The marine diatom Amphora sp. was grown at a light intensity of 7.0 x 10(15) quanta centimeter(-2) second(-1). Light saturation of photosynthesis for these cells was between 6.0 and 7.0 x 10(16) quanta centimeter(-2) second(-1). At light intensities greater than saturation, photosynthetic (14)CO(2) fixation was depressed, while P700 unit size (chlorophyll a concentration/P700 activity) increased and number of P700 units per cell decreased. After a 1-hour exposure of Amphora sp. to a photoinhibitory light intensity of 2.45 x 10(17) quanta centimeter(-2) second(-1), there was a 45 to 50% decrease in the rate of (14)CO(2) fixation relative to the rate at the culture light intensity. There also was a 25% increase in P700 unit size and a 30% reduction in the number of P700 units per cell but no change in total chlorophyll a concentration. Following this period of photoinhibition, the cells were returned to a light regime similar to that in the original culture conditions. Within 1 hour, both number of P700 units per cell and P700 unit size returned to levels similar to those of cells which were kept at the culture light intensity. The rates of photosynthesis did not recover as rapidly, requiring 2 to 3 hours to return to the rate for the nonphotoinhibited cells. Our results indicate that a decrease in P700 activity (with a resultant increase in P700 unit size) may be partially responsible for the photoinhibition of algal photosynthetic carbon dioxide fixation.