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

The interaction between iron and light limitation was investigated in 3 mar ine diatom species: 2 Antarctic diatom isolates, Chaetoceros brevis and C. dichaeta, and an isolate from temperate waters, C. calcitrans. In C. calcit rans and C. brevis, grown in the laboratory using trace metal buffered medi um, both iron and light limitation affected growth rates, cellular chloroph yll 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- a nd light-limited cells, The large diatom C. dichaeta, grown in natural Sout hern Ocean water (without ethelenediaminetetraace tic acid [EDTA]) showed q ualitatively similar responses to co-limitation by iron and light. C. dicha eta only grew under long-day light conditions, This response was further mo dified by the availability of iron, Addition of iron resulted in higher gro wth rates. In contrast, ambient iron concentrations did not limit the growt h rate of the small Antarctic diatom C. brevis, However, iron limitation co uld be induced by addition of the natural iron binding ligand desferrioxami ne 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 (with in 24 h) in photochemical quantum efficiency (F-v/F-m) and decreases in eff ective absorption of the cros-section of photosystem II (sigma (PSII)), the turnover time of the photosynthetic unit (tau) and the electron transfer r ate (1/ tau), followed by an increase in growth rates after 48 h. The inter actions between iron and light in Antarctic diatoms are sufficient to expla in 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 s pecies (i.e., the large C. dichaeta) can bloom only under conditions of rel atively high iron concentrations and favorable high light intensities or a long-day light period. These differences in physiological responses will ha ve consequences for primary production, the carbon cycle and biogeochemical cycles.