Resting spore formation in the marine diatom Thalassiosira nordenskioeldii under iron- and nitrogen-limited conditions

Resting spore formation was investigated in the neritic and oceanic strains of Thalassiosira nordenskioeldii under iron- and nitrate-depleted conditions at 5 degrees C and 10 degrees C. Both strains immediately formed resting spores under nitrate-depleted conditions with almost 100% composition after 4-8 and 3-6 d cultivation periods at 5 degrees C and at 10 degrees C, respectively. However, resting spore formation in both strains under iron-depleted conditions increased with incubation time more gradually, and after 15 d of cultivation, spore composition ranged from 60% in the neritic strain at 5 degrees C to 1% in the oceanic strain at 10 degrees C. In addition, chlorotic cells with smaller cell volume compared with vegetative cells were observed under iron-depleted conditions. Sinking rates of vegetative cells, iron-limited cells and spores and nitrate-limited resting spores cultivated at 5 degrees C were 1.24 +/- 0.14, 3.41 +/- 0.43 and 9.22 +/- 1.04 m d(-1), respectively, slightly faster than those at 10 degrees C. The faster sinking rates in iron-limited resting cells and resting spores than in vegetative cells may prevent their habitat from expanding to high-nitrate low-chlorophyll oceanic regions with low iron concentrations.