Iron assimilation in Chlamydomonas reinhardtii involves ferric reduction and is similar to Strategy I higher plants

Abstract

The mechanism of adaptation to Fe-deficiency stress was investigated in the unicellular green alga, Chlamydomonas reinhardtii. Upon removal of nutritional Fe, the activity of a cell surface Fe(III)-chelate reductase was increased by at least 15-fold within 24 h. This increase was negatively correlated with the Fe concentration in the growth media. Incubation of cells in the presence of the Fe2+-specific chelator, bathophenanthrolinedisulphonic acid, led to an increased Fe3+ reductase activity, even when sufficient Fe was present. Growth of cells in Cu-free media for 48 h led to no statistically significant increase in Fe3+ reductase activity. The Fe(III)-chelate reductase activity in Fe-starved cells was saturable with an apparent K(m)of 31 μM and was inhibited by uncouplers of the transmembrane proton gradient but not by SH-specific reagents. Fe uptake was only observed in Fe-deficient cells. Uptake was specific for Fe in that a 100-fold excess of a number of metal ions in the transport assay did not inhibit uptake activity. However, a 100-fold excess of Cu resulted in a 87% inhibition of Fe uptake. The V(max) for Fe3+ reduction activity was 250-fold greater than for Fe uptake; although the K(m) values for both processes differed by only 10-fold. Thus, the rate limiting step in Fe assimilation was transport and not reduction. These results indicate that Fe assimilation in C. reinhardtii involves a reductive step and thus resembles the mechanism of Fe uptake in Strategy I higher plants.