Effects of biosynthesis and physiology on relative abundances and isotopiccompositions of alkenones

The number of double bonds in long-chain, unsaturated ketones (alkenones) p roduced by some members of the Haptophyceae is correlated with the ambient temperature at the time of synthesis. For these same organisms the depletio n of carbon-13 in biosynthetic products relative to dissolved inorganic car bon is related directly to the specific growth rate and inversely to the co ncentration of dissolved carbon dioxide. This report summarizes issues rela ting to the physiology, metabolism, and biochemistry of alkenone producers and how they affect the abundances and isotopic compositions of alkenones. These considerations show that an understanding of cellular responses to pa rameters governing uptake of inorganic carbon (C-i), isotopic fractionation , growth under diverse nutrient conditions, and genetic variability, both i n the field and in culture, is necessary for developing a conceptual unders tanding of the biological significance of the epsilonp and U-37(K') indices . Emiliana huxleyi is the best known alkenone producer and can serve as a m odel organism for these studies. This report identifies knowledge gaps and appropriate objectives for both field- and laboratory-based research.