Decreased bacterial growth on vascular plant detritus due to photochemical modification

We investigated the effects of UV radiation on abiotic decomposition and dissolution of leaf Litter from the aquatic macrophyte Phragmites australis. Dead leaves were autoclaved and incubated in quartz tubes with autoclaved Milli-Q water, in darkness, under WA or under UVA+UVB radiation, using fluorescent tubes with intensities similar to solar radiation. More DOM was produced in irradiated samples compared to dark ones. After exposure, the water was inoculated with a natural assemblage of bacteria and incubated in the dark. Free bacteria reached higher densities in cultures with detritus pre-treated in darkness than in cultures with irradiated substrates. No significant differences in numbers of attached bacteria were found. We suggest that W-irradiated detritus from P. australis has a negative effect on the growth of free living bacteria, possibly due to the leaching of inhibitory substances from the leaf Litter into the water and/or a decrease in DOM bioavailability. Similar experiments, using natural solar radiation, indicated that visible Light has only a minor effect on leaching of inhibitory substances.