Phylogenetic effects of alkenone unsaturation in cultured Group II and Group III haptophytes

Certain haptophyte algae produce a recalcitrant suite of long-chain (C₃₇-C₃₉) methyl and ethyl ketones called alkenones. These compounds are widely applied in paleoclimate studies due the temperature sensitivity of the ratio of di- to tri- unsaturated alkenones, most commonly quantified in the U^K’₃₇ index. However, phylogenetic effects and other physiological effects can alter the intercept of the U^K’₃₇ relationship to temperature, complicating the application to past climates. Here we present results of two strains of Group II (brackish) and four strains of Group III (open marine) haptophytes batch cultured under different temperatures, light levels, and CO₂ (aq) concentrations. One Group III strain was also continuously cultured in a turbidostat.

The alkenone response to temperature differs per strain, as has previously been found in culture studies. Additionally, when applying core top calibrations commonly used in paleoclimate studies, U^K’₃₇ consistently under-predicts batch culture growth temperature. We further find no systematic control on the offset between alkenone unsaturation calibrations from core top and expected values in batch culture for a given strain. When considering data from multiple strains, the offset from expected values in U^K’₃₇ and U^K_38Me, but not U^K_38Et, correlate to ratio of the alkenone concentration relative to particulate organic matter. In cells harvested with a higher proportion of alkenones relative to particulate organic carbon, this cold offset is diminished and temperature prediction for U^K’₃₇ and U^K_38Me is more consistent with core top calibrations.

In addition, we compare nutrient replete continuous culture results to batch culture of the same strain to assess if the alkenone unsaturation response to temperature similar.