The effect of humic acid on uptake/adsorption of copper by a marine bacterium and two marine ciliates

The effect of humic acid (HA) on Cu uptake by a bacterium and two bacterivorus ciliates was investigated. The presence of HA resulted in a statistically significant (p<0.001) decrease in Cu associated with bacteria that were exposed to 67 μg Cu L−1. Complexation of Cu appears to lower the availability of Cu with respect to bacterial cell surface binding and uptake. For ciliates, 10 mg HA L−1 significantly reduced uptake of Cu by Uronema, but did not reduce uptake of Cu by Pleuronema. Uronema exposed to 67 μg Cu L−1 accumulated 54% less Cu when 10 mg HA L−1 was present (0.50 pg ciliate−1 vs 0.23 pg ciliate−1). Uronema feeding on V. natriegens, took up less than half as much Cu as unfed Uronema when exposed to Cu without HA (0.41 pg Cu fed ciliate−1 vs 0.86 pg Cu unfed ciliate−1, but only 40% less when exposed to Cu and HA (0.31 pg Cu fed ciliate−1 vs 0.51 pg Cu unfed ciliate−1). The lower % reduction attributable to fed ciliates in the presence of HA suggests that some of the Cu associated with HA is available through trophic processes.