Iron acquisition by the green alga Selenastrum minutum: Growth in iron-limited chemostats and effects of chelator stability constant

Abstract

The unicellular green alga Selenastrum minutum was successfully cultured in an iron-limited chemostat system, under a wide range of iron-limited growth rates. Steady-state biomass was a function of the concentration of the limiting nutrient (Fe 3+ ) in the medium. Respiratory and photosynthetic activities both increased with increasing iron-limited growth rate of S, minutum, as did the amount of chlorophyll per cell. Contrary to a Strategy I green alga, Chlamydomonas reinhardtii, which displays greatly enhanced plasma membrane ferric chelate reductase activity as a result of iron limitation, cells of S. minutum displayed low activity of this enzyme under both iron-limited and iron-sufficient culture conditions. As well, the addition of BPDS, a strong chelator of Fe 2+ , to iron-limited S. minutum chemostat cultures had no discernable effect on culture biomass. These results strongly suggest that S. minutum does not use a Strategy I mechanism of iron acquisition. In contrast to C. reinhardtii, there was an inverse relationship between the stability constant of the ferric chelator (HEDTA, EDDHA, HBED) and steady state S. minutum culture biomass. However, the calculated free Fe 3+ levels provided a poor estimate of iron-limited S. minutum biomass, suggesting that S. minutum has the ability to directly access chelated Fe 3+ . There was no evidence for production of chelators or siderophores as a response to iron limitatio, possibly suggesting that Strategy II iron acquisition is also not involved. The experiments described in this paper represent the first description of a successful iron-limited chemostat culture system of a non-Strategy I organism, and clearly indicate that the iron acquisition mechanism of S. minutum differs from the wellcharacterized Strategy I system of C. reinhardtii.