Heterosigma akashiwo, a red tide alga, was grown in Fe-deficient and Fe-replete batch cultures. Cell final yields and the growth rate were limited when Fe was below 10 nM and alleviated with 100 nM Fe. By comparison with the results under Fe-replete conditions, chlorophyll a-specific and cell-specific light saturated net photosynthetic capacity (P-m(chl a) and P-m(cell)), dark respiration rate (R-d(chl a) and R-d(cell)) and apparent photosynthetic efficiency (alpha(chl a) and alpha(cell)) decreased proportionately, whereas the cells became light saturated at higher irradiance under Fe stress (Fe-limited conditions).