Influence of N substrate on Fe requirements of marine centric diatoms

Abstract

The interaction between Fe requirements and N metabolism in centric diatoms was investigated to determine whether use of nitrate (NO3-) imparts a higher cellular Fe demand for growth than use of ammonium (NH4+), and thus reduces fitness under Fe deficiency. Six species of the genus Thalassiosira from a variety of habitats were examined. Coastal and central gyre representatives grew faster in Fe-sufficient media containing NH4+, but isolates from the equatorial Pacific, an oceanic high-nutrient, low- biomass region, achieved maximum rates with NO3-. Iron quotas ranged from 26 to 102 μmol Fe mol-1 C and were not affected in a predictable manner by N source or habitat. Relative growth rates were diminished in Fe-deficienl media, particularly in coastal species which grew at less than 25% of their maximum rates (μ(max)). All oceanic species maintained fast rates of growth (0.8 μ(max)) under the same 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 μmol mol-1 and were significantly greater (by ~ 1.8 times) in all species when NO3- was the N source (p < 0.05). Steady-state Fe uptake rates were also faster in NO3- dependent ceils at low Fe. Nitrogen source had different effects on Fe-limited growth rates. Surprisingly, T. oceanice (clone 1003) and T. weissflogii grew faster with NO3- even though higher Fe requirements for use of oxidized N were expected to reduce division rates relative to NH4+-grown cells. When total Fe concentrations in the medium were decreased to 1 nM, growth rates of T. oceanica (clone 1003) decreased to 0.2 μ(max) and were significantly faster (25 %) in NH4+ than in NO3- -amended media. Under these more stressful Fe-limiting conditions, Fe quotas were the same in cells cultured in both N- based media. Our results thus demonstrate that phototrophic phytoplankton require significantly 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 that 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 requirements.