The motivation for this work is the observation of higher than previously thought consumption of terrestrial dissolved organic carbon (TDOC) in inland and coastal waters. One possible mechanism contributing to enhancing consumption of TDOC from soil and plant litter is the "priming effect," which has been demonstrated for soil but not in aquatic systems. The priming effect in soils consists in an increase in the rate of soil mineralization by the addition of labile organic matter. The proponents want to investigate if the priming effect is a general phenomenon of aquatic systems, which would influence the CO2 emissions of both aquatic and terrestrial ecosystems, and affect their nutrient cycles. Their working hypothesis is that microbial consumption of TDOC will be enhanced by the availability of priming substrates derived from available dissolved organic carbon (ADOC), resulting in detectable effects using chemical assays and microcosm experiments. Thus, this study will examine microbial utilization of TDOC in Mississippi waters collected at Baton Rouge where dissolved labile substrates from ADOC mix with TDOC.
Determining whether the priming effect affects the process of carbon cycling in aquatic systems and thereby atmospheric C concentrations and the delivery of C to the oceans is of outmost importance for the global carbon budget and making predictions. The PIs want to insert the findings into the US Carbon Cycle Plan. Undergraduate students will be involved in the research.
The goal of this project was to develop a test of the idea that low molecular weight dissolved organic substrates can promote degradation of complex, high molecular weight substrates by enhancing production of key enzymes that breakdown polymers. The concept is known as "priming." While this concept has been demonstrated in soils, there is less supporting evidence for aquatic systems, even though these are also characterized by high molecular weight substrates. To test the potential for priming, a set of 6 common low molecular weight substrates were added to surface water samples from the Mississippi and Atchafalaya Rivers. The responses of three hydrolytic enzymes were then observed. The results showed that several of the low molecular weight substrates consistently increased activity of at least two of the three enzymes, in particular the two involved in polysaccharide degradation. These results provide clear support for the most fundamental requirement of the priming concept, which is that production of hydrolytic enzymes should be enhanced by substrates that might not be direct products of enhance hydrolytic activity. For example, in this study, cellobiose, a product of cellulose degradation, enhanced the activity of an enzyme involved in degradation of an entirely different polymer, chitin (found in fungal cell walls, and crustacean and insect exoskeletons). In addition, an effort was undertaken to isolate a freshwater bacterium for use in model priming studies. This was done using a polymeric substrate, thus ensuring enrichment of an isolate that produces extracellular hydrolytic enzymes. The isolate obtained, a close relative of Acinetobacter bouvetii, was provided to a collaborator (Dr. T. Bianchi and his collaborators) for a recently completed pilot study. This study has provided new information that is important for understanding controls of riverine carbon, which is a globally significant reservoir of organic matter that is sensitive to climate change. In addition, this study provide support and training for a research assistant in the PI's laboratory.
