Stimulation of Growth in Scenedesmus Obliquus (Chlorophyceae) by Humic Acids Under Iron Limited Conditions¹,²

SUMMARY Stimulatory affects of humic acids of molecular weight 30,000 or greater on iron‐starved Scenedesmus obliquus (Turp.) Kütz. in association with bacteria were studied by growth and Fe uptake experiments. Humic acids stimulated growth of Fe‐starved cells by decreasing the lag phase and extending the growth phase. Humic acids stimulated increased algal growth in medium containing EDTA as well as in medium containing no EDTA, indicating humic acids are not stimulating algal growth under Fe limiting conditions by creating a soluble Fe pool. Humic acids decreased Fe availability to Fe‐starved S. obliquus. Iron bound to humic acids is unavailable for uptake by Fe‐starved cells indicating growth stimulation is not due to chelation effects alone. Stimulation of growth is not a membrane phenomenon as humic acids show the same stimulatory effect when in contact with cells or separated by dialysis membrane. Humic acids also stimulate growth in the dark, with and without aeration, indicating use as a heterotrophic substrate. A photoheterotrophic mechanism is indicated by increased algal growth caused by illuminating cultures, containing humic acids but excluding CO 2 .