| Marine diatoms play a predominant role in the global carbon cycle but their growth is often limited by iron availability, especially in some oceanic regions. Diatoms from oceanic waters have lower iron requirements than coastal species, but the biochemical basis for this difference is unknown. The photosynthetic apparatus is a probable source of interspecific variability in iron requirements because it is both iron-rich and highly plastic, but it has not been examined in any oceanic diatom species.; I examined the phenotypic and genotypic variability in diatom iron requirements and measured for the first time the cellular concentrations of photosystems in a coastal and an oceanic diatom (Thalassiosira weissflogii and T. oceanica, respectively) acclimated to a range of irradiances and iron concentrations. Growth and photosynthetic rates, elemental composition, and photosynthetic pigments were measured. Photosynthetic electron transport chain complexes were quantified to construct a photosynthetic iron budget.; In iron-limited diatoms, nearly all the cellular iron was required for photosynthetic electron transport. Consequently, cellular and photosynthetic iron requirements co-varied with growth irradiance. Growth and photosynthetic measurements established that the increased iron requirement of low light acclimated cells did not appreciably heighten their susceptibility to iron limitation, or necessarily result in iron-light co-limitation, because the reduction in growth rate (Fe demand) at low light was greater than the increase in Fe requirements. The diatoms acquired comparatively more iron at low light by uncoupling rates of steady state iron uptake from growth and, in the coastal diatom, by reducing cell volumes. Instead, diatoms were more iron-stressed growing near their maximum capacity under high irradiances.; The photosynthetic iron requirements of the oceanic diatom were substantially diminished: the cytochrome b6f complex and photosystem I, the most iron-rich complexes, were present at extraordinarily low concentrations compared to those found in coastal diatoms. The concentrations of these complexes were comparably low in Fe-replete cells, demonstrating that their abundance was not a consequence of iron deficiency. These results provide the first biochemical explanation for the low cellular iron requirements of an oceanic phytoplankton species. |
