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Prorocentrurn mariae-lebouriae, a dinoflagellate which commonly forms red tides in Chesapeake Bay (USA), is shown to exhibit diel periodicity of photosynthesis and pronounced photoadaptive responses to changes in its photic environment. These 2 physiological characteristics are both manifested as shifts in the photosynthesis-irradiance (P-I) relationship. A new culture methodology is used to separate changes in the P-I curve parameters (a and P,) that are associated with rhythmicity of photosynthesis from those accompanying light adaptation. This culture approach provides populations of cells that are individually phased to distinct light-dark schedules, allowing periodicity and light adaptation to be studied concurrently. P. mariae-lebourjae shows diel periodicity of photosynthesis comparable to the time-dependent patterns detected for many other phytoplankton species. The amplitude of diel oscillations in P-I curves ranged from 1.4 to 2.5, depending on the photic regime. Maxima in a and P, occurred in mid-morning on a high light cycle, but shifted to midday following a reduction in light availability. This species also adapts rapidly to changes in irradiance. Chlorophyll (chl) a increased from 2.6 to 5.1 pg cell-', chl cfrom 1.8 to 3.0 pg cell-', and peridinin from 2.2 to 3.1 pg cell-' within 72 h of a shift to low light. A n accompanying increase in the ability to harvest light, exhibited as an increase in a cell-', enabled the cells to fix carbon at the same rate in low light as occurred in high light. During this 72 h interval, however, cells did not maintain a maximal division rate as p, +O. A combination of strategies to cope with perturbations in the photic regime involving changes in photosynthesis, pigmentation, and cellular carbon is documented and discussed in the context of temporal specificity of individual photoadaptive responses.

Photoadaptation and diel periodicity of photosynthesis in the dinoflagellate Prorocentrum mariaelebouriae