We conducted isothermal (15¿C) batch culture experiments with the coccolithophorid Emiliania huxleyi (strain NEPCC 55a) to evaluate the extent to which nutrient and light stress contribute to variability in the alkenone unsaturation index U37K'. Alkenone content and composition were constant throughout exponential growth in both experiments when nutrients (nitrate and orthophosphate) were replete. Stationary phase (nutrient-starved) cells continued to produce alkenones, amassing concentrations (ΣAlk) ≥ 3 times higher than those dividing exponentially (1.5--2 pg cell-1), and the U37K' of excess alkenone dropped by 0.11 units. In contrast, 5 days of continuous darkness resulted in a 75% decrease in cellular ΣAlk and a significant U37K' increase (+0.11 units). Given an established 0.034 unit/¿C response for exponentially growing cells of this strain, the observed range of U37K' variability at 15¿C corresponds to an uncertainty of ¿3.2¿C in predicted growth temperature. This level of variability matches that of the global U37K' annual mean sea surface temperature calibration for surface marine sediments, begging the question: What is the physiological condition of alkenone-producing cells exported to marine sediments? Comparison of our laboratory results for a strain of E. huxleyi isolated from the subarctic Pacific Ocean with depth profiles for alkenones in surface waters from two contrasting sites in the northeast Pacific Ocean suggests that the answer to this question depends on the ocean regime considered, a possibility with significant bearing on how stratigraphic U37K' records in marine sediments are to be interpreted paleoceanographically.