Collaborative Research: Microbial Communities at the Cariaco Redox Interface: Coupling of Sulfur, Carbon, and Metal Cycles

Project summary The interface between oxic and anoxic environments plays an important role in cycling of many elements in the ocean, particularly those with several redox states, including carbon, sulfur, nitrogen manganese and iron. The interface lies within a redox transition zone where reduced (sulfide, reduced metals, organic) and oxidized (oxygen, nitrate, metal oxides) chemical species coexist in opposing concentration gradients. Many important reactions in the transition zone are biologically mediated (e.g., sulfide oxidation and MnO2 reduction) and these reactions support microbial populations unable to exist anywhere else. The Cariaco Basin is the world's second largest pelagic anoxic systems and is the only large, truly marine, permanently anoxic basin. It is also the site of the international CARIACO (CArbon Retention In A Colored Ocean) time series program, which is examining long-term variability in ocean productivity and sedimentation. The major long term goal of this project is discovering and characterizing yet undescribed microorganisms in the redox transition zone of the Cariaco Basin at the CARIACO time series site, and laying groundwork for understanding their significance in carbon, sulfur and metal cycling. The following experimental objectives are being pursued: 1. Describing the prokaryotic community with the highest feasible vertical resolution within the redox transition zone using 16S rRNA libraries/T-RFLP or DGGE techniques; 2. Designing oligonucleotide probes targeting specific groups of prokaryotes in each of the three microbial guilds (aerobic, microaerophilic and anaerobic) on the basis of 16S rDNA libraries. Using fluorescent in situ hybridization with these probes (FISH) and samples collected during first two years of the project to test whether prokaryotes found in 16S rDNA libraries/DGGE gels are numerically dominant within their respective guilds; 3. Identifying autotrophic components of the three guilds using a combination of 14C autoradiography with in situ hybridization; 4. Characterizing the three guilds by the diversity of ribulose-1,5-bisphosphate carboxylase/oxygenase and citrate liase genes; 5. Producing concentration profiles of sulfide, thiosulfate, sulfite, elemental sulfur, MnII and MnO2, dissolved and particulate Fe, and FeS to interpret possible biogeochemical properties of these Cariaco guilds; 6. Isolating prokaryotes with novel types of metabolism--chemoautotrophs oxidizing sulfide and thiosulfate using manganese oxide or iron oxide/hydroxide as electron acceptors and creating enrichment cultures of other metabolisms: (a) aerobic sulfide oxidizers, (b) (micro)aerophilic metal oxidizers, (c) anaerobic denitrifying sulfide oxidizers, (d) anaerobic MnO2/S0 reducers (Geobacteraceae and physiologically similar bacteria) and (e) S0 and sulfite/thiosulfate disproportionating bacteria; 7. Identifying enrichment (or pure) cultures by FISH with probes designed from 16S rDNA libraries. 16S rDNA and CO2 fixing genes from these isolates are being compared to the molecular information (16S rDNA, CO2 fixing genes, FISH) from their respective guilds to validate their importance in nature. Physiological properties of isolated bacteria are being studied for their ability to grow on potential electron donors, acceptors, and sources of carbon. This project includes a substantial educational component, an international collaboration with Venezuelan scientists (Simon Bolivar University and Fundacion La Salle de Ciencias Naturales) and close collaboration with the ongoing CARIACO project funded by NSF. At least two graduate students and two undergraduates are being trained during the course of the project. Collaboration with one of the leading Latin American institutions is offering continuing opportunities to recruit students of Venezuelan origin and opportunities for forming formal collaborations.