EFFECTS OF Si:P CONCENTRATION RATIOS AND NUTRIENT LIMITATION ON THE CELLULAR COMPOSITION AND MORPHOLOGY OF ASTERIONELLA FORMOSA (BACILLARIOPHYCEAE)

The freshwater diatom Asterionella formosa Haas. was grown in semicontinuous culture at 20°C under continuous cool‐white fluorescent light of ca. 20 μEin · m⁻²· s ·⁻¹ in a medium containing Si: P in various concentration ratios. The cell quotas of P and Si changed in relation to the available concentrations of P and Si at constant μ= 0.11 and 0.16 d⁻¹. Under Si‐limitation, the P cell quota increased by over an order of magnitude as the influent [Si:P] decreased. The Si cell quota increased with increase in [Si] in the influent medium, and it increased as [P] increased at a specific [Si]. Under P‐limitation, the P cell quotas were fairly constant and low; the Si cell quotas were relatively high and decreased slightly as influent [P] and [Si] increased. Asterionella stored up to 28 times more P and 2 times more Si than needed. The number of Asterionella cells per colony varied as a function of the influent [Si:P] and nutrient limitation being usually less than or equal to 6 when P‐limited, and greater than 10 when Si‐limited.