A new method for estimating growth rates of alkenone-producing haptophytes

Brian N. Popp, Robert R. Bidigare, Bryan Deschenes, Edward A. Laws, Fredrick G. Prahl, Jamie K. TanimotoRichard Wallsgrove

Limnol. Oceanogr. Methods 4:114-129 (2006) | DOI: 10.4319/lom.2006.4.114

ABSTRACT: Laboratory culture experiments were performed to establish the range of growth conditions where 13C labeling of di- and tri-unsaturated C37 methyl ketones yields reliable growth rates for alkenone-producing algae. Results document that even at low growth rates and short time intervals, 13C labeling of the di-unsaturated C37 alkenone provides reasonable estimates of growth rate for Isochrysis galbana, Isochrysis sp., and three strains of Emiliania huxleyi. These findings suggest that although alkenone biosynthesis almost certainly involves a complex combination of intermediate pools, those pools must turn over at a rate sufficiently fast that the labeling of the di-unsaturated C37 alkenone is not greatly biased. However, bias was noted for the tri-unsaturated alkenone, suggesting that either growth rates in the field should be based on K37:2 labeling or that long incubations should be used. Specific growth rates calculated from alkenone 13C labeling experiments conducted in the subarctic Pacific decreased as a function of depth in the euphotic zone and were linearly correlated with photosynthetically active radiation below ~50 µEin m–2 s–1. These observations indicate that at depths greater than ~25 m, growth rates of the alkenone-producing algae were light-limited. Determination of the specific growth rates of the alkenone-producing algae was motivated by our present knowledge of the controls on stable carbon isotopic fractionation in marine microalgae. Development of our alkenone 13C labeling technique for in situ growth rate determinations allows evaluation of the effect of growth rate on carbon isotopic fractionation in natural populations of E. huxleyi and Gephyrocapsa oceanica so that laboratory-based microalgal stable isotope fractionation hypotheses may be evaluated in the field.