Blooms of the Pelagophyte, Aureococcus anophagefferens are responsible for the brown tides that occur in the Peconic estuary on Long Island, NY. To understand the role of iron in controlling these blooms, the growth of A. anophagefferens was examined in artificial seawater supplemented with f/2 nutrients using trace metal clean techniques. Two different conditions of iron availability were utilized; (1) iron was complexed with twice its concentration using EDTA or NTA, and (2) cultures were grown in trace-metal buffered media using 100 µM excess EDTA. Cultures grown on 117 nM added iron (total Fe concentration = 140 nM) gave maximum cell numbers (9 x 10e9 cell / L), maximum fluorescence (~ 850), and growth rate (~1 div per day) similar to that obtained in iron-replete (11 µM) cultures. Thus the iron-quota for A. anophagefferens was lower than 100 nM. Even at 0 µM added iron, the maximum cell yield of Aureococcus was not significantly different from the 11 µM iron-replete controls. The use of trace-metal buffered conditions did not significantly affect the growth of A. anophagefferens in culture. The estimated iron quota (QFe) for these cultures, <5 amol/cell, is in agreement with the possible oceanic origin of these cultures and the observed drop in iron concentration during bloom formation. To understand how A. anophagefferens obtains its needed iron, the enzyme ferric chelate reductase was characterized from this organism. This constitutive enzyme shows a broad pH optimum, is bispecific for NADPH (preferred) and NADH, and has a Vmax (0.57-1.3 µmol/min/mg protein) and Km (370-900 µM) for chelated iron typical of most eukaryotic algae. The observed rates of enzyme activity (average 20 µmol/min/mg protein using NADPH) are sufficient to support the maximum growth rate of the organism. Laboratory cultures showed no evidence for siderophore or chelator formation under low iron conditions. We conclude that A. anophagefferens CCMP1708 has a very low iron requirement and the necessary mechanisms to obtain that iron. It is unlikely that this trace metal would limit bloom formation. This work was supported by New York State Sea Grant and the Suffolk County Department of Health.