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Ladderanes as tracers for present and past anaerobic ammonium oxidation

In this thesis, three novel environmental settings were investigated for the presence of ladderane lipids, and thus the anammox bacteria performing the anammox process. Anammox was found to be an import process in two marine settings (the eastern Tropical North Pacific oxygen minimum zone and the euxinic Cariaco Basin). Anammox bacteria were also found to occur in soil environments (as seen from a laboratory enrichment culture).

Ladderane lipids are membrane lipids produced exclusively by anaerobic ammonium oxidising (anammox) bacteria. Anammox bacteria are key players in the marine nitrogen cycle, performing the anammox reaction, converting equal parts of ammonium and nitrite into dinitrogen gas. This process is responsible for a significant loss of bio-available nitrogen from the ocean. Ladderanes can be used to trace present-day anammox activity. However, the fate of ladderane lipids as they undergo structural alterations during diagenesis and catagenesis is unknown, and the influence of these processes must be constrained before confidently applying these biomarkers to infer past anammox activity.

To investigate the effect of oxidation on ladderane lipids, laboratory aerobic degradation experiments were performed. This led to the transformation of C18 and C20 ladderane fatty acids into C14 and C16 short-chain ladderane fatty acids. This transformation was likely microbially mediated. Based on the structures of these biodegraded ladderane fatty acids, a β-oxidation, which removes two carbon atoms from the alkyl-side chain, was proposed as the degradation pathway. This process stopped at the ladderane ring structure.

A new method was applied on marine suspended particulate matter and sediments to determine the natural occurrence of the C14 lipids. Short-chain ladderanes were shown to be abundant in settings where lipids from anammox bacteria had been exposed to oxic conditions after cell death, but not in settings where ladderane lipids were deposited in an euxinic water column. It was determined that short-chain ladderanes are present in marine sediments for a longer period of time (107 years) than the original ladderanes, which had been used previously as tracers for past anammox activity. They, therefore, may be more suitable for detecting past anammox activity in ancient marine sediments.

The fate of ladderane lipids during thermal maturation was investigated via hydrous pyrolysis experiments mimicking natural catagenetic processes. Anammox cell material was thermally matured at temperatures ranging between 120 and 365 °C. At experimental temperatures >200 °C, oil was generated. Two novel lipid classes were identified in the saturated aliphatic fractions of these oils. Representatives from both classes were isolated, and identified. The first class contained C19 and C20 ladderane-like hydrocarbons with tricyclododecane moieties. The second class contained branched long-chain (C28 – C30) alkanes. Using compound specific δ13C isotopic values, both lipid classes were confirmed to be the result of the thermal maturation of anammox lipids. These lipids were not detected in marine sediments, due to either the low selectivity of the GC/MS method used, the screened sediments were too immature for the thermally mature lipids to have been generated, or that these lipids do not accurately represent the natural thermal maturation of anammox biomass.

Date and time: 3/12/2012 14:30
Location: Academiegebouw, Domplein 29, Utrecht
PhD student: Darci Rush
Faculty: Faculty of Geosciences
Thesis: Ladderanes as tracers for present and past anaerobic ammonium oxidation
Supervisor 1: Prof. dr. ir. J.S. Sinninghe Damsté
Supervisor 2: Prof. dr. ir. S. Schouten