Meristematic oxygen variability in eelgrass (Zostera marina)

We examined the variability in oxygen content of meristematic tissues in eelgrass in order to evaluate its potential role in events of sudden mass mortality within eelgrass beds. Oxygen content within intact eelgrass plants could be described by use of microelectrode techniques at high temporal and spatial resolution in the laboratory and in the field. Under both situations, the meristematic oxygen content was highly variable, ranging from 0 to 200% of air saturation depending on environmental conditions. Changes from steady-state maximum oxygen content to steady-state minimum content occurred within 30 min following experimental manipulation. The internal oxygen content exceeded water column oxygen concentration in the light and was intimately coupled to changes in irradiance because of the photosynthetic oxygen release within the leaves. The photosynthetically produced pool of oxygen could, however, not function as an efficient storage to support nighttime respiration. In the dark, oxygen was primarily supplied from the water column via diffusion into leaves, and the meristem quickly turned anoxic if the water column was anoxic. Experimental reduction of oxygen conditions immediately around the basal plant meristem had no major effect on internal oxygen content. High temperatures had a dramatic effect on the internal oxygen balance of eelgrass. Increasing temperature stimulated plant respiration more than photosynthesis, and the meristem went anoxic, even in the light, at water temperatures above 308C. We hypothesize that low meristematic oxygen content is a key factor in events of seagrass die-off. Stochastic events with massive plant mortality in beds of eelgrass (Zostera marina L.) are frequently observed in shallow Danish coastal waters. Mass mortality always occurs in late summer under warm, calm weather conditions and is often associated with periods of low oxygen concentrations in the water column. The dying plants are characterized by apparently healthy looking leaves, roots, and rhizomes, but with necrotic tissues in the intercalary meristematic region. A similar scenario can be observed in less successful laboratory cultures of eelgrass at high temperatures or with poor stirring. Within a few days, leaf bundles start to detach from the rhizomes and the plants die. These observations suggest that events of mass mortality among eelgrass stands are caused by degradation of meristematic tissues, which could be induced by poor meristematic oxygen conditions from high temperatures or low water column oxygen concentrations. The intercalary meristem of eelgrass and other seagrasses is situated in the transition zone between water and sediment, and it initiates the formation of both leaves and rhizome segments. Oxygen must be supplied by radial diffusion from neighboring tissues, especially from the fully developed lacunae of older leaves surrounding the meristematic tissue. Meristems are sites of high metabolic activity, with high oxygen demands that support cell division and growth (Brix 1 To whom correspondence should be addressed. Present address: Department of Marine Ecology, National Environmental Research Institute, Vejlsøvej 25, DK-8600 Silkeborg, Denmark (tmg@dmu. dk). 2 Present address: Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand.