Insoluble prokaryotic membrane lipids in the geosphere : implications for organic matter preservation

• Main Author: Torres, Lidia Chaves
• Published: University of Bristol 2015
• Subjects: 577
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.682182

Annually, 0.5-1 % of the OM productivity escapes mineralization to be preserved in the geosphere, the largest reservoir of organic matter (OM) on Earth. This preservation of OM has a direct impact on carbon bioavailability, on the carbon cycle and also on the formation of fossil fuels. This thesis aims to provide insights into the processes that lead to the preservation of OM on Earth by examining recalcitrant OM at early stages of diagenesis and, to a lesser extent, its stability with time. This is achieved by examining the p311itioning of prokaryotic biomarkers between solvent-extractable and insoluble OM fractions from a Sphagnum peat bog, geothermally-derived silica sinters and SE continental slope marine sediments. The insoluble organic matter (IOM) was examined using stepwise chemical degradation involving base and acid hydrolysis, and biomarkers that occur in the IOM fractions are expected to have an enhanced preservation potential. Iso- and anteiso- Cl5 and C 17 fatty acids (F As), hopanoic acid and bishomohopanol - diagnostic for· the Bacteria domain - and archaeol - diagnostic for Archaea - were quantified via GCIMS, and glycerol dialkyl glycerol tetraethers (GDGTs), including both branched and isoprenoid form, were examined using HPLC/MS. Results clearly reveal that a large proportion of prokaryotic biomarkers - up to 80 % - occur in insoluble fractions at early diagenesis. Different factors seem to dete1111ine the partitioning of biomarkers between extractable and insoluble fractions. First, intact polar (IP) head groups, whose phosphate or glycoside groups contain motifs likely promote covalent or sorptive interactions that enhance lipid insolubility. This factor seems particularly important in peat. Second, esterification reactions with time. Such mechanisms for prokaryotic lipid insolubilization seems particularly imp0l1ant in the marine sediments analysed, with larger proportions of base and acid hydrolysis-released biomarkers at deeper sections of the core. Third, physical encapsulation or sorptive interactions. These mechanisms are unlikely in peat, but account for at least 40 % of the marine branched F As, most likely via interactions with CaC03. Likewise, it is suggested that sorptive interactions of prokaryotic biomarkers with the silica matrix of the geothermal sinters promote their insolubilization. However, the specific distribution of IP head groups in such settings might additionally bias the partitioning of prokaryotic lipids, with archaeol also present in IOM pools as opposed to marine and peat sediments. An additional novel experiment was performed on the Sphagnum peat bog. A pulse of 13CH4 was introduced, targeting methanotrophic bacteria. Subsequently, I3C-Iabelled bacterial F As were traced among the extractable and insoluble fractions over 8 months and under different conditions, anaerobic vs aerobic and UV-Vis vs dark. This allowed to examine first, whether prokaryotic lipids are inherently resistant to solvent-extraction, and second, whether IOM is further increased in situ during the experiment. Up to 5 % of I3C_ labelled lipids were insoluble initially after the I3C-pulse; thus, indicating a ce11ain inherent recalcitrance of prokaryotic cells. Moreover, no in situ transfer from extractable to insoluble fractions was observed during the course of the experiment, which is likely due to the short timescale. However, direct evidence is provided of the preferential protection of insoluble 13C-labelled FAs against oxidative mineralization. Collectively, these results show that putatively labile compounds are entrained into insoluble forms essentially immediately; with mechanisms that range from the inherent recalcitrance of cells to covalent condensation reactions and sorptive interactions, all of which strongly depends on the depositional environment. Insolubility affords some degree of protection to these biomolecules, but there is some evidence that this IOM is dynamic and lipids can also be released from the IOM during early diagenesis, challenging their stability over geological time.