Unusual C 35 and C 36 alkenones in a paleoceanographic benchmark strain of Emiliania huxleyi

Abstract

Our specimen of the cultured Emiliania huxleyi strain (CCMP1742, also known as NEPCC55a) that provides the benchmark for U 37 K ′ -based paleothermometry has started producing, for reasons yet unclear, major amounts of three new alkenones identified as ω15,22-C 35 methyl ketone, ω15,22-C 36 ethyl ketone and ω16,23-C 36 methyl ketone. Comparison of these structures with those established now by the same OsO 4 derivatization method applied to the di-unsaturated C 37, C 38 and C 39 alkenones typically found in this organism provides insight into the possible pathway for their biosynthesis. Isothermal batch culture experiments also show the content and composition of these new compounds change systematically and quite significantly in cells when subjected to environmental conditions such as nutrient depletion, variation in light availability and prolonged darkness. Alkenones of similarly unusual short-chain length are evident in suspended particulate materials from present day surface waters in the Ligurian Sea (Mediterranean) and in two different Holocene time horizons (Unit I and Unit II deposits) in Black Sea sediments. However, the positions of the double bonds are different from those that we now report in our culture, implying a different biosynthetic sequence. These alkenones are most likely derived from another, as yet unknown, haptophyte species. If this other organism accounts for all documented occurrences of these compounds in natural samples, then either it has a capacity for growth over a remarkably wide salinity range or surface water salinity in the early Holocene Black Sea may not have been as low as is currently believed.