Controls on the molecular distribution and carbon isotopic composition of alkenones in certain haptophyte algae

Although the biochemical functions and biosynthetic pathways of alkenones a re still largely unknown, alkenone unsaturation ratios are now used extensi vely to infer ancient sea surface temperature, and their isotopic compositi ons have been used to reconstruct ancient atmospheric CO levels. The inferr ed relationships between alkenone unsaturation ratios, isotopic composition s, and growth conditions are based on empirical laboratory and field studie s and, in the case of isotope fractionation, on simple models of carbon acq uisition and fixation. Significant uncertainty still exists concerning the physiological and ecological factors affecting cellular production of alken ones, unsaturation ratios, and isotopic composition. Phytoplankton culture conditions have been shown to affect alkenone unsaturation ((K'U)(37)), cel lular alkenone content, intracellular isotopic compositions (Delta delta), and changes in fractionation (epsilon (P)) as a function of the quotient of algal growth rate and aqueous carbon dioxide concentration (mu /CO2). Such studies imply that plant physiology can affect the interpretation of envir onmental signals. The factor(s) controlling cellular alkenone concentration s and unsaturation ratios are reviewed, as well as the carbon isotopic comp osition of the alkenone-producing algae. A new technique is presented to de termine growth rates of the alkenone-containing algae in natural settings t hat will facilitate testing laboratory-based hypotheses concerning the carb on isotopic fractionation and its relationship to growth rate/growth status of alkenone-producing algae in the field.