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Project Progress

Sterols are required by almost all eukaryotes, but their structural diversity varies across different lineages. It is unclear if there are any systematic variations in sterol structures between metazoa and their unicellular relatives. MIT graduate student Jonathan Grabensatter analyzed sterols from four unicellular relatives of metazoa. For comparison, sterols from the basal metazoans hexactinellid sponges and ctenophores were analyzed as well. Sterols were identified by gas chromatography and mass spectrometry. Choanoflagellate sterol profiles are largely similar, containing two or three C27 compounds with 2, 3 and 4 double bonds and one C28 with three double bonds. Ichthyospora are dominated by a C27Δ5, 24 but also produce cholesterol, three C28 sterols and two 4-methyl C27 sterols. Capsaspora produce cholesterol, a tetraunsaturated C27 and two C28 sterols. The hexactinellid sponges and ctenophores were dominated by cholesterol. Genomic searches were performed to identify changes in genes encoding enzymes for sterol biosynthesis. These data suggest that there was an evolutionary drive to decrease the use of side-chain methylated sterols as time progressed until, eventually, all metazoa evolving after the ctenophores produce only cholesterol.

The Pearson group continues to focus on identifying unique lipid signatures (compositional and isotopic) to distinguish between prokaryotic and eukaryotic life. Post-doc Lindsay Hays has been working on the Tayshir (and/or Trezona) anomalies that occur between the Sturtian and Marinoan glaciations of the Cryogenian. She will present a poster at Fall AGU (2011), showing the results of a comparative analysis of sample preparation and cleaning. This work is critical to understanding the syngeneity of the biomarkers extracted from these rocks, and it will be prepared as a publication for Organic Geochemistry.

Post-doc Dan Rogers prepared a comprehensive suite of samples for sequencing hopanoid synthesis genes (28 samples spanning a range of environmental gradients), and the samples were submitted for sequencing in January 2011. For this project, he developed a new set of primers for amplification of squalene-hopene cyclase genes. With the improved results that these primers give, he prepared his samples for high-throughput pyrosequencing (http://www.454.com/). This approach should enable us to generate at least ten times the amount of data obtained relative to our earlier work, which used conventional cloning techniques. Unfortunately our pyrosequencing run failed – this work will be repeated by a new post-doc, Stephanie Kusch, and a technician to be hired.

Graduate student Hilary Close published her paper addressing the “Logan hypothesis” for carbon isotopic enrichment in Proterozoic bitumens (Close et al., 2011). This paper presents a model that suggests carbon isotopic signatures preserved in the Neoproterozoic could have been highly controlled by the relative ratio of eukaryotic to prokaryotic export production. Her current efforts involve testing this model by examining lipid export and degradation in the water column of the Eastern Tropical North Pacific ocean. She will present preliminary results in a talk at Fall AGU.

In collaboration with Rosanna Alegardo and Nicole King (UC Berkeley) Jon Grabenstatter characterized the lipids on an unusual bacterium, Algoriphagus sp. a bacterial prey that induces colony formation and a distinctive feeding behavior in a choanoflagellate.