Structural and functional adaptations of eelgrass (Zostera marina L.) to the anaerobic sediment environment

Structural and functional adaptations of the seagrass Zostera marina L. to the anaerobic sediment environment were examined both experimentally and under natural conditions along a transect including environmentally different sites in Izembek Lagoon, Bering Sea, Alaska. Aerobic root respiration rates, end products of anaerobic metabolism in roots and rhizomes, lacunal development, and internal gas volume and gas composition were compared among plants from three sites. Root respiration decreased across the transect from the intertidal site to the two subtidal stations of increased sediment anaerobiosis. Ethanol concentrations of roots and rhizomes were very low in all cases and shikimate could not be detected. Malate concentrations in the rhizome tissues were high in the midbed portion of the transect and lower at both the intertidal and deep edges of the bed; malate concentrations of root tissue followed a reciprocal pattern. The internal gas volume (lacunal space) of leaves and roots increased along the transect of increased sediment anaerobiosis. Anatomical observations of roots showed a similar trend with enhanced lacunal development at the most anaerobic site. Data collected in field and laboratory experiments in which the hydrogen sulfide concentration surrounding the roots and rhizomes was enhanced showed physiological adaptations characteristic of tolerance to anaerobiosis. The collective data indicate that Zostera marina is capable of responding to markedly different microenvironments.