Effect of single-species and mixed-species leaf leachate on bacterial communities in biofilms

Dissolved organic matter in the form of leaf leachate represents an important carbon and energy source in many lotic ecosystems. In this study, we investigated utilization of mono-specific and mixed-species leaf leachate and impacts on biofilm bacterial community structure. Ceramic tiles were incubated in a Northeast Ohio stream to allow for biofilm development and then exposed in the laboratory to glucose or leachate from: sugar maple (Acer saccharum), pin oak (Quercus palustris), maple + oak, American beech (Fagus grandifolia), witch hazel (Hamamelis virginiana), or beech + witch hazel. Bacterial responses to these amendments were compared to un-amended controls based on fluorescent in situ hybridization (FISH) targeting selected taxa and terminal restriction fragment length polymorphism (T-RFLP) of bacterial 16S rRNA genes; also changes in DOC concentrations were quantified. Generally, there were limited differences among communities as a result of leachate amendment, although specific taxa monitored by FISH exhibited differential responses. There was no evidence that mixing of leachate from different leaf species created an effect different than what could be expected based on monospecific experiments. Witch hazel solicited the greatest response, based on T-RFLP data, regardless of whether the community was exposed to witch hazel alone or witch hazel + beech, accounting for 19% of the variability in Jaccard distance (P < 0.05) and 27% of the variability in Hellinger distance among profiles. In conclusion, we found that leaf leachate can be readily degraded but only in some cases did differences in leaf leachate among tree species cause an alteration in community structure. Mixing of leachate from different leaf species did have an impact on DOC loss but did not alter community structure. The occurrence of particular compounds, such as those in witch hazel, may alter community structure suggesting that the presence and abundance of specific plant taxa can impact bacterial communities.