||The alkenone unsaturation index (U-37(K) or U-37(K)#39;) 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)#39; index failed to do so, with U-37(K)#39; 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 C-37:4 alkenone when reconstructing temperatures from P. paradoxa-derived alkenone records. (C) 2013 Elsevier Ltd. All rights reserved.