Ferric and cupric reductase activities in the green alga <i>Chlamydomonas reinhardtii</i> : experiments using iron-limited chemostats

Cells of the green alga Chlamydomonas reinhardtii Dangeard were grown in Fe-limited chemostat culture over a range of growth rates (0.15–1.5 d −1 ). Greater cell densities and culture chlorophyll levels were achieved using an excess of chelator [ethylenediamine di-( o -hydroxyphenylacetic acid)] relative to FeCl 3 (80:1), compared to growth using a 1:1 chelator:FeCl 3 ratio. The C. reinhardtii cells reduced extracellular ferric chelates, and ferric chelate reductase activity increased with increasing Fe-limited growth rates. However Fe-sufficient cells exhibited a low rate of ferric chelate reductase activity, similar to severely Fe-limited cells. Iron-limited cells were capable of reducing a wide variety of ferric chelates, representing a wide range of stability constants, at similar rates, suggesting that the stability constants of ferric complexes are not important determinants of ferric reducing activity. Cupric reductase activity also increased with increasing Fe-limited growth rates, and Cu(II) was preferentially reduced compared to Fe(III). These results suggest that both reductase activities may represent the same plasma-membrane enzyme. The rate of cupric reduction was a function of the free [Cu 2+ ], not the total [Cu(II)], suggesting that free Cu 2+ is the actual substrate for cupric reductase activity.