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The interaction between iron and light limitation was investigated in 3 marine diatom species: 2 Antarctic diatom isolates, Chaetoceros brevis and C. dichaeta, and an isolate from temperate waters, C. calcitrans. In C. calcitrans and C. brevis, grown in the laboratory using trace metal buffered medium, both iron and light limitation affected growth rates, cellular chlorophyll a fluorescence, spinal index (i.e., the number and size of spines) and cell size. Growth rates and cell size declined at lower iron concentrations and lower light intensities. Cellular chlorophyll a fluorescence increased with lower light, but decreased due to iron limitation. The spinal index, based on the ratio of side scatter to forward scatter, increased in iron- and light-limited cells. The large diatom C. dichaeta, grown in natural Southern Ocean water (without ethelenediaminetetraacetic acid [EDTA]) showed qualitatively similar responses to co-limitation by iron and light. C. dichaeta only grew under long-day light conditions. This response was further modified by the availability of iron. Addition of iron resulted in higher growth rates. In contrast, ambient iron concentrations did not limit the growth rate of the small Antarctic diatom C. brevis. However, iron limitation could be induced by addition of the natural iron binding ligand desferrioxamine B (DFOB). Addition of iron to Fe-depleted cultures of C. dichaeta and C. brevis reversed the effects of DFOB, as evidenced by rapid increases (within 24 h) in photochemical quantum efficiency (Fv/Fm) and decreases in effective absorption of the cross-section of photosystem II (σPSII), the turnover time of the photosynthetic unit (τ) and the electron transfer rate (1/ τ), followed by an increase in growth rates after 48 h. The interactions between iron and light in Antarctic diatoms are sufficient to explain the observation that in the Southern Ocean some species (i.e., the small C. brevis) thrive under low iron and low light conditions, whereas other species (i.e., the large C. dichaeta) can bloom only under conditions of relatively high iron concentrations and favorable high light intensities or a long-day light period. These differences in physiological responses will have consequences for primary production, the carbon cycle and biogeochemical cycles

Co-limitation by iron and light of Chaetoceros brevis, C. dichaeta and C. calcitrans (Bacillariophyceae)