Hydrogen and Oxygen Isotopic Fractionation During Heterotrophic Cellulose Synthesis

Hydrogen and oxygen isotopic fractionation relative to mediumwater for two different carbohydrate metabolic pathways leadingto cellulose synthesis were measured. This was accomplishedby analysing stable hydrogen and oxygen isotope ratios of waterand cellulose for seedlings. The seedlings had been germinatedand heterotrophically grown in closed vessels from species havingstarch (Triticum aestivum L. and Hordeum vulgare L.) and lipids(Ricinus communisL. and Arachis hypogaea L.) as the primarysubstrate. Isotopic fractionation factors occurring during enzyme-mediatedexchange of carbon-bound hydrogen with water or the additionof carbon-bound hydrogens from water during the synthesis ofcellulose from either starch or lipids were similar (rangingfrom +144 to +166%). About 34% and 67% of carbon-bound hydrogenswere derived from water during the synthesis of cellulose fromstarch and lipid, respectively. Thus, the greater deuteriumenrichment in cellulose from oil seed species associated withgluconeogenesis was caused by a greater proportion of water-derivedcarbon-bound hydrogens and not because of differences in fractionationfactors. The proportion of carbon-bound hydrogens derived fromwater during these metabolic pathways was similar to that ofoxygen derived from water. These results may explain the variabilityin D/H ratios of cellulose nitrate from terrestrial and aquaticplants. Key words:     

Decreased precipitation exacerbates the effects of sea level on coastal dune ecosystems in open ocean islands

The alteration of fresh and marine water cycling is likely to occur in coastal ecosystems as climate change causes the global redistribution of precipitation while simultaneously driving sea‐level rise at a rate of 2–3 mm yr^?1. Here, we examined how precipitation alters the ecological effects of ocean water intrusion to coastal dunes on two oceanic carbonate islands in the Bahamas. The approach was to compare sites that receive high and low annual rainfall and are also characterized by seasonal distribution (wet and dry season) of precipitation. The spatial and temporal variations in precipitation serve as a proxy for conditions of altered precipitation which may occur via climate change. We used the natural abundances of stable isotopes to identify water sources (e.g., precipitation, groundwater and ocean water) in the soil–plant continuum and modeled the depth of plant water uptake. Results indicated that decreased rainfall caused the shallow freshwater table on the dune ecosystem to sink and contract towards the inland, the lower freshwater head allowed ocean water to penetrate into the deeper soils, while shallow soils became exceedingly dry. Plants at the drier site that lived nearest to the ocean responded by taking up water from the deeper and consistently moist soil layers where ocean water intruded. Towards the inland, decreased rainfall caused the water table to sink to a depth that precluded both recharge to the upper soil layers and access by plants. Consequently, plants captured water in more shallow soils recharged by infrequent rainfall events. The results demonstrate dune ecosystems on oceanic islands are more susceptible to ocean water intrusion when annual precipitation decreases. Periods of diminished precipitation caused drought conditions, increased exposure to saline marine water and altered water‐harvesting strategies. Quantifying species tolerances to ocean water intrusion and drought are necessary to determine a threshold of community sustainability.