INFLUENCE OF N SUBSTRATE ON FE REQUIREMENTS OF MARINE CENTRIC DIATOMS

The interaction between Fe requirements and N metabolism in centric di atoms was investigated to determine whether use of nitrate (NO3-) impa rts a higher cellular Fe demand for growth than use of ammonium (NH4+) , and thus reduces fitness under Fe deficiency. Six species of the gen us Thalassiosira from a variety of habitats were examined. Coastal and central gyre representatives grew faster in Fe-sufficient media conta ining NH4+, but isolates from the equatorial Pacific, an oceanic high- nutrient, low-biomass region, achieved maximum rates with NO3-. Iron q uotas ranged from 26 to 102 mu mol Fe mol(-1) C and were not affected in a predictable manner by N source or habitat. Relative growth rates were diminished in Fe-deficient, media, particularly in coastal specie s which grew at less than 25% of their maximum rates (mu(max)) All oce anic species maintained fast rates of growth (0.8 mu(max)) under the s ame Fe-Limiting conditions, despite having 4 times less intracellular Fe than the coastal species. Fe:C ratios of Fe-deficient Thalassiosira spp. ranged from 0.7 to 14 mu mol mol(-1) and were significantly grea ter (by similar to 1.8 times) in all species when NO3- was the N sourc e (p < 0.05). Steady-state Fe uptake rates were also faster in NO3- de pendent cells at low Fe. Nitrogen source had different effects on Fe-l imited growth rates. Surprisingly, T. oceanica (clone 1003) and T. wei ssflogii grew faster with NO3- even though higher Fe requirements for use of oxidized N were expected to reduce division rates relative to N H4+-grown cells. When total Fe concentrations in the medium were decre ased to 1 nM, growth rates of T. oceanica (clone 1003) decreased to 0. 2 mu(max) and were significantly faster (25 %) in NH4+ than in NO3--am ended media. Under these more stressful Fe-limiting conditions, Fe quo tas were the same in cells cultured in both N-based media. Our results thus demonstrate that phototrophic phytoplankton require significantl y more cellular Fe to grow on NO3- than NH4+. Nitrate-grown cells are able to obtain this extra Fe, even when Fe is Limiting, suggesting tha t Fe acquisition is somehow linked to NO3- metabolism. Under severe Fe deficiency however, NO3- utilization reduces division rates compared to NH4+, because cells are unable to fulfill their extra Fe requiremen ts.