ACCUMULATION OF NATURAL ORGANIC-MATTER ON THE SURFACES OF LIVING CELLS - IMPLICATIONS FOR THE INTERACTION OF TOXIC SOLUTES WITH AQUATIC BIOTA

Reference humic substances (fulvic and humic acid) were used to demons trate the adsorption of natural dissolved organic matter (DOM) onto th e surfaces of phytoplankton (Chlorella pyrenoidosa, Synechococcus leop oliensis) and isolated fish gill cells (Salmo salar-) at environmental ly 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 nega tive surface charge of cell surfaces increased, as monitored using the electrophoretic mobility of living cells. This effect was much greate r at pH 4 than at circumneutral pH, suggesting that the interaction of DOM with cell surfaces involves either a hydrogen-bonding sorption me chanism, 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 D OM by cell surfaces has been confirmed by direct measurements of DOM l oss from solution on contact with phytoplankton cell suspensions and b y the observation of the association of humic substances with phytopla nkton cell surfaces using transmission electron microscopy. These resu lts demonstrate that DOM interacts with living surfaces and may thereb y influence chemical and physical processes at the cell-solution inter face.