ABSTRACT
Many major river deltas in the world are subsiding and consequently become increasingly vulnerable to flooding and storm surges, salinization and permanent inundation. For the Mekong Delta, annual subsidence rates up to several centimetres have been reported. Excessive groundwater extraction is suggested as the main driver. As groundwater levels drop, subsidence is induced through aquifer compaction. Over the past 25 years, groundwater exploitation has increased dramatically, transforming the delta from an almost undisturbed hydrogeological state to a situation with increasing aquifer depletion. Yet the exact contribution of groundwater exploitation to subsidence in the Mekong delta has remained unknown. In this study we deployed a delta-wide modelling approach, comprising a 3D hydrogeological model with an integrated subsidence module. This provides a quantitative spatially-explicit assessment of groundwater extraction-induced subsidence for the entire Mekong delta since the start of widespread overexploitation of the groundwater reserves. We find that subsidence related to groundwater extraction has gradually increased in the past decades with highest sinking rates at present. During the past 25 years, the delta sank on average â¼18 cm as a consequence of groundwater withdrawal. Current average subsidence rates due to groundwater extraction in our best estimate model amount to 1.1 cm yr-1, with areas subsiding over 2.5 cm yr-1, outpacing global sea level rise almost by an order of magnitude. Given the increasing trends in groundwater demand in the delta, the current rates are likely to increase in the near future.
ABSTRACT
The formation of glucosylceramide, the predominant sphingolipid in plant tissues, was examined in microsomes from wax bean hypocotyls. Membranes were incubated with UDP-[14C]glucose in an assay mixture. The lipid extracts obtained from the assays were separated by thin-layer chromatography, and the radioactivity incorporated into glucosylceramide, steryl glucoside, and acylated steryl glucoside was determined. Although the formation of glucosylceramide was detected and characterized, several lines of evidence contradicted the assumption that UDP-glucose is the immediate glucose donor for glucosylceramide formation in plants: PDMP (DL-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol), an inhibitor of ceramide glucosyltransferase in animal tissues, did not inhibit glucosylceramide formation in bean microsomes. Addition of UDP-glucose pyrophosphorylase during the assay to degrade UDP-[14C]glucose blocked the further production of labeled steryl glucoside, but did not prevent the continued formation of labeled glucosylceramide. Omitting UDP-[14C]glucose and including steryl [14C]glucoside in the assay resulted in the formation of labeled glucosylceramide. Collectively, these results suggest that glucosylceramide formation in plants does not utilize UDP-glucose as the immediate glucose donor, as has been demonstrated for the reaction in animal tissues, and that steryl glucoside serves as glucose donor for ceramide formation. This study, the first to examine glucosylceramide formation in plants, provides evidence for a novel enzymatic reaction in sphingolipid synthesis as well as a new, metabolic role for steryl glucoside in plant tissues.
