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 system atically 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 fo r 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 - epsil on(p))C-e. The correlation found between b and [PO4] is due to effects linking nutrient levels to growth rates and cellular carbon budgets f or alkenone-containing algae, most likely by trace-metal limitations o n algal growth. The relationship reported here is characteristic of 39 samples (r(2) = 0.95) from the Santa Monica Basin (six different time s 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 Sargas so 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), ep silon(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. According ly, before concentrations of dissolved CO2 can be estimated from isoto pic 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.