Very interesting paper describing hoe Humic Acid can influence algae growth.  For related product see our Humic and Fluvin Acid product offered by Attogene:

ABSTRACT
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 prodtion.

A. Prakash and M. A. Rashid Fisheries Research Board of Canada and Atlantic Oceanographic Laboratory, Bedford Institute, Dartmouth, Nova Scotia