Coastal Seawater Herbicide Degradation

Nearshore coastal seawater was collected at the Australian Institute of Marine Science (19°16’ S, 147° 03’ E), Cape Cleveland, QLD under the permit G12/35236.1 issued by the Great Barrier Reef Marine Park Authority. The seawater was filtered to 0.45 μm to remove all particulates and added to individual 500 ml Erlenmeyer flasks (300 ml final volume). The sample treatments were spiked to a final concentration of ~10 μg l^-1 for each herbicide (Table 2) and the flasks stoppered with autoclaved cotton bungs to allow for aerobic conditions. Herbicide standards (98.5–99.9%) were purchased from Sigma-Aldrich, added to 2 ml of the carrier solvent ethanol (to assist in solubility), and made to 5 mg l^-1 concentration with Milli-Q water. The same volume of ethanol (final less than 0.03% v/v) was added to all flasks, including controls for consistency between treatments. Triplicate flasks were shaken at 25°C and 100 rpm in the dark using an Innova 44, incubator shaker. One series of flasks contained a mixture of the six PSII herbicides (ametryn, atrazine, diuron, hexazinone, simazine, tebuthiuron) and the second series of flasks the same herbicide mixture with the addition of 45 mg l^-1 mercuric chloride (MC) to eliminate microbial activity (Table 2) [28 ]. Sample treatment flasks were weighed before sampling to monitor evaporation losses for concentration adjustments. Flasks were topped up with fresh sterile water (Milli-Q) and any losses were compensated for during calculations. Experiment 1 (pilot) examined the degradation of six PSII herbicides over 60 d (Table 2). The 60 day experiment length was set as the maximum by following the OECD method. The purpose of this experiment was to test whether bacteria contributed to biodegradation of these herbicides. Microbial activity is eliminated in the presence of MC and to inform the second experiment which was to be conducted over a longer period.