Bioavailability and trophic transfer of humic bound metals from bacteria to zooplankton

Humic acids (HA) are known to affect the bioavailability of adsorbed chemicals, both positive and negative effects have been reported. Research investigating the effects of HA on representatives of the lower trophic levels has been limited, especially in the estuarine/marine environment. This research investigated the effects of salinity on heavy metal binding to HA. Further studies then concentrated on defining the effect of HA on bioavailability and transfer of Cu among several trophic levels: a bacterium, Vibrio natriegens; ciliates (Pleuronema sp. and Uronema sp.); and the copepod Acartia tonsa. The complexation of Cd, Cr, Zn, and Cu with Suwannee River Humic Acid (SRHA) was estimated by reduced passage of the metals through 1000 dalton dialysis membranes in the presence of 10 mg {dollar}rm SRHA{lcub}cdot{rcub}Lsp{lcub}-1{rcub}{dollar} at salinities of 0, 1, 3, 10 and {dollar}15perthous .{dollar} The complexation of all metals except Cu decreased significantly at all salinities above zero. Complexation of Cu decreased initially, but then decreased at salinities above {dollar}3perthous .{dollar} Cu binding by dissolved organic matter from Perdido Bay was also measured for comparison and found to be generally higher. When Vibrio natriegens was exposed to Cu with and without SRHA at {dollar}rm 10 mg{lcub}cdot{rcub} Lsp{lcub}-1{rcub}{dollar} the total Cu associated with bacterial cells decreased by 60 and 41% in two different experiments. When Pleuronema was added to these pre-exposed bacteria and allowed to feed, the presence of SRHA caused an insignificant increase in ciliate Cu uptake calculated on a per ciliate basis. In contrast, SRHA significantly reduced Cu uptake in two similar tests with Uronema by 54 and 39%. SRHA also significantly reduced Cu uptake by Uronema that were not fed during exposure, but only by 25%. Uptake of Cu was significantly reduced by SRHA in Acartia tonsa that were feeding on Uronema (61 and 48% in two different experiments) and improved survival and reproduction when compared to Cu exposure without SRHA. In Acartia tonsa that were not fed, SRHA reduced Cu uptake approximately 40-50%. In conclusion, SRHA reduced Cu uptake by lower trophic-level organisms, but some humic-bound Cu was available through feeding processes.