Physiological impacts on alkenone paleothermometry

Abstract

[1] 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 U 37 K 0. Alkenone content and composition were constant throughout exponential growth in both experiments when nutrients (nitrate and orthophosphate) were replete. Stationary phase (nutrientstarved) cells continued to produce alkenones, amassing concentrations (AEAlk) ! 3 times higher than those dividing exponentially (1.5-2 pg cell À1 ), and the U 37 K 0 of ''excess'' alkenone dropped by 0.11 units. In contrast, 5 days of continuous darkness resulted in a 75% decrease in cellular AEAlk and a significant U 37 K 0 increase (+0.11 units). Given an established 0.034 unit/°C response for exponentially growing cells of this strain, the observed range of U 37 K 0 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 U 37 K 0 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 U 37 K 0 records in marine sediments are to be interpreted paleoceanographically.

Carbon isotopic fractionation during a mesocosm bloom experiment dominated by Emiliania huxleyi: Effects of CO2 concentration and primary production

“…In exponentially growing cells alkenones comprise between 2% and 20% of total cellular carbon (Prahl et al, 1988;Pond and Harris, 1996;Conte et al, 1998). Under nutrient stress, values up to 39% have been reported (Prahl et al, 2003). This is in very good agreement with the results of our experiment, assuming that cellular concentrations of total organic carbon in E. huxleyi range from 6 to 15 pg cell À1 in exponentially growing cells (Riebesell et al, 2000;Zondervan et al, 2002;Prahl et al, 2003) and up to 22 pg cell À1 under stationary phase conditions Van Bleijswijk et al, 1994;Riegman et al, 2000).…”

“…Physiological factors such as nutrient availability (e.g., Epstein et al, 1998;Prahl et al, 2005) as well as genetic differences (e.g., Conte et al, 1998;Popp et al, 2006) have been suggested in order to explain this phenomenon. In a recent culture experiment at constant temperature (15°C), Prahl et al (2003) found a decline of U K 0 37 values by 0.11 units following growth limitation by nutrient depletion. This decrease in U K 0 37 was accompanied by an increase in cellular alkenone concentration from 1.5-2 to 6 pg cell À1 which is nearly identical to the values observed in the present study (Fig.…”

Bathypelagic particle flux signatures from a suboxic eddy in the oligotrophic tropical North Atlantic: production, sedimentation and preservation

“…Loebl et al, 2010), and might result in an increase in the production and storage of alkenones (e.g. Eltgroth et al, 2005;Prahl et al, 2003). Alkenone temperature records from the Subtropical Front at the Chatham Rise, south-western Pacific Ocean (Sikes et al, 2005), showed that biases occurred during times of highest lipid fluxes and low nutrient conditions in the surface mixed layer.…”

Modelling an alkenone-like proxy record in the NW African upwelling

“…Batch culture experiment have demonstrated that the cell alkenone content, as well as the U K 37 , may be significantly affected by environmental conditions like nutrient or light stress, as well as stationary or exponential growth phases (e.g. Epstein et al, 1998;Prahl et al, 2003). The quantity of alkenone per cell may vary as a function of growth conditions and therefore be unequal from one habitation depth to another.…”