Effect of iron limitation on photosynthesis in a marine diatom

The response of the marine diatom Phaeodactylum ,tricornutum to Fe deficiency was evaluated in the context of fundamental physiological models of growth and photosynthesis. Fe deficiency induced chlorosis, which decreased Chl a : C ratios and Chl a-specific light-saturated photosynthesis (P,“). In contrast to Pme, (Ye was slightly increased under Fe deficiency, and photosynthesis in Fedeficient cells became light-saturated at lower irradiances than in Fe-replete cells grown at the same irradiance. Fe deficiency increased the in vivo absorption cross section normalized to Chl a (a*), but decreased the maximum quantum yield of photosynthesis (4,). Thus, the product a* &,,, which equals the Chl a-specific initial slope of the photosynthlesis-irradiance curve (@), was less sensitive to Fe limitation than was a* or 4, alone. Using a pump-and-probe fluorometer, we found that Fe deficiency reduced the maximum fluorescence yield (A@,,), which is consistent with the reduction in &, but increased the absorption cross section of photosystem 2 (gps2). Immunoassays of proteins separated electrophoretically indicated that the reduction in maximum fluorescence yields was accompanied by a reduction in the relative abundance of D 1, the photosystem 2 reaction center protein. Light-harvesting chlorophyll proteins (LHCP) and the large and small subunits of ribulose bisphosphate carboxylase were not affected by Fe deficiency. Changes in the abundance of Dl relative to LHCP suggest an increase in the fraction of nonfunctional reaction centers under Felimited conditions. Fe-deficient cells, growing at ~20% of their maximum growth rate, had reduced cellular C, N, and P contents, but maintained C : N : P ratios at the Redfield proportions. These results imply that C : N : P ratios do not provide an unequivocal index of relative growth rate. The underlying goal in a variety of phytoplankton growth models is to provide a general and internally consistent “mechanistic” description of phytoplankton growth, photosynthesis, and respiration (Bannister 1979; Shuter 1979; Kiefer and Mitchell 1983; Laws et al. 1983, 1985; Sakshaug et al. 1989). Although varying in form, all the models are based on an energy balance for phytoplankton growth that equates the gross photosynthesis rate to the sum of growth and respiration (Geider et al. 1986), assuming other terms such as excretion of disAcknowledgments We are grateful to Z. Kolber and K. Wyman for technical assistance and discussions, D. Henry for purification of LHCP, and J. Hirschberg for the gift of the Dl antibody. This research was supported by the U.S. DOE contract DE-AC02-76CHOOO 16 to P.G.F. and by NSF grant OCE 89-15084 to R.J.G. solved organic material are minimal. The energy balance can be written simply as p + .R = P,” 8 tanh( -aBZ/PmB) (1) where p is the growth rate (d-l), R the respiration rate (d-l), 8 the Chl a : C ratio, P,” the Chl a-specific, light-saturated photosynthesis rate, (xtl the Chl a-specific initial slope of the PI curve, and Z the irradiance. It is sometimes informative to simplify Eq. 1 and consider cases of light limitation (Eq. 2) or saturation (Eq. 3) of photosynthesis and growth: p + R = 9aBZ (2) piR=8P,“. (3) 14pplications of Eq. 1 most often consider phytoplankton growth in response to limitation by light, temperature, or nutrient (particularly N03-) availability. Recent experimental evidence, however, suggests that Fe may limit phytoplankton production in