Alkenone δD as an ecological indicator: A culture and field study of physiologically-controlled chemical and hydrogen-isotopic variation in C37 alkenones

A combined culture and field study was conducted in order to (1) more firmly identify the physiological controls on hydrogen isotopic composition of C<inf>37</inf> alkenones produced by open-ocean coccolithophores and (2) determine the degree to which these controls are manifested in a natural water column. Nutrient-limitation experiments in culture, combined with previously published data, show that net fractionation between the growth medium and alkenones (α<inf>K37</inf>) varies with cellular alkenone content and production rate, and, by extension, growth phase. It is hypothesized that the relationship of α<inf>K37</inf> with cellular alkenone content and production rate is due to increased use of anabolic NADPH in response to high rates of lipid synthesis. Euphotic zone profiles of δD<inf>K37</inf>, measured in suspended material from the Gulf of California and Eastern Tropical North Pacific, decreased with depth and light availability, and did not correlate in any expected way with previously-suggested controls on α<inf>K37</inf>. It is possible that the field data are driven by behavior in light-limited cells that is not represented by the available, nutrient-limited culture data. If true, δD<inf>K37</inf> may have utility as an indicator of production depth in settings prone to subsurface production maxima. Relationships between α<inf>K37</inf>, cell density, and the carbon-isotopic fractionation term ε<inf>p</inf>, however, suggest that α<inf>K37</inf> acts an indicator of growth rate, which in this setting is only partially dependent on light, consistent with our interpretation of the culture data. If this latter interpretation proves correct, δD<inf>K37</inf> may be a powerful ecological proxy specific to these climatically-important, calcifying, temperature-encoding species.