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Humic substances, in small amounts, exert a stimulatory effect on marine dinoflagellates that is reflected in increased yield, growth rate, and uptake. Humic acid was found to be more active than fulvic acid; in both cases the growth responses were dependent on concentration. Of the various fractions isolated, the low molecular weight fractions of humic acid produced the greatest growth response in unialgal cultures of dinoflagellates. The positive effect of humic substances on phytoplankton growth is, for the most part, independent of nutrient concentration and cannot be attributed entirely to chelation processes. It appears that growth enhancement in the presence of humic substances is linked with stimulation of algal cell metabolism. Because of their high concentrations in coastal waters, humic substances may thus be regarded as an ecologically significant entity influencing phytoplanktonic production.
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Prakash et al. (1968) studied this question.
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