An Investigation into the Biomarker Potential of Highly Branched Isoprenoids in Northern Minnesota Lacustrine Sediments

Abstract

Highly Branched Isoprenoids (HBIs) are isoprene-based lipids synthesized by diatoms. HBIs are commonly used as sea-ice proxies in marine environments where their presence is indicative of marginal ice zones. Historical data on Ice cover in the Laurentian Great Lakes is sparse and only spans from the mid 1960’s to present day. This study aims to expand the biomarker potential of HBIs to apply them as a proxy for ice cover on Lake Superior. Seven study sites around Lake Superior were selected and studied to determine which freshwater HBIs were present, and at what concentration they were found. Each site was sampled twice, in the summer and in the winter to determine if seasonal conditions affect HBI production of diatom communities. The HBI suite at each site is characterized by gas chromatography mass spectrometry (GC-MS) analysis of the nonpolar extractable fraction of the sediments and the genera of diatoms present at each site are characterized by visual identification. In addition, microcosm studies of Nitzschia and Fragilaria are isolated from the Two Harbors study site and are analyzed to determine the HBI suites produced by them. Overall, eight distinct HBIs were identified from the seven study sites, in concentrations ranging from 0.1-0.9 µg g-1 dry sediment. Four distinct HBIs were identified from the microcosm cultures including three distinct C30 and one distinct C25 HBIs. HBIs show a potential to be correlated to environmental conditions, though future experiments must be conducted to develop this relationship.