Effect of sulfide and the role of root mass on metabolic fluxes in the seagrass Zostera marina

Abstract

While seagrass communities serve important ecological and economic purposes, their health has been declining worldwide. Sediment sulfide toxicity might be an important stressor of seagrasses, but isolating single variables is difficult in the field and few studies have investigated direct physiological effects. We used flow-through respirometry to investigate whether sulfide affects fluxes of metabolic gases (O2, CO2, NH3, H2S) by the shoots and roots of the North American native seagrass Zostera marina. O2 transport from the shoots to the roots occurs in Z. marina and might also be involved in sulfide detoxification. We therefore hypothesized that such O2 transport would occur to a greater extent in plants with higher shoot-to-root mass ratios, improving sulfide detoxification, thereby maintaining nitrogen uptake by the roots. We found that the roots primarily consumed O2, which increased with increasing shoot-to-root mass ratio, but this relationship was dominated by root mass and was disrupted by sulfide. In response to sulfide, root O2 consumption decreased by more than 50 %, whereas shoot photosynthesis (O2 production; light) and aerobic respiration (O2 uptake; dark) were not significantly affected. H2S concentrations decreased in root chambers, indicating either oxidation from shoot-supplied O2 or uptake of sulfide by roots. Besides O2 and H2S, no other gas flux was affected by sulfide exposure. Our data suggest that root mass controls the physiological space of Z. marina between sulfide-free and sulfide-rich conditions and this space increases with decreasing seagrass size.