MOLECULAR AND ISOTOPIC INVESTIGATIONS OF THE BIOGEOCHEMISTRY OF ARCHAEAL ETHER LIPIDS

Open Access
- Author:
- Turich, Courtney Hanna
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 29, 2006
- Committee Members:
- Katherine Haines Freeman, Committee Chair/Co-Chair
Maryann Victoria Bruns, Committee Member
Michael Allan Arthur, Committee Member
Christopher Howard House, Committee Member
Daniel Jones, Committee Member - Keywords:
- Archaea
Lipids
TEX86
isotopes - Abstract:
- Once thought to inhabit only extreme environments, the Archaea are now known to occur globally in oceans, marshes, lakes, sediments, and soils. Archaea abundance and metabolic diversity link these microbes to important biogeochemical transformations. Archaea also generate abundant and diagnostic membrane lipids that are widespread in modern environments as well as the sedimentary record. These lipid compounds offer important evidence for past Archaea distribution and activity, and are a key means for understanding archaeal contributions to biogeochemical cycles, especially carbon and nitrogen, over Earth’s history. To link records of naturally occurring lipid to their biological and metabolic origins, I studied archaeal lipid distributions from a global set of modern waters in order to test the hypothesis that specific lipid assemblages will correspond to specific genotypic groups. Cluster analysis showed that marine lipid distribution patterns fell into two groups: the epipelagic zone and mesopelagic/upwelling zones. Multivariate analysis with environmental data showed that nitrate concentrations largely explained the variation observed within these two marine groups, thereby linking metabolism to the archaeal lipid distributions ultimately in preserved sediments. In the more extreme gradient from marine to hypersaline environments, differences in lipid assemblages form the basis of a novel paleosalinity indicator, the “archaeaol/caldarchaeol ecometric” (ACE). The robust linear relationship between ACE and salinity in modern environments can be applied to ancient sedimentary organic matter deposited in the Mediterranean prior to the Messinian Salinity Crisis (5.7 Ma). ACE-based salinity values are consistent with mineral and faunal evidence of salinity increases, and complement bulk organic and isotopic geochemical records of differences in productivity and carbon cycling in the lead up to the Messinian Salinity Crisis. Finally, targeting the hydrogen (for hydrological reconstruction) and carbon (for metabolic reconstruction) isotope compositions of archaeal lipids requires a method capable of measuring isotope ratios in these large and non-volatile lipid compounds. I describe the method development for the analysis of isolated compounds using continuous flow elemental analysis isotope ratio mass spectrometry, with the critical step of concentrating the gas effluent through cryogenic refocusing in order to reduce sample size requirements. Thus far, sample sizes have been reduced by several orders of magnitude. Memory effects and isotopic fractionation within the thermal conversion-elemental analyzer for hydrogen isotope analysis currenly hinder precision and accuracy. However, isotope analysis in the nanomolar range is now possible for carbon and nitrogen. This novel technique, Nano-EA-irMS, will allow studies of the isotopic properties of compounds which could not previously be analyzed.