Prolonged darkness and diatom mortality .2. Marine temperate species

The effect of prolonged darkness was tested on the diatom species Ditylum brightwellii (West) Grunow, Thalassiosira punctigera (Castracane) Hasle and Rhizosolenia setigera Brightwell, isolated from the North Sea. All species survived dark incubation in the vegetative state. D. brightwellii and T. punctigera survived 35 days in the dark, whereas R. setigera survived 21 days. Mass mortality occurred, at latest, after 49 days of darkness. Chlorophyll-a decreased slightly with time in the dark. Particulate organic carbon and particulate organic nitrogen decreased negligibly indicating that maintenance respiration is low. With increasing time in the dark, photosynthetic carbon assimilation, after re-exposure to light for 6 h, was strongly affected in all three species. After 35 days of dark exposure, photosynthetic carbon assimilation was almost zero and did not recover during the subsequent growth experiments. Growth rates of D. brightwellii, achieved in former light conditions, decreased slightly with increasing time in the dark, growth rates for T. punctigera remained constant, while R. setigera increased its growth rate slightly. In all three species rapid growth recovery was reduced by lag phases occurred before growth was resumed.