• adsorption;
  • algae;
  • carbon assimilation;
  • humic substances;
  • pH;
  • surface

Adsorption of Suwannee River fulvic acid (SRFA) to algal surfaces of three green algae was studied at environmentally relevant pH values (4 –7) and SRFA concentrations (5–100 mg·L1). The influence of adsorbed SRFA on carbon uptake of Scenedesmus subspicatus Chodat was also examined. Although no adsorption was observed at neutral pH values (pH 6 and 7), at pH 4 up to 31 mg SRFA·m2 and at pH 5 up to 4 mg SRFA·m2 was adsorbed to the algal surfaces. Electrophoretic mobility measurements of S. subspicatus demonstrated an increase in the negative surface charge of the alga in the presence of SRFA at pH 4. The adsorbed SRFA also influenced 14C uptake in S. subspicatus; in this case, enhanced carbon uptake could be related to the amount of adsorbed SRFA. The binding of humic substances by algal surfaces was interpreted as the result of hydrogen bonding and hydrophobic interactions.