Present and past contribution of anaerobic ammonium oxidation to nitrogen cycling as revealed by ladderane lipids

Abstract

Abstract Anammox, the anaerobic oxidation of ammonium to dinitrogen gas with nitrite as the electron acceptor, constitutes a novel route to convert biologically available (fixed) nitrogen to gaseous N2. This process is mediated by specific bacteria belonging to the Planctomycetes that were initially discovered in waste water systems. Within the nine years after their discovery, anammox bacteria have been identified as key players in the global nitrogen cycle. They have been found in different suboxic to anoxic environments, including oxygen minimum zones, marine and freshwater sediments, tropical lakes and even in sea ice. Anammox is now acknowledged as an important process for the removal of fixed inorganic nitrogen from the oceans, freshwater and wastewater treatment systems, which was so far solely attributed to heterotrophic denitrification. However, relatively little is known about anammox bacteria, specifically their unusual biology and the mechanisms regulating their occurrence in the natural environment. The potential importance of anammox in past settings is also an open question. Anammox bacteria contain a specific intracellular compartment, the anammoxosome, in which anammox catabolism is thought to take place. The membrane of this organelle is composed of unique ladderane lipids containing either three or five linearly fused cyclobutane rings. Ladderane lipids provide an usually dense membrane which is thought to protect the remainder of the cell from the toxic intermediate of the anammox reaction, hydrazine. Their unusual chemical structure which is so far unprecedented in nature, make ladderane lipids ideal markers for the detection of anammox bacteria. This thesis aimed to get a better understanding of the presence and distribution of ladderane lipids as a marker for anammox in different present day environmental settings, to examine processes of transport, preservation and early diagenesis of ladderane lipids as well as to elucidate their potential suitability as a tracer for past anammox processes. The present study showed the general applicability of ladderane lipids as tracers for anammox bacteria in different present-day environmental settings. It also contributed to a better understanding of the behavior of ladderane lipids upon microbial degradation and thermal maturity and showed that diagenetic products of ladderane lipids are probably suitable biomarkers for the detection of anammox in ancient sediments, such as Cretaceous black shales. The detection of ladderane lipids in Arabian Sea sediments provided already insight in past anammox activity.