Inverse relationship between D/H fractionation in cyanobacterial lipids and salinity in Christmas Island saline ponds

Sediments from 28 saline and hypersaline (salinity 13.6-149.2) ponds on Christmas Island (Kiritimati), in the Central tropical Pacific Ocean, were investigated for the effect of salinity on the D/H ratios of lipid biomarkers. Hydrogen isotope ratios (expressed as delta D values) of total lipid extracts, and individual hydrocarbons heptadecane, heptadecene, octadecane, octadecene, diploptene, and phytene from cyanobacteria, became increasingly enriched in deuterium as salinity increased, spanning a range of 100%, while lake water delta D values spanned a range of just 12 parts per thousand. Net D/H fractionation between lipids and source water thus decreased as salinity increased. Isotope fractionation factors (alpha(lipid-water)) were strongly correlated with salinity, and increased in all compound classes studied by up to 0.0967 over a salinity range of 136. Differences in the hydrogen isotopic composition of lipids derived from three biosynthetic pathways (acetogenic, mevalonate, and non-mevalonate) remained similar irrespective of the salinity. This suggests that the mechanism responsible for the observed alpha(lipid-water)-salinity relationship originates prior to the last common biosynthetic branching point, the Calvin Cycle. We propose that a decrease in the exchange of intra- and extracellular (ambient) water resulting from down-regulation or closure of water channels (aquaporins) within cyanobacterial cell membranes, and subsequent isotopic enrichment of the intracellular water, likely resulting from metabolic reactions. These findings imply that caution must be exercised when attempting to reconstruct source water delta D values using lipid delta D values from environments that may have experienced salinity variations. On the other, hand our results can be used to establish a paleo-salinity proxy based on lipid delta D, if additional constraints on source water delta D values can be made. (c) 2007 Elsevier Ltd. All rights reserved.