap tissue of plants with epiphyte-cover during darkness, leaving the plants more susceptible to sulphide intrusion. Epiphyte growth on seagrass leaves thus negatively affects the light climate and O2 uptake in darkness, hampering the plants performance and thereby reducing the oxidation capability of its below-ground tissue.
Highlights
Seagrasses are angiosperms that form coastal habitats of prime importance for marine biodiversity and carbon sequestration (Duarte, 2001; Duarte et al, 2005)
The width and the mass transfer impedance of the diffusive boundary layer (DBL) depends on factors such as the surface topography and the flow velocity, where e.g., relative low flow rates and uneven surfaces increases the thickness of the DBL (Jørgensen and Des Marais, 1990); both parameters are highly affected by epiphyte growth on the leaf surface
Light Climate Our observations on the light microclimate around the leaves of Z. marina revealed that epiphyte cover affect the quantity and quality of light reaching the seagrass leaf
Summary
Seagrasses are angiosperms that form coastal habitats of prime importance for marine biodiversity and carbon sequestration (Duarte, 2001; Duarte et al, 2005). If H2S reaches the root tissue surface it may enter the lacunar system of the seagrass plant via lipid-solution permeation of the plasmalemma (Raven and Scrimgeour, 1997) Such H2S intrusion into the below-ground tissue of seagrasses has mainly been related to inadequate internal aeration during night-time, as a result of a low water-column O2 content and a decrease in the diffusive O2 supply from the surrounding water-column (Pedersen et al, 2004; Borum et al, 2005). We used electrochemical and fiber-optic microsensors to investigate effects of epiphyte-cover on seagrass leaves on the below-ground aeration of the rhizosphere of the seagrass Zostera marina kept in a custom-made split flowchamber with natural sediment This microenvironmental approach allowed us to (i) analyse the DBL and light microclimate around seagrass leaves with- and without epiphytes, and (ii) correlate changes in these above-ground micro-environmental parameters with changes in the ROL from the root-caps, and thereby, the oxidation capacity of the below-ground tissue
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