New possibilities for discovering importance nitrogen in the past
26-11-2012 A new way has been found to investigate past changes in the marine nitrogen cycle. Darci Rush of the Royal Netherlands Institute for Sea Research (NIOZ) found that although it is difficult to find the biomarker molecules of the nitrogen-processing bacteria, there is the potential to detect the transformation products of these molecules in sediments up to 1 million years old. Rush will defend her thesis on December 3th at the Utrecht University.
Ladderane molecules are lipids which occur only in bacteria that transform ammonium and nitrite to nitrogen gas in the absence of oxygen, a key process in the global marine nitrogen cycle. However, not much is known about the importance of this process in the geological past. Rush first investigated whether the ladderane molecules could be used to detect ammonium oxidising bacteria (anammox) in present day environments. Indeed, anammox was found to be an important process in the large eastern Tropical North Pacific oxygen minimum zone, where oxygen concentrations are low, and the Cariaco Basin, where there is no oxygen present. This showed that anammox is truly a ubiquitous process in the present day ocean.
It is important to understand what has happened to the marine nitrogen cycle in the past to predict what will happen in the future. In the past there have been several occurrences of oceanic anoxic events (OAEs) when temperatures were warm, ocean circulation was altered, and less oxygen was freely available in the ocean. During OAEs, anammox is believed to have been an important process. If this is the case, then we can predict that in the future with climate change heading towards warmed less circulating oceans, anammox may well be a major process determining nitrogen availability.
Rush investigated the fate of ladderane molecules, for example, how they are transformed when they sink to the bottom of the ocean. For this, laboratory experiments were performed by incubating large amounts of anammox biomass, from a waste water treatment plant, with Wadden Sea sediments for several weeks. This led to the transformation of the ladderane molecules into shorter-chain ladderane molecules. This transformation was likely performed by micro-organisms living in the Wadden Sea sediment.The short-chain ladderanes were shown to be abundant in settings where lipids from anammox bacteria had been exposed to oxygen after cell death and were present in marine sediments of up to 100,000 years old. The potential for these short chain ladderanes to be found in even older sediments, based on experimental calculations, is high. These transformation products, therefore, may be more suitable for detecting past anammox activity in ancient marine sediments.
This thesis was accomplished with financial support from the Darwin Center for Biogeosciences and the Royal Netherlands Institute for Sea Research NIOZ.
Darci Rush was born in Canada and studied Biotechnology and Biochemistry at the University of Ottawa. She moved to Marseille in 2004 to study Marine Biogeochemistry at l’Université de la Méditerranée. Darci started her PhD studies at the Royal NIOZ in August 2008. She is now working at Newcastle University, investigating bacteriohopanepolyols as tracers for aerobic methane oxidation.
Picture above: Incubation experiment of anammox biomass with Wadden Sea sediment.
Ladderane lipids (1-4) and their transformation products (I, II and III)