Abstract:
To investigate potential variability in the biosynthetic fractionation of hydrogen isotopes between environmental water and plant lipids, the cord grass Spartina alterniflora was sampled from a single location in a coastal marsh over a period of 16 months. Values of ?D for a variety of lipids were measured by gas chromatography/pyrolysis/isotope ratio mass spectrometry. S. alterniflora grows partially submerged in seawater, so it has a virtually unlimited supply of water with nearly unvarying isotopic composition. Temporal changes in the ?D values of lipids can thus be interpreted as representing mainly variations in biosynthetic fractionation. Fatty acids, n-alkanes, and phytol extracted from S. alterniflora have nearly constant ?D values from ?October through May, but exhibit marked decreases of up to 40? during summer months. These shifts in lipid ?D values are interpreted as representing a change in the source of organic substrates, principally acetate, used for their biosynthesis. Lower summertime ?D values for lipids are consistent with an increasing reliance on current photosynthate as feedstock for biosynthesis, whereas stored carbohydrate reserves are utilized more extensively during other times of the year. Regardless of the specific mechanism, the data emphasize that overall fractionations between water and plant lipids depend on biological as well as environmental variables, and that the biosynthetic fractionation is not necessarily constant even for a single plant. Because lipids such as fatty acids are present in all cells and turn over on timescales of weeks to months, measurements of ?D values in fatty acids may also provide useful constraints for distinguishing biologic versus environmental controls on cellulose ?D values in trees. ? 2006 Elsevier Inc. All rights reserved.
