SEASONAL VARIATION IN SUN-SHADE ECOTYPES OF PLECTONEMA NOTATUM (CYANOPHYTA)1

Clonal cultures of the thermophilic alga Plectonema notatum Schmidle were grown from cells collected in winter from the surface and subsurface zones of the cell mat in Jerry Johnson Hot Spring, Idaho. Both surface and subsurface clones exhibited light saturation curves and concentrations of photosynthetic pigments typical of ecotypes highly adapted to shade environments. These winter clones were compared to surface and subsurface clones collected during summer months. This comparison illustrates the seasonal rotation of ecotypes. The shade adapted winter surface and subsurface types dominated the mat during the low light intensity, short photoperiod winter season. With increasing light intensities and photoperiods the winter ecotypes were replaced by the high light intensity adapted summer ecotypes.