Metabolic adaptation of Zostera marina (eelgrass) to diurnal periods of root anoxia

Abstract

The temperate seagrass Zostera marina L. is common in coastal marine habitats characterized by the presence of reducing sediments. The roots of this seagrass grow in these anoxic sediments, yet eelgrass is highly productive. Through photosynthesis-dependent oxygen transport from leaves to roots, aerobic respiration is supported in eelgrass roots only during daylight; consequently, roots are subjected to diurnal periods of anoxia. Under anoxic root conditions, the amino acids alanine and γ-amino butyric acid accumulate within a few hours to account for 70% of the total amino acid pool, while glutamate and glutamine decline. Little ethanol is produced, and the pool size of the organic acid malate changes little or declines slowly. Upon the resumption of shoot photosynthesis and oxygen transport to the roots, the accumulated γ-amino butyric acid declines rapidly, glutamate and glutamine pools increase, and alanine declines over a 16-h period. These adaptive metabolic responses by eelgrass to diurnal root anoxia must contribute to the successful exploitation of shallow-water marine sediments that have excluded nearly all vascular plant groups. A metabolic scheme is presented that accounts for the observed changes in organic and amino acid pool sizes in response to anoxia.