Epiphyte-cover on seagrass (Zostera marina L.) leaves impedes plant performance and radial O2 loss from the below-ground tissue

The O-2 budget of seagrasses is regulated by 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 O-2 availability of the seagrass plant and its capability to aerate its rhizosphere as a defense against plant toxins. We used electrochemical and fiber-optic microsensors to quantify the O-2 flux, DBL, and light microclimate around leaves with and without filamentous algal epiphytes. We also quantified the below-ground radial O-2 loss (ROL) from roots (similar to 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 (similar to 21% areal cover) resulted in reduced light quality and quantity for photosynthesis, thus leading to reduced plant fitness. A similar to 4 times thicker DBL around leaves with epiphyte-cover impeded gas (and nutrient) exchange with the surrounding water-column and thus the amount of O-2 passively diffusing down to the below-ground tissue through the aerenchyma in darkness. During light exposure of the leaves, radial oxygen loss from the below-ground tissue was similar to 2 times higher from plants without epiphyte-cover. In contrast, no O-2 was detectable at the surface of the root-cap tissue of plants with epiphyte-cover during darkness, leaving the plants more susceptible to sulfide intrusion. Epiphyte growth on seagrass leaves thus has a negative effect on the light climate during daytime and O-2 supply in darkness, hampering the plants performance and thereby reducing the oxidation capability of its below-ground tissue.