The greatest value of humic and fulvic acids are their roles as chelators. Humic and fulvic acids have functional groups that act as claws, holding mineral ions strongly enough to keep them from reacting with each other and becoming unavailable to the plant, but weakly enough so that they can be released to the plant cells on demand. Humic acid transports the minerals to the outside of the cell membranes, and releases the minerals for uptake by the plant. The fulvic acid fraction consists of small organic molecules that easily penetrate cell membranes. Fulvic acid transports minerals through the cell membranes and releases them directly to the plant cells.
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INFLUENCE OF HUMIC SUBSTANCES ON THE GROWTH
OF MARINE PHYTOPLANKTON: DINOFLAGELLATES
A. Prakash and M. A. Rashid
Fisheries Research Board of Canada and Atlantic Oceanographic Laboratory,
Bedford Institute, Dartmouth, Nova Scotia
Humic substances, in small amounts, exert a stimulatory effect on marine dinoflagellates
that is reflected in increased yield, growth rate, and ‘“C 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 prod&ion.