Experimental and field evidence for thiyl radical-induced stereomutation of alkenones and other lipids in sediments and seawater

Possible origins were investigated for unusual cis/trans alkenones previously identified after incubation of a marine haptophyte Emiliania huxleyi strain with bacterial consortia isolated from cyanobacterial microbial mats. Diunsaturated alkenones and alkenoates isolated from the strain were incubated in the presence of ethanethiol to examine whether double bond stereomutation (i.e., cis/trans equilibration without double bond migration) could be induced by thiyl radicals. Significant stereomutation was observed after 4–5 days of incubation. The reaction was strongly attenuated when incubation was carried out in the presence of a free radical quencher (2,4,6-tri-tert-butylphenol), confirming the involvement of a thiyl radical-dependent mechanism. Room temperature incubation of E. huxleyi in natural seawater amended with ethanethiol also resulted in stereomutation of the alkenones and a significant increase in the measured value for the palaeotemperature proxy . In this case, production of thiyl radicals is likely initiated by peroxyl and hydroxyl radicals arising from the homolytic decomposition of photochemically or autoxidatively produced hydroperoxides often found in senescent algal cells. Thiyl radical-induced stereomutation of alkenones also acted in the presence of sulfides and transition metals (e.g., Fe3+). Based on these results, alkenone cis/trans isomerization previously observed during bacterial incubations of E. huxleyi cells under sulfate-reducing conditions is attributable to formation of thiyl radicals either from methanethiol produced by bacterial degradation of DMSP (β-dimethylsulfoniopropionate) or from oxidation of thiolate ions by transition metals. Analysis of a sediment interval from the Unit II deposit in a Black Sea core revealed the presence of stereomutated isomers, confirming that such a reaction can be geochemically significant. Stereomutation could explain previous observations of a surprising increase in values with depth in the anoxic water column of the Black Sea. Since this stereomutation process is induced under sulfate-reducing conditions, it may occur widely in anoxic environments, a prospect that should be checked as part of palaeotemperature studies. Other commonly found lipids such as phytol and unsaturated fatty acids can also be affected by these reactions, thereby providing additional means for recognizing the occurrence of this phenomenon.