Optimality-based modeling of nitrogen allocation and photoacclimation in photosynthesis

Abstract

Abstract The ability to predict phytoplankton growth rates under light and nutrient limitation is fundamental to modeling the ocean carbon cycle. Equally fundamental is the ability to predict chlorophyll:carbon ratios, since satellite-based chlorophyll estimates are one of the few data sets to which model output can be compared globally. Because the Geider et al. [Geider, R.J., MacIntyre, H.L., Kana, T.M., 1998. A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients, and temperature. Limnology and Oceanography 43, 679–694] model addresses both desiderata, it has become the model of choice for representing photosynthesis in a wide range of ecosystem models. The Geider et al. [Geider, R.J., MacIntyre, H.L., Kana, T.M., 1998. A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients, and temperature. Limnology and Oceanography 43, 679–694] model follows previous models in positing that maximum photosynthetic rate can be reached only when nitrogen cell quota (nitrogen:carbon ratio) reaches a fixed maximum value q N , max C . Empirically, this assumption is contradicted by the extremely thorough data set of Laws and Bannister [Laws, E.A., Bannister, T.T., 1980. Nutrient- and light-limited growth of Thalassiosira fluviatilis in continuous culture, with implications for phytoplankton growth in the ocean. Limnology and Oceanography 25, 457–473] and by other studies: maximum growth rate does not seem to require maximum nitrogen cell quota. To the extent that existing models do not reflect this key characteristic, they may fail to yield reliable predictions of chlorophyll:carbon ratios as functions of nitrogen:carbon ratios. They also may fail to reflect differences in growth rates of competing phytoplankton species, an essential feature of state-of-the-art ecosystem models used in biogeochemistry simulations. In the present paper I replace the nitrogen limitation function of previous models by one that does not require maximum nitrogen cell quota to produce maximum photosynthetic rate. This new nitrogen-limitation function permits derivation of a steady-state optimality-based relationship between chlorophyll:carbon ratios and nitrogen:carbon ratios; the predictions of this new model are shown to be at least as good as predictions based on the “chlorophyll a synthesis regulation term” of Geider et al. [Geider, R.J., MacIntyre, H.L., Kana, T.M., 1996. A dynamic model of photoadaptation in phytoplankton. Limnology and Oceanography 41, 1–15; Geider, R.J., MacIntyre, H.L., Kana, T.M., 1998. A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients, and temperature. Limnology and Oceanography 43, 679–694]. The Laws and Bannister [Laws, E.A., Bannister, T.T., 1980. Nutrient- and light-limited growth of T. fluviatilis in continuous culture, with implications for phytoplankton growth in the ocean. Limnology and Oceanography 25, 457–473] data suggest that the relationship between chlorophyll:carbon ratio and nitrogen cell quota is independent of nitrogen source (nitrate vs. ammonium) for nitrogen-limited cells. Finally, a full set of parameters for the Laws and Bannister [Laws, E.A., Bannister, T.T., 1980. Nutrient- and light-limited growth of T. fluviatilis in continuous culture, with implications for phytoplankton growth in the ocean. Limnology and Oceanography 25, 457–473] data set is estimated and used to predict chlorophyll:carbon and nitrogen:carbon ratios as functions of growth rate. This improved conceptualization of nitrogen:carbon and chlorophyll:carbon relationships in photosynthesis should provide a robust theoretical underpinning for a new generation of models of multiple-nutrient limitation.