Aquatic hypoxia mediated by the decomposition of allochthonous leaf litter from the invasive shrub Amur honeysuckle (Lonicera maackii)

Background/Question/Methods Our preliminary studies suggest that allochthonous leaf litter of Amur Honeysuckle (Lonicera maackii) decrease dissolved oxygen (DO) to hypoxic levels. This finding indicates a high likelihood of some oxygen-binding substance on or leached from L. maackii leaves or as the result of microbial leaf decomposition. To determine the agent responsible, teas were made from both Amur Honeysuckle leaves and native riparian tree leaves. Teas were then assessed to determine the characteristics of the leachates in binding oxygen or the role of microbial activity in oxygen consumption. The presence of oxygen-binding plant proteins were discounted early in our study with negative results under a series of protein assays. Both fresh dried leaves (microwave digested) and leaf teas were run through an ICP-OES to determine if any potential oxygen-binding chelating metals were apparent. Standard plate counts of bacterial colonization, gram stain identification, and thioglycolate  media incubation were used to enumerate and characterize teas specific bacteria. Two forms of microbial sterilization (UV radiation to leaves, sterile filtration of teas) were used to assess the impact on oxygen depletion levels. In addition, microbial metabolic activities of leaf decomposition were monitored in teas over time using closed system gas analysis (O₂ depletion and CO₂ production).

Results/Conclusions Elemental analysis using the ICP-OES reveal that several metals (Ca, K, Mg) were leached in greater quantities from L. maackii than native riparian leaves. These metals have the potential to chelate with humics from decomposing leaves to bind oxygen. Bacterial colonies were also found to be 1000x more abundant in L. maackii than native riparian leaf teas. Isolated honeysuckle bacterial colonies were mostly gram negative, with one 3-species super society. All honeysuckle bacteria were found to be aerobic in thioglycolate media. Sterilization through UV radiation of leaves was not completely successful, and spore-producing bacteria (gram-positive) and a yeast survived. Sterile filtration of teas produced the greatest divergence in DO levels (similar to water controls) in comparison to non-filtered teas. Metabolic activities of microbes degrading L. maackii leaves in teas were also significantly greater than decomposition activities in riparian leaf teas or in water controls.