Does growth rate affect ketone unsaturation and intracellular carbon isotopic variability in Emiliania huxleyi?

Abstract

We present the first results of U37K′ and εalkenone from Emiliania huxleyi grown in chemostat cultures at constant temperature but at variable growth rates. A small change in U37K′ with growth rate was found for a noncalcifying strain of E. huxleyi, whereas results from a calcifying strain showed no systematic variation between U37K′ and growth rate. We conclude that nutrient‐limited growth rate effects will not produce serious errors in paleotemperature determinations using U37K′ or U37K′. However, U37K′ values for the two strains differed significantly and apparent temperatures calculated using unsaturation ratios obtained from chemostat experiments are very different from those derived from batch culture experiments. Results of isotopic analyses of alkenones and bulk cells indicate no effect of growth rate on εalkenone (εalkenone=4.2±0.2‰). Constancy of εalkenone values is a requirement for environmental and paleoenvironmental interpretations. These results indicate that isotopic analyses of alkenones are ideal markers for paleo‐[CO2(aq)] determinations.