Nitrate Regulation of Fe Reduction and Transport by Fe-Limited Thalassiosira oceanica

Under Fe-limiting conditions, nitrate (NO3 -)-grown marine diatoms have higher intracellular Fe requirements, but divide as fast or faster than ammonium (NH4 +)-grown cells by maintaining faster steady-state Fe uptake rates. Here we report that Thalassiosira oceanica, clone 1003, possesses an Fe reductase that reduces Fe(III) bound to a variety of organic ligands, including the siderophore desferrioxamine B (DFB), a high affinity, Fe(III)-specific ligand. Reduction is mediated extracellularly and is induced by Fe deficiency. Cellular rates of Fe(III) reduction are significantly faster in NO3 --than in NH4 +-grown cultures suggesting a link with N metabolism. At subsaturating Fe concentrations, short- and long-term Fe uptake rates are also significantly faster in NO3 --than in NH4 +-grown cells. The results suggest that when Fe is limiting, faster rates of reduction of organically bound Fe(III) by phytoplankton promote faster rates of Fe transport and growth. The implications of these findings could be significant for understanding phytoplankton Fe nutrition in oceanic waters where organic complexation dominates the speciation of Fe. We hypothesize that the reductive Fe transport pathway may enable phytoplankton to directly utilize Fe bound to strong organic ligands in the sea.