Prolonged nitrate exhaustion and diatom mortality: A comparison of polar and temperate Thalassiosira species

The survival of two diatom species, Thalassiosira antarctica Comber isolated from the Southern Ocean and Thalassiosira rotula Meunier isolated from the North Sea, was investigated during a 21 day nitrate-exhaustion period, both in the light and in darkness. Both species endured 3 weeks of nitrate exhaustion in the light by forming a physiological resting state, characterized by the development of shrunken, chlorotic protoplasts, a decrease in photosynthetic capacity and the accumulation of particulate organic carbon (POC). These resting cells of both species still resumed growth after 21 days of nitrate exhaustion although the viability of the cells decreased with time of nitrate exhaustion. Growth rates during subsequent growth experiments decreased with lengthening pre-incubation time in nitrate exhaustion. The viability of T.antarctica resting cells was lower than that of T.rotula. After 21 days in nitrate exhaustion, T.antarctica grew at 31% of the initial division rate, whereas the growth rate of T.rotula was 61%. When growth was resumed, accumulated POC decreased and particulate organic nitrogen (PON), chlorophyll a and photosynthetic carbon assimilation increased during the subsequent 5 day growth periods, but never reached initial values. Darkness had no effect on cellular POC, PON and chlorophyll a content of both species. In T.rotula, the capacity for photosynthetic carbon assimilation decreased to almost zero during the 21 day dark period, although survival capacity was not affected. After a lag phase of 1-5 days, growth was resumed at initial growth rates, indicating that the bulk of cells of both diatom species were still viable. Thus, in the dark, both Thalassiosira species survived nutrient exhaustion without physiological impairment.