Due to the input of humic substances from freshwater run-off into the marine habitat, the influences of such substances on marine organisms should not be neglected. We here investigate the effect of a humic acid (HA) extract from the North Sea, characterized by spectroscopic techniques and carboxylic and phenolic group content, on the growth of different algae. Two benthic pennate diatoms isolated from the Baltic Sea, Navicula ramosissima (C. Agardh) Cleve, 1895 and Entomoneis paludosa (W. Smith) Reimer, 1975, as well as two tychopelagic centric diatoms isolated from the North Sea, Melosira nummuloides C. Agardh, 1824 and Paralia sulcata (Ehrenberg) Cleve, 1873, were employed. The concentrations of pigments (fucoxanthin, diadinoxanthin, chlorophyll-a and β-carotene) and nutrients were also measured. Adding low concentrations of naturally humic substances (5.5 mg of C l−1) to the algae cultures resulted in enhanced growth rates compared to the control experiments, possibly due to the increase in the bioavailability of trace metals or other nutrients.