Continent-ocean margins are regions of intense biogeochemical cycling and play a major role in global carbon cycle. Tropical mobile mud belts and delta topset beds represent one class of continental margin environments where these various potential properties simultaneously combine to an extreme, producing massive, diagenetic reactors of global significance. Oxidation of organic matter in this type of sediments involves coupling of the carbon and the Fe(Mn) cycles by reactions of sulfur cycle and involves chemolithoautotrophic sulfide oxidizing bacteria. As a consequence, two principal processes are responsible for the net CO2 production by the topic mobile sediments: oxidation of carbon matter, which produces CO2, and chemolithotrophic CO2 fixation, which consumes CO2. The primary goal of the present proposed project is to establish the relative importance of these two processes of the carbon cycle in tropical suboxic deltaic and mobile mud environments, and to determine the role eubacteria play in them. This proposal is designed around the specific hypothesis that heterotrophic organic oxidation and chemolithotrophic carbon dioxide fixation co-occur in mobile muds and contribute to overall CO2 production by these sediments. We propose to examine deposits from a migrating mud wave in coastal French Guiana, downdrift of the Amazon River delta. To test the primary project hypothesis we propose to use a combination of radiotracer, microbiological and molecular biological techniques to: (I) Identify reactions of the carbon cycle and their rates in the field and laboratory experiments. Using radioactive carbon tracers, we will determine the relative importance of heterotrophic oxidation of organic substrates (acetate, algal biomass) and CO2 fixation and/or production. To determine whether CO2 fixation is connected with autotrophic activity the genes for RubisCO I and RubisCO II will be amplified and cataloged. (II) To model possible role of chemolithotrophic and heterotrophic reactions in the carbon cycle and ascertain responses of the system to outside disturbances, we will use a combined approach of metabolic inhibitors in radiotracer experiments and measurements of RubisCOI and RubisCOII gene expression. The number of bacteria potentially able to carry out specific reactions of the cycle will be estimated and compared with estimates of total sedimentary bacterial production (using oligonucleotide probes and measurement of rRNA).
The novelty of the project include determination of the rate of CO2 fixation and organic carbon oxidation in tropical mobile sediments, which allow us to answer the question: Are they significant on a global scale?. The role of chemolithotrophic carbon fixation in suboxic deltaic environments will also be elucidated. The fact that CO2 might be fixed in massive deltaic environments would have tremendous impact on the carbon cycle. Depending on the enzymes participating in CO2 fixation in mobile mud belts (RubisCO I, RubisCO II or CO dehydrogenase), 13C of biomass may be affected. We will address the mechanism by which CO2 is fixed and determine whether RubisCO I or RubisCO II or neither (ATP-dependent citrate lyase or CO dehydrogenase) mediate this fixation. Microorganisms involved in the above described biogeochemical processes will be identified and how the inhibition of biological activity of these microorganisms affects the overall carbon cycle will be ascertained. As a part of broader impacts of the project, one graduate student will be trained. One K12 teacher or an undergraduate student will be involved in our research every summer that the project is funded. Results and progress on project will be publicly disseminated through research meeting, publications and the Department of Biology Web site. Usefulness for the society includes the understanding of how the diagenetic processing is occurring in the dynamic mid wave and allowing to evaluate changes in the processing due to present and anticipated changes in the environment as a result of anthropogenic activity.
