Factors affecting the photosynthetic capacity of laboratory cultures of the diatom Phaeodactylum tricornutum

The light-saturated photosynthetic capacity of cultures of Phaeodactylum tricornutum Bohlin grown under different conditions has been measured. In batch cultures grown in a regime of alternating light and dark periods, the photosynthetic capacity reaches a maximum before the end of the exponential phase of growth, and declines thereafter. In cultures illuminated at 0.7 mW (milliwatt)/cm², there is a 75% falloff in photosynthetic capacity per cell over an 8 day period following the time of maximum photosynthetic capacity. At 1.75 mW/cm², the corresponding fall-off is 85% over a 4 day period. Cultures exposed to a prolonged period of darkness (up to 16 days at 18°C) maintain a high photosynthetic capacity. Incubation in darkness also protects the cells from the deleterious effects of high temperature (28°C) upon photosynthetic capacity. The various fluctuations of photosynthetic capacity occur without any accompanying major changes in the concentration of chlorophyll a. Evidence from estimations of total protein and of the gross pattern of photosynthetic assimilation under different conditions suggests that the changes in photosynthetic capacity are largely controlled by the enzymic component of the photosynthetic machinery. By carefully controlling the conditions of dark incubation, the photosynthetic capacity can be reduced to a very low level without significantly affecting chlorophyll a concentration. Since the effect on photosynthetic capacity is reversible, it is possible to study aspects of chloroplast development without the complication of an associated synthesis of chlorophyll.