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| - Permeability Changes in Model andPhytoplankton Membranes in thePresence of Aquatic HumicSubstances
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| - Aquatic humic and fulvic acids can increase thepermeability of biological membranes to lipophilic solutes.In in vivo experiments, passive diffusion of fluoresceindiacetate (FDA) into the green alga Selenastrum capricornutum increased in the presence of Suwannee Riverhumic and fulvic acids at pH 5 (humic > fulvic) but not atpH 7. The observation of enhanced diffusion at thelower pH is consistent with adsorption measurements,which showed that the association of humic and fulvic acidswith the algal surface was greater at pH 5 than at pH 7.Permeability experiments were also performed on modelmembranes to investigate the interaction of these humicsubstances with membrane lipids. In these in vitroexperiments, we followed leakage of the fluorescentprobe sulforhodamine-B (SRB) that had been encapsulatedwithin 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine(POPC) vesicles; this model phospholipid is representativeof those found in the plasmalemma of green algae. Releaseof SRB from the vesicles was markedly accelerated inthe presence of Suwannee River humic and fulvic acids(humic > fulvic); for the humic acid, lowering the pH from7.6 to 5.7 enhanced this surfactant-like effect. Thedemonstration that humic substances can alter thepermeability of phytoplankton and model membranes atnatural concentrations and pH values has potentialimplications for the uptake and regulation of toxic andessential solutes by the phytoplankton community.
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