EFFECT OF LIGHT HISTORY ON THE ULTRASTRUCTURE AND PHYSIOLOGY OF PROROCENTRUM MARIAE‐LEBOURIAE (DINOPHYCEAE)1

Ultrastructural and physiological responses of Prorocentrum mariae-lebouriae (Parke & Ballantine) Faust are reported for cultures maintained at growth irradiances (Ig) ranging from 20.6 to 0.3 E m−2.d−1 and following downward shifts in light intensity. We tested the hypothesis that Prorocentrum grown under light regimes that elicit different responses in photosynthesis and pigmentation exhibit distinctive cell ultrastructures. Prorocentrum from high-light conditions had high saturation intensities for photosynthesis (Ik) and low levels of Chl a, Chl c and peridinin-cell−1 These cultures were morphologically distinguished by a large starch volume fraction (Vv), small chloroplast Vv and fewer thylakoids lamella−1. Ik values were lower and pigment concentrations higher in low-light treatments, and cells showed reduced starch Vv, large chloroplast Vv, and higher numbers of thylakoids · lamella −1. Cells grown under extremely low-light conditions appeared stressed as indicated by the absence of starch reserves and the presence of large vacuoles within the cytoplasm. Results for presence of large vacuoles within the cytoplasm. Results for quantiative electron microscopy, photosynthesis-irradiance (P-I) relations and cell pigmentation indicate that photoadaptation in P. mariae-lebouriae involves a strategy that encompasses changes in both the “size” and “number” of photosynthetic units.