A large fraction of microbial life is living kilometers deep inside the Earth. The majority of these microbes were thought not to be bacteria but archaea, based on the abundant presence of their membrane lipids. Sabine Lengger of the NIOZ Royal Netherlands Institute for Sea Research found, however, that these lipids are poor indicators for living cells and that the amount of living archaeal biomass in the deep biosphere has been vastly overestimated. Lengger will defend her thesis on 11 July at Utrecht University.

Microbes dominate life

Microbes dominate life on Earth and are ubiquitously present in all kinds of environments on land as well as at sea. It is estimated that there are ca. 10³⁰ microbial cells on our planet. Recent studies have indicated that microbial life is not only dominating environments on the surface of the Earth but is also present deep inside the Earth, in sediments and rocks buried for millions of years. Estimates suggest that there are at least as many microbes living in the subsurface as on the Earth’s surface. This vast microbial world has also been called ‘the deep biosphere’.

Membrane lipids to investigate the ‘deep biosphere’

Many studies have tried to investigate what types of microbes are living deep inside the Earth. Unfortunately, most of the genomic techniques used for investigating surface microbes work poorly due to the interference of the sediment and rock minerals. Therefore, scientists have turned to membrane lipids rather than DNA as a tool to investigate the ‘deep biosphere’. They surprisingly found that that it was not dominated by membrane lipids from bacteria but from another group, the archaea, single-celled microorganisms that inhabit the most extreme environments of the Earth. Therefore, it is thought that these microbes dominate microbial life deep inside the Earth.

PhD research Sabine Lengger
Lengger studied the origin and fate of these archaeal membrane lipids by examining their distributions in marine sediments from, amongst others, Arabian Sea and Iceland, taken by the NIOZ research vessel ‘Pelagia’. On board of the research vessel she also incubated the marine sediments with isotopically labelled carbon to investigate how active the microbes were and what kind of carbon they would eat.

Lipids persisted millions of years

Lengger found strong indications that bacterial lipids in sediments rapidly disappeared when the bacteria died. Thus, the presence of bacterial membrane lipids is good evidence that the sediment contains living bacterial cells. Surprisingly, most of the membrane lipids of the archaea were hardly degraded after the death of the archaea and persisted for thousands to millions of years. The results thus show that archaeal membrane lipids are poor indicators for living cells and cannot be used to investigate microbial life inside the Earth. It therefore remains unknown who is ruling the ‘deep biosphere’.

This thesis was accomplished with financial support from the Darwin Center for Biogeosciences and the NIOZ Royal Netherlands Institute for Sea Research.

About Sabine Lengger

Sabine Lengger was born in Leoben, Austria and studied Technical Chemistry - Biochemistry at Graz University of Technology. Lengger started her PhD studies at the Royal NIOZ in 2008. She is now working at the University of Plymouth, investigating organic pollutants using multidimensional gas chromatography techniques.

Thesis

Production and preservation of archaeal glycerol dibiphytanyl glycerol tetraethers as intact polar lipids in marine sediments: Implications for their use in microbial ecology and TEX₈₆ paleothermometry

PhD Defence ceremony:

Sabine Lengger will defend her PhD Thesis on July 11^th 2013 at 12:45 h at the Academiegebouw Utrecht University.

Supervisors: prof. dr. ir. Stefan Schouten and prof. dr. Ir. Jaap S. Sinninghe Damsté.