Living on the “energy” edge, microorganisms in deeply buried sediments

PhD defence, Friday 25 August 2017. Marion Jaussi.

2017.08.25 | Steffi Hjerrild Iversen

Marion Jaussi

As humans, microorganisms need enough food (energy over time) to be able to grow, move, and repair damages to keep a healthy body. However, deep below the oceans, in marine sediments, the food (organic matter) available is scarce. Furthermore, this gets worse as we go deeper into the sediment. In her PhD studies, Marion Jaussi investigated (i) if the number of bacteria and archaea cells in the sediments is controlled by the energy available and (ii) whether, at deeper depths, these microorganisms are living close to the absolute minimum energy flow required to survive.


Marion discovered that the two majors groups of microorganisms living in sediments have very different fates in energy-depleted sediments. Sulfate-reducing cells, which feed on small organic molecules and sulfate, gain approximately the same amount of energy at any depth in the sediments. This is because their number declines as the organic matter availability drops. In contrast, the population size of fermentative microorganisms does not decrease as fast as the sulfate-reducers, although both groups get approximatively the same amount of energy. Thus, in average, the fermentative microorganisms have access to less energy per cell as the depth increases. This different strategy suggests that fermentative cells far need less energy to be active and/or use the energy more efficiently.

These findings shed new light on an undiscovered aspect of microbial life deep below the sediment surface and will open new avenues for further researches on the overlooked fermentative microorganisms’ abilities to survive under extremely low-energy conditions.

The PhD degree was completed at the Center for Geomicrobiology, Department of Bioscience, Science and Technology, Aarhus University.

This résumé was prepared by the PhD student.

Time: Friday 25 August 2017 at 13.00
Place: Building 1110, Room 223, C.F. Møllers Alle 8, Aarhus University, 8000 Aarhus C
Title of dissertation: Microbial metabolism in the deep biosphere
Contact information: Marion Jaussi, e-mail: marion.jaussi@bios.au.dk, tel.: +45 91943935
Members of the assessment committee:
Professor J. Gijs Kuenen, Department of Biotechnology, Delft University of Technology, The Netherlands
Professor Erik Kristensen, Department of Biology, University of Southern Denmark, Denmark
Associate Professor Nils Risgaard-Petersen (chair), Center for Geomicrobiology, Aarhus University, Denmark
Main supervisor:
Associate Professor Hans Røy, Center for Geomicrobiology, Aarhus University
Co-supervisor:
Professor Bo Barker Jørgensen, Center for Geomicrobiology, Aarhus University
Language: The PhD dissertation will be defended in English

The defence is public.
The dissertation is available for reading at the Graduate School of Science and Technology/GSST, Ny Munkegade 120, building 1520, rooms 128-134, 8000 Aarhus C.

PhD defence
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Revised 24.11.2017