CAUSES OF DAILY RHYTHMS IN PHOTOSYNTHETIC RATES OF PHYTOPLANKTON

¹ Department of Biological Sciences, State University of New York at Albany, Albany, New York 12222

The cause for the daily rhythm in the photosynthetic capacity of phytoplankton has been examined. Alternative hypotheses have been modeled with the Michaelis-Menten equation. In one model rates of photosynthesis oscillate in response to a forcing from the external concentration of limiting nutrient while photosynthetic potential (U_max) of the assemblage remains constant. In the alternative model photosynthetic potential oscillates in response to intrinsic organization of the cell. The alternatives may be deduced from the photosynthetic response to added nutrient.

The photosynthetic response to added phosphate was tested with water from Lake George, New York. Rates of carbon assimilation in the unenriched controls described a daily oscillation with an amplitude of approximately two and a phase maximum in midmorning. The degree of stimulation to added phosphate was also rhythmic. The maximum and minimum corresponded with the daily maximum and minimum, respectively, in unenriched controls. The oscillating intensity of response was interpreted as a changing potential in the intrinsic capacity of the algal assemblage.

The display of intrinsic rhythms by at least one dominant component in the algal assemblage infers the entrainment of activity cycles to the daily cycle. Additional evidence to support the inference is described. Although the algae may be entrained, there is also evidence from the literature that nutrient concentrations undergo daily oscillations. Photosynthetic rhythms could result from both an intrinsic and a nutrient (forcing) oscillation. A changing phase relationship between the two could explain the decline in amplitude of the photosynthetic rhythm with increase in latitude.