Effects of nitrogen source on the physiology and metal nutrition of Emiliania huxleyi grown under different iron and light conditions

Emiliania huxleyi, a small oceanic coccolithophore, was isolated from the NE subarctic Pacific and maintained under oceanic conditions. Coccolith-forming cultures were grown with either NO3- or NH4+ as the primary nitrogen source. Fe-stress was induced experimentally, and physiological parameters including metal quotas (Fe, Mn, Zn, Cu) were measured for both NO3-- and NH4+-grown cells to determine whether it was advantageous for the cells to grow on NH4+ rather than NO3- under Fe-stressed conditions. The parameters used to observe the cell's physiological status were specific growth rate (mu) cell volume (CV), carbon (C), chlorophyll a (chl a), and nitrogen (N) per cell volume. Under Fe-replete conditions (100 nM Fe), no physiological parameters were significantly different (p < 0.05) between NO3-- and NH4+-grown cells. However, under Fe-stressed conditions, CV (NH4+ > NO3-), chi a CV-1 (NH4+ < NO3-), Mn CV-1 (NH4+ < NO3), and Mn:C (NH4+ < NO3-) were all significantly different p < 0.05) for NO3-- and NH4+-grown cells. Under Fe-stressed conditions, NO3--grown cells of E. huxleyi maintained the same or greater levels of chl a, N, Fe, Mn, and Cu as NH4+-grown cells, largely due to the drastic decrease in CV of NO3--grown cells under Fe-stress. Although NO3--grown cells substantially decreased their CVs under Fe-stressed conditions (whereas NH4+-grown cells did not), both NO3-- and NH4+-grown cells reduced their CVs equally under limiting-irradiance, Fe-replete conditions. A reduction in CV by the NO3--grown cells is particularly advantageous for cells living in a low Fe environment, since it reduces the cellular requirements for photosynthate, N, and Fe. The fact that these already small cells become smaller, along with their unique ability to maintain chlorophyll synthesis at Fe levels limiting to cell division, may help explain why E. huxleyi is a member of the numerically dominant size class in the NE subarctic Pacific.