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

The alkenone unsaturation index (U37K or U37K′) 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 U37K temperature proxy to lake sediment records. The haptophyte Pseudoisochrysis paradoxa produces alkenones but its U37K 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 U37K index to temperature for a culture of P. paradoxa. We found that the U37K index accurately captured the alkenone response to temperature whereas the U37K′ index failed to do so, with U37K′ values below 0.08 projecting to two different temperature values. Our results add a fifth species-specific U37K calibration and provide another line of evidence that different haptophyte species require different U37K calibrations. The findings also highlight the necessary inclusion of the C37:4 alkenone when reconstructing temperatures from P. paradoxa-derived alkenone records.