Photochemical reactivity of aquatic macrophyte leachates: abiotic transformations and bacterial response

We evaluated the influence of UV radiation exposure time on abiotic transformations of leachates from 2 aquatic macrophytes: Phragmites australis and Hydrocaris morsus-ranae. In addition, we studied the influence of phototransformations of DOM on bacterial growth. Sterile filtered leachates were exposed to UV radiation for 5, 10, 24, 48, 72 and 120 h or kept in the dark. Dissolved inorganic carbon (DIC), hydrogen peroxide and low molecular weight carboxylic acids (oxalic, formic, malonic and acetic acids) were measured. After UV or dark treatment, the leachates were inoculated with bacteria and incubated in the dark. Based on the observed DIC and hydrogen per oxide formation rates, we concluded that phototransformations are initially more rapid in H. morsus-ranae DOM than in P. australis DOM. On the other hand, production of DIC after more than 48 h of UV exposure was strongly reduced for H. morsus-ranae DOM, whereas DIC production was only slightly reduced for P. australis DOM. The differences in the photoreactivity between DOM from P. australis and H. morsus-ranae DOM were also reflected in the bacterial growth response to DOM phototransformations. The lowest bacterial growth efficiencies (BGE) were found in the irradiated leachate of P, australis exposed for more than 48 h and in the irradiated leachate of H, morsus-ranae exposed for 10 h, which coincided with the highest concentrations of hydrogen peroxide for both types of leachates.