The influence of iron availability on nitrogen assimilation in the marine diatom Thalassiosira weissflogii

Since the recognition of iron-limited, high nitrate, low chlorophyll (HNLC) regions of the ocean, iron limitation has been hypothesized to limit the assimilation of nitrate by diatoms. To determine the influence of iron availability on nitrogen assimilatory enzymes, in vitro enzyme assays were developed and optimized for urease and nitrite reductase. Both assays were able to account for 100% or more of the incorporated nitrogen when growing on urea or nitrate as the sole nitrogen source. Cultures of Thalassiosira weissflogii (Grunow) Fryxell et Hasle (clone Actin) were grown under non-steady-state Fe-limited and Fe-replete conditions using artificial seawater medium. Iron-limited cultures suffered from decreased efficiency of photosystem II as indicated by variable fluorescence (F_v/F_m). Under Fe-replete conditions, in vitro nitrate reductase (NR) activity was rate limiting to nitrogen assimilation and in vitro nitrite reductase (NiR) activity was 100-fold higher. Under iron limitation, cultures excreted up to 100 fmol NO2 − cell−1 d−1, and NiR activities declined by 50-fold while internal NO₂− pools remained relatively constant. Activities of both NR and NiR remained in excess of nitrogen incorporation rates throughout iron limited growth indicating that the supply of reductant to NiR may be responsible for the limitation of nitrogen assimilation at the nitrite reduction step. Urease activity showed no response to iron limitation. Carbon:nitrogen ratios were equivalent in both iron conditions indicating that relative to carbon, nitrogen was incorporated at similar rates whether iron was limiting growth or not. Steady-state long-term iron limitation of T. weissflogii using the artificial culture medium Aquil produced similar results, although NO₂− efflux rates were much lower (10 fmol NO₂− cell−1 d −1). If inferences from a single species may be applied to a mixed assemblage then, according to this work, diatoms in HNLC regions are not deficient in their ability to assimilate nitrate when Fe-limited. Rather, it appears that diatoms are limited in their ability to process photons within the photosynthetic electron transport chain which results in nitrite reduction becoming the rate limiting step in nitrogen assimilation. This appears to be the case whether limiting iron is supplied either in non-steady or steady states.