Oceanic Phytoplankton Need Less Iron

Abstract

Growth studies were undertaken with two diatoms, the oceanic diatom Thalassiosira oceanica and the estuarine species Thalassiosira pseudonana, using radiolabelled ⁵⁵Fe in Gulf Stream seawater having an EDTA-trace metal buffer. Intracellular Fe in exponentially growing cells was determined following treatment with titanium(III) EDTA-citrate reducing solution to dissolve adsorbed and noncellular iron. The two species showed different growth responses. T. oceanica grows at its maximum rate over a wide range of Fe concentration. Growth rate is reduced slightly (less than 20%) at the lowest Fe levels. Such low levels result in the growth rate of T. pseudonana dropping to almost zero. T. pseudonana shows a graded response to increased Fe, and a maximum growth rate that is 50% greater than that of T. oceanica. Fast growth by T. oceanica at lowest Fe levels is explained almost entirely by a much lower cell Fe requirement, or quota, rather than by more effective transport at low Fe conditions. T. oceanica cells at near-maximum specific growth rate of 1 day⁻¹ have a cellular iron:carbon ratio of about 2 × 10⁻⁶. T. pseudonana at the same growth rate requires several times this amount, and even more at its highest growth rate. Cellular Fe required for rapid growth of the oceanic diatom is substantially lower than values for coastal or estuarine species in this and other studies. Computations concerning the phytoplankton carbon that can result from use of a particular amount of Fe give different results; therefore, the Fe: Cratio for a genetically adapted low-iron oceanic phytoplankton species is used, rather than values for coastal species as were applied in the past.