Induction of the resting phase in the planktonic diatomAulacoseira subarcticain very low light

Aulacoseira subarctica was incubated in darkness under aerobic conditions at 16°C. Carbohydrate reserves were utilized within 7 days, then lipid. Chlorophyll a increased, and photosynthetic capacity was maintained for 4 weeks. From then on, photosynthetic capacity began to decline and this was accompanied by rounding of the chloroplasts — the classic “resting phase”. The reduction of photosynthetic capacity was due to a reduction of photosynthetic efficiency; the saturation characteristic was unaltered. Experiments in a light gradient showed that 1·2 μmol m-1 s-1 continuous light was sufficient to keep the cells in the vegetative stage. Under lower illumination the resting phase was induced.