Dissolved organic carbon and its impact on microbial communities of rivers

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Dissolved organic carbon is the carbon fraction of dissolved organic matter. Dissolved organic carbon is important as it serves as food for aquatic microbial communities. Dissolved organic carbon produced within aquatic systems (autochthonous DOC) such as photosynthetic exudates are thought to be bioavailable to microbes, whereas sources from outside of aquatic systems (allochthonous DOC) such as catchment derived humic substances and leachates are thought to be more recalcitrant. Aquatic microbial communities are important for the decomposition of organic matter and carbon and nutrient cycling. They may also play a role in transfer of energy to higher trophic levels. Despite this, bacteria may consume these less labile sources of allochthonous DOC when available and may respond differently depending on the source. To understand how differing sources of allochthonous DOC such as humic acid, willow leachate and redgum leachate, and glucose (as a simulated autochthonous carbon source) impact microbial communities, microcosm amendment experiments were run in the Williams River Estuary and in the Manly Dam Reservoir. Sampling occurred on days 0, 1, 2, 5 and 8 of the experiment with bacterial density/mL measured by flow cytometry and community structure determined via 16s rRNA sequencing on days 1, 5 and 8. Bacterial abundance was significantly different between treatments and the control in the Williams River experiment but not in the Manly Dam experiment. Bacterial community composition was significantly different between treatments after 1 and 5 days but was more similar after 8 days at both locations. Ciliate abundance was significantly different between treatments in both experiments with all treatments showing higher cell counts than the control. Bacteria in the redgum and humic acid treatments responded most strongly. The experimental results show evidence for short-term bacterial community structuring in response to DOC sources and trophic transfer of these resources from bacteria to ciliates. To observe if bacteria and ciliates responded to flow events delivering allochthonous carbon, a monthly monitoring study of bacteria and ciliate abundance was undertaken in two inland rivers (Namoi and Macquarie) and two coastal river estuaries (Williams and Manning). Low flows dominated during the inland river monitoring while some moderate flow events occurred on the estuaries. DOC was found to be related to flow at on at least one of two sites on the Namoi River, the Macquarie River and the Williams River. No relationship between DOC and bacteria was observed at any sites, however ciliates were positively related with bacteria on the Macquarie River. Bacteria was positively related to phytoplankton on the Namoi. It is possible that the lack of flow events that occurred during the sampling period on the inland rivers did not allow the opportunity to adequately test responses to allochthonous carbon resources. Further, monthly monitoring may not have provided sufficient temporal resolution to detect rapid changes in resource concentrations and bacterial growth at all sites. The downward control of microbial biomass by zooplankton grazing may have also masked growth responses in bacteria and ciliates. This thesis has found experimental evidence for the utilisation of different DOC sources by bacteria and transfer of this energy to ciliates. In the monitoring part of the study, DOC concentration was increased at some sites suggesting allochthonous input. However, bacteria were not found to respond to these resources. Future studies should monitor for relationships between bacteria, ciliates and DOC with at least weekly sampling intervals. These studies should also investigate the role of both nanoflagellates as intermediaries between bacteria and ciliates and the role of grazing by higher consumers such as zooplankton.
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