Insoluble prokaryotic membrane lipids in a Sphagnum peat: Implications for organic matter preservation

Preservation of organic matter (OM) in the geosphere has a direct impact on carbon bioavailability, the carbon cycle and the formation of fossil fuels. We have examined some of the processes that lead to the preservation of OM by characterising insoluble OM in a Sphagnum peat bog. We focussed on the partitioning of prokaryotic biomarkers between solvent-extractable and insoluble OM fractions and how that partitioning changed with depth. The insoluble organic matter (IOM) was examined using stepwise chemical degradation involving base and acid hydrolysis. Iso- and anteiso-C15 and C17 fatty acids (FAs), hopanoic acid and bishomohopanol, and branched glycerol dialkyl glycerol tetraethers (GDGTs) - diagnostic for Bacteria - were targeted as well as archaeol and isoprenoidal GDGTs - diagnostic for Archaea. High percentages of these compounds - up to 65% - occur in IOM pools, indicating that archaeal- and bacterially derived OM is prone to insolubilization even in shallow sediments (< 5 cm depth). Differences in functionalities - likely related to intact polar (IP) head groups - seem to determine the insolubilization of prokaryotic lipids; specifically, we propose that compounds bearing rapidly degradable phosphate-based polar head groups are less likely to be incorporated into the insoluble OM pools. Collectively, these data indicate that microbial membrane lipids are rapidly incorporated into peat IOM during early diagenesis. We suggest that this is due to an inherently recalcitrant character of some prokaryotic cells which affords protection to their membrane lipids. However, this large proportion of insoluble prokaryotic lipids does not appear to be entirely stable at shallow burial depths, indicating that the peat IOM is a dynamic reservoir, at least during early diagenesis.