Permeability changes in model and phytoplankton membranes in the presence of aquatic humic substances

Aquatic humic and fulvic acids can increase the permeability of biological membranes to lipophilic solutes. In in vivo experiments, passive diffusion of fluorescein diacetate (FDA) into the green alga Selenastrum capricornutu m increased in the presence of Suwannee River humic and fulvic acids at pH 5 (humic > fulvic) but not at pH 7. The observation of enhanced diffusion a t the lower pH is consistent with adsorption measurements, which showed tha t the association of humic and fulvic acids with the algal surface was grea ter at pH 5 than at pH 7. Permeability experiments were also performed on m odel membranes to investigate the interaction of these humic substances wit h membrane lipids. In these in vitro experiments, we followed leakage of th e fluorescent probe sulforhodamine-B (SRB) that had been encapsulated withi n 1w-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) vesicles; t his model phospholipid is representative of those found in the plasmalemma of green algae. Release of SRB from the vesicles was markedly accelerated i n the presence of Suwannee River humic and fulvic acids (humic > fulvic); f or the humic acid, lowering the pH from 7.6 to 5.7 enhanced this surfactant -like effect. The demonstration that humic substances can alter the permeab ility of phytoplankton and model membranes at natural concentrations and pH values has potential implications for the uptake and regulation of toxic a nd essential solutes by the phytoplankton community.