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

Abstract

Resting spore formation was investigated in the neritic and oceanic strains of Thalassiosira nordenskioeldii under iron- and nitrate-depleted conditions at 5°C and 10°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°C and at 10°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°C to 1% in the oceanic strain at 10°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°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°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.