Effect of iron limitation on photosynthesis in a marine diatom

Abstract

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 (PmB). In contrast to PmB, αb 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 (ϕm). Thus, the product a* ϕm, which equals the Chl a-specific initial slope of the photosynthesis-irradiance curve (αB), was less sensitive to Fe limitation than was a* or ϕm alone. Using a pump-and-probe fluorometer, we found that Fe deficiency reduced the maximum fluorescence yield (Δϕsat), which is consistent with the reduction in ϕm, but increased the absorption cross section of photosystem 2(σPS2). Immunoassays of proteins separated electrophoretically indicated that the reduction in maximum fluorescence yields was accompanied by a reduction in the relative abundance of D1, 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 D1 relative to LHCP suggest an increase in the fraction of nonfunctional reaction centers under Fe-limited 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.