New culturing studies of various haptophyte algae: The role of phylogeny on the alkenone paleothermometer

Alkenone paleothermometry (via the U^K₃₇ and U^K'₃₇ indices) is widely used to reconstruct sea surface temperature and, more recently, lake water temperature. Genetic analyses indicate that there is a diversity of different alkenone-producing lacustrine haptophytes, and differences among U^K₃₇-temperature calibrations suggest that unique calibrations might be required to quantify past temperature variation from individual lakes. The only term needed to quantify U^K₃₇-inferred temperature relative to a reference period (e.g., modern temperature, or 20^th Century mean temperature) is the slope of the calibration regression: U^K₃₇-temperature sensitivity (i.e., the change in U^K₃₇ per °C temperature change). Here, we present new data developed by culturing four different species of alkenone-producing haptophyte algae across a range of temperatures (6-30 °C) and light levels (20-200 µE). The simultaneous culture of four distinct species allows direct comparison of the absolute quantities of alkenones and alkenoates, as well as other lipids, produced by different species of haptophytes under identical environmental conditions. Our results indicate that algal growth rate, when controlled by light intensity, has no impact on U^K₃₇ values. As expected, we find that growth temperature controls both the degree of alkenone unsaturation and the relative production of alkenones vs. alkenoates in all four species. Importantly, comparison of the four U^K₃₇-temperature calibrations resulting from our experiments with preexisting calibrations supports the hypothesis that U^K₃₇-temperature sensitivity is controlled by phylogeny. Therefore, even in the absence of a site-specific calibration, this term can be used to quantify past temperature variation from lake sediments if the genetic identity of the lake’s alkenone-producer is known.