Light-shade adaptation by the oceanic dinoflagellates Pyrocystis noctiluca and P. fusiformis

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Species-specific rates of photosynthetic carbon uptake (P), chlorophyll a content and P versus irradiance (P-I), have been measured for cells of Pyrocystis noctiluca and P. fusiformis isolated from natural populations collected in the euphotic zone within and below the surface mixed layer in the Sargasso Sea. These same measurements and the assay for ribulose bis-phosphate carboxylase (RuBP-Case), have been made for cultures of P. noctiluca in a 12 h L: 12 h D photoperiod at 9 different constant or at changing light intensities. In nature chl a cell-1 was constant throughout the euphotic zone. The photosynthetic capacity (Pmax), of cells captured below the surface mixed layer was lower by a factor of 10 compared with cells collected from the surface mixed layer. The Pmax for P. noctiluca collected and incubated within the surface mixed layer was the same as for cell cultures grown under high light, non nutrient-limiting conditions, suggesting that photosynthesis in the natural system was not nutrient limited. In laboratory cultures under constant low light intensities, chl a cell-1 increased by a factor of 5 while both Pmax and RuBPCase activity decreased by a factor of ca 4 compared with high light intensities. In changing light intensities both Pmax and RuBPCase activities were decreased by factors of 4 during low light intervals while chl a cell-1 approached a constant intermediate value. The change in chl a cell-1 in response to prolonged exposure to constant low light intensities was first order with a rate constant of 0.33 d-1. For all irradiance conditions in culture, the P-I dependence could be described by the simple Michaelis-Menten formula. The ratio of Pmax to KI, (the light intensity where P=Pmax/2) was a constant with a Coefficient of Variation of 12%: The constancy of this ratio, the parallel changes in RuBPCase activity with Pmax and the constant chl a cell-1 in the Sargasso Sea imply that for P. noctiluca and presumably P. fusiformis in nature, a dark enzymatic step rather than changes in photosynthetic pigment concentrations may regulate the photosynthetic capacity in the changing photic environment.