Production and temperature sensitivity of long chain alkenones in the cultured haptophyte Pseudoisochrysis paradoxa

The alkenone unsaturation index ( U 37 K or U 37 K ′ ) serves as a critical tool for reconstructing temperature in marine environments. Lacustrine haptophyte algae are genetically distinct from their ubiquitous and well studied marine counterparts, and the unknown species-specific genetic imprints on long chain alkenone production by lacustrine species have hindered the widespread application of the U 37 K temperature proxy to lake sediment records. The haptophyte Pseudoisochrysis paradoxa produces alkenones but its U 37 K calibration has never been determined. It has an alkenone fingerprint abundant in tetraunsaturated alkenones, a hallmark of lacustrine environments. We present here the first calibration of the U 37 K index to temperature for a culture of P. paradoxa. We found that the U 37 K index accurately captured the alkenone response to temperature whereas the U 37 K ′ index failed to do so, with U 37 K ′ values below 0.08 projecting to two different temperature values. Our results add a fifth species-specific U 37 K calibration and provide another line of evidence that different haptophyte species require different U 37 K calibrations. The findings also highlight the necessary inclusion of the C37:4 alkenone when reconstructing temperatures from P. paradoxa-derived alkenone records.