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Reference humic substances (fulvic and humic acid) were used to demonstrate the adsorption of natural dissolved organic matter (DOM) onto the surfaces of phytoplankton (Chlorella pyrenoidosa, Synechococcus leopoliensis) and isolated fish gill cells (Salmo salar) at environmentally relevant pH values (4-7) and DOM concentrations (0-20 mg L - 1 ) and various ionic strengths (0.02-0.1 M). In the presence of DOM the negative surface charge of cell surfaces increased, as monitored using the electrophoretic mobility of living cells. This effect was much greater at pH 4 than at circumneutral pH, suggesting that the interaction of DOM with cell surfaces involves either a hydrogen-bonding sorption mechanism, between electronegative functional groups present in the DOM and on the cell surface, or the formation of hydrophobic bonds between the cell surface and the hydrophobic domain of the DOM. Sorption of DOM by cell surfaces has been confirmed by direct measurements of DOM loss from solution on contact with phytoplankton cell suspensions and by the observation of the association of humic substances with phytoplankton cell surfaces using transmission electron microscopy. These results demonstrate that DOM interacts with living surfaces and may thereby influence chemical and physical processes at the cell-solution interface.