Wed 2nd October @ noon, Rm 312.222

Abstract:

Deep marine sediments host ~2.9 × 10²⁹ microbial cells, equivalent to up to 3.6% of the total living biomass on the planet with Archaea accounting for ~37% of that population. These subsurface microbial communities are generally considered to be structured through in situ environmental conditions such as the availability of electron acceptors and donors, porosity and sediment lithology. Yet, recent studies have shown that a subset of subseafloor microbial communities were present at the time of deposition and formed a genetic archive originally referred to as “the Paleome” and reflect the paleo-depositional environment. However, additional highly resolved profiling of subseafloor microbiomes are required to substantiate this claim. In this talk I will show that up to 15% of subseafloor bacterial communities in the Arabian Sea subsist through fermentation but originated from the overlying water column during the last glacial interglacial cycle where they were involved in denitrification during periods of strong oxygen minimum zone conditions. This fraction of subseafloor Bacteria underwent weak postburial selection and therefore formed a living long-term genomic record of the paleodepositional environment. To explore the global potential of sedimentary Archaea to also record changes in the paleodepositional environment, we performed a highly-resolved 16S rDNA survey of subseafloor archaeal communities in up to 140 kyr-old sediments of the northern Red Sea covering the last six Marine Isotope Stages (MIS) when alternating glacial low stands and interglacial highstands caused drastic changes in sea level and salinity in the Red Sea. Our results revealed a strong significant response of subseafloor archaeal communities to changes in paleodepositional conditions associated with MIS stages and their transitions and only a moderately significant response to changes in sediment lithology. This suggest that a substantial part of the subseafloor archaeal communities would have been present at the time of deposition and that they no longer play a highly active role in the cycling of sedimentary organic matter. This claim was substantiated by the parallel finding that the majority of buried archaeal lipid biomarkers (GDGTs) that comprise the TEX₈₆ paleothermometry were derived from ancient marine Thaumarchaeota and not from e.g. methanogenic archaea that were identified from the Red Sea paleome.

Short bio:

Associate Prof. Marco Coolen is a geomicrobiologist who is best known for his studies on ancient DNA signatures preserved in lake and marine sediments to reconstruct terrestrial and aquatic ecosystem responses to long-term climate variability or to more recent human environmental impact. In addition, Coolen studies the activity, limitations, and evolution of the deep biosphere and is exploring to what extent microbes buried in the geologic record can form long-term genomic records of ecosystem-climate interactions. He is also using similar technologies to study the diversity and functioning of active microbes in modern environments such as the role that bacteria play in human gallstone formation, the functioning of Shark Bay microbial mats, and the degradation of oceanic plastic waste as well as industrial-derived persistent organic pollutants.