Growth-dependent hydrogen isotopic fractionation of algal lipid biomarkers in hypersaline Isabel Lake (México)

• Published in: Geochimica et cosmochimica acta 2013-04, Vol.106, p.490-500
• Main Authors: Romero-Viana, Lidia, Kienel, Ulrike, Wilkes, Heinz, Sachse, Dirk
• Format: Article
• Language: English
• Subjects: biomarkers; carbon; cytosol; deuterium; isotope fractionation; lakes; lipids; oxidation; paleoclimatology; physiological state; polysaccharides; rain; salinity; seasonal variation; sediments
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2012.12.017

In this study, we evaluated the potential of the hydrogen isotopic composition of algal lipid biomarkers as a proxy for past hydroclimatic variability in hypersaline Isabel Lake, Mexico (Eastern Pacific). We compared rainfall variability recorded in the region over the last 65years with changes in δD values of the most abundant compounds preserved in the uppermost 16cm of lake sediment. Changes in δD values of the 1,15-C32 diol (δDdiol), a specific biomarker of algal populations, were related to rainfall variability; specifically, n-alkyl diols were more deuterium-enriched (depleted) during wetter (drier) periods. Strikingly, neither the magnitude of lipid biomarker isotopic changes over interannual timescales (of up to 70–80‰) nor the direction of that variability can be explained by changes in δD values of the water source or salinity fluctuations (approximately 30 on the practical salinity scale) controlled by seasonal rainfall. However, changes in sedimentary biomarker composition, higher total organic carbon content and less negative δ13C values of the 1,15-C32 diol indicate enhanced algal growth during wetter periods. We find that these conditions result in less negative δD values of n-alkyl diols. We hypothesize that due to higher lipid demand during enhanced algal growth, an increasing proportion of hydrogen for lipid synthesis is derived from the cytosol via oxidation of polysaccharides, which may cause a deuterium enrichment of the acetogenic compounds. This study has significant implications for paleohydrological reconstructions using algal lipid δD values, particularly in highly seasonal environments such as Isabel Lake. In such environments, δD values of specific algal lipid biomarkers may not record the full seasonal cycle in source water δD but appear to be mainly controlled by the physiological state of algal populations. Our data provide the first evidence that changes in D/H fractionation due to algal growth conditions can be recorded in sediments. For paleoclimate reconstructions in such environments, algal growth conditions should be constrained with additional proxy data (δ13C values of the same biomarkers), as the net D/H fractionation between water and lipids may not have been constant over time.