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

We evaluated the influence of UV radiation exposure time on abiotic transfo rmations of leachates from 2 aquatic macrophytes: Phragmites australis and Hydrocaris morsus-ranae. In addition, we studied the influence of phototran sformations of DOM on bacterial growth. Sterile filtered leachates were exp osed to UV radiation for 5, 10, 24, 48, 72 and 120 h or kept in the dark. D issolved 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 an d incubated in the dark. Based on the observed DIC and hydrogen per oxide f ormation rates, we concluded that phototransformations are initially more r apid in H. morsus-ranae DOM than in P. australis DOM. On the other hand, pr oduction of DIC after more than 48 h of UV exposure was strongly reduced fo r 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 g rowth response to DOM phototransformations. The lowest bacterial growth eff iciencies (BGE) were found in the irradiated leachate of P, australis expos ed for more than 48 h and in the irradiated leachate of H, morsus-ranae exp osed for 10 h, which coincided with the highest concentrations of hydrogen peroxide for both types of leachates.