Carbon isotopic fractionation by the marine diatom Phaeodactylum tricornutum under nutrient- and light-limited growth conditions

A theoretical model was developed to explain the characteristics of carbon isotopic fractionation (ε_P) by the marine diatom Phaeodactylum tricornutum under nutrient- and light-limited growth conditions. The model takes into consideration active transport and diffusion of inorganic carbon through the cell membrane and chloroplast membrane and the energetic tradeoff between production of Rubisco and operation of a carbon-concentrating mechanism to achieve a given growth rate. The model is able to explain 88% of the variance in experimental ε_p data reported in this study and in previous work and is able to account for the observed pattern of Rubisco activity in nitrate-limited chemostats. Two important implications of the model include the fact that ε_p is not a unique function of the ratio of growth rate to external CO₂ concentration (as opposed to the predictions of several previous models) and that changes in light-limited and nutrient-limited growth rates have opposite effects on the fraction of CO₂ taken up by the chloroplast that is lost to diffusion and hence on certain patterns of carbon isotopic fractionation.