Consistent fractionation of C-13 in nature and in the laboratory: Growth-rate effects in some haptophyte algae

The carbon isotopic fractionation accompanying formation of biomass by alkenone-producing algae in natural marine environments varies systematically with the concentration of dissolved phosphate. Specifically, if the fractionation is expressed by epsilon(p) approximate to delta(e) - delta(p), where delta(e) and delta(p) are the delta(13)C values for dissolved CO2 and for algal biomass (determined by isotopic analysts of C-37 alkadienones), respectively, and if C-e is the concentration of dissolved CO2, mu mol kg(-1), then b = 38 + 160*[PO4], where [PO4] is the concentration of dissolved phosphate, mu M, and b = (25 - epsilon(p))C-e. The correlation found between b and [PO4] is due to effects linking nutrient levels to growth rates and cellular carbon budgets for alkenone-containing algae, most likely by trace-metal limitations on algal growth. The relationship reported here is characteristic of 39 samples (r(2) = 0.95) from the Santa Monica Basin (six different times during the annual cycle), the equatorial Pacific (boreal spring and fail cruises as well as during an iron-enrichment experiment), and the Peru upwelling zone. Points representative of samples from the Sargasso Sea ([PO4] less than or equal to 0.1 mu M) fall above the b = f[PO4] line. Analysis of correlations expected between mu (growth rate), epsilon(p), and C-e shows that, for our entire data set, most variations in epsilon(p) result from variations in mu rather than C-e. Accordingly, before concentrations of dissolved CO2 can be estimated from isotopic fractionations, some means of accounting for variations in growth rate must be found, perhaps by drawing on relationships between [PO4] and Cd/Ca ratios in shells of planktonic foraminifera.