GENETIC AND PHYSIOLOGICAL INFLUENCES ON THE ALKENONE ALKENOATE VERSUSGROWTH TEMPERATURE RELATIONSHIP IN EMILIANIA-HUXLEYI AND GEPHYROCAPSA-OCEANICA/

Selected warm and cold water strains of the coccolithophorid Emiliania huxleyi and the closely related species Gephyrocapsa oceanica were cu ltured under controlled temperature conditions to assess genetic and p hysiological variability in the alkenone/alkenoate vs. temperature rel ationship. Differences in the strains' growth rates over the 6-30 degr ees C experimental temperature range were small but consistent with th eir cold or warm water origins. E. huxleyi and G. oceanica had similar alkenone/alkenoate biochemistry, justifying the extension of alkenone stratigraphy to sediments predating the appearance of E. huxleyi. The se species could not be distinguished by C-38/C-37 alkenone or alkenoa te/alkenone ratios as previously suggested (Volkman et al., 1995; Sawa da et al., 1996) but given samples from a range of temperatures may be distinguished by a plot of the C-38 ethyl vs. C-38 methyl unsaturatio n ratios (U-38Et(K) and U-38Me(K), respectively). Biochemical response s to temperature and the C-37 alkenone-based (U-37(K')) temperature ca librations differed significantly among the strains. The U-37(K') temp erature calibration was nonlinear for five of the six strains examined . A reduction in slope of the calibration at temperatures < 12 degrees C and >21 degrees C suggests the cell's alkenone-based adaptation to temperature is limited at the extremes of its growth temperature range . The unsaturation ratios of the C-38 methyl and ethyl alkenones (U-38 Me(K) and U-38Et(K)) varied similarly with temperature and were strong ly intercorrelated. The experiments also documented an influence of ce ll physiological state on both alkenone and alkenoate composition and on alkenone unsaturation. Cells in late logarithmic and stationary gro wth had significantly increased abundance of alkenoates and C-38 ethyl alkenones relative to C-38 methyl alkenone abundance. In some strains the unsaturation ratios of both C-37 and C-38 alkenones also signific antly decreased when cells entered the late log phase. Comparison of c ulture results with field data indicates that the average physiologica l state of alkenone-synthesizers in the open ocean differs from cultur ed cells growing under exponential growth and appears to be more simil ar to cells in late log or stationary growth phases. Differences in al kenone/alkenoate ratios between cultured cells and sediments underlyin g waters of a similar temperature most probably reflect a difference i n cell physiology between cultured cells and oceanic populations and n ot greater diagenetic losses of alkenones relative to alkenoates, as p reviously suggested (Prahl et al., 1995). Our experiments confirm that biogeographical variations observed in the alkenone vs. temperature r elationship in natural waters reflect, at least in part, differences i n genetic makeup and physiological status of the local alkenone-synthe sizing populations. Hence, alkenone-based paleo sea surface temperatur e estimates are subject to errors, albeit small, which arise from gene tic differences between modern-day and paleo-populations. The reductio n in slope of the U-37(K') temperature calibration for most strains at T > 24 degrees C indicate that linear U-37(K') temperature calibratio ns (e.g., Prahl et al., 1988) which are currently used to estimate pal eo SST, and which are poorly constrained at higher temperatures, proba bly underestimate the magnitude of SST change for tropical and subtrop ical regions. Copyright (C) 1998 Elsevier Science Ltd.