Modern marine stromatolites are a highly organized, yet compact ecosystem that is uniquely appropriate for studies of coupled biosphere-geosphere interactions. Results from recently funded NSF research indicate that these living examples of Earth's oldest macrofossi s form by repeated transitions among three distinct microbial communities, whose predominance appears to be driven by interactions between microbial population dynamics, biogeochemical cycling, and mineral deposition. The compact nature of this ecosystem allows components to be sampled, experimentally manipulated, and modeled, such that the sensitivity of the system to internal and external perturbations can be evaluated and biogeochemical cycling and minera formation can be quantified. It is thus an ideal model system for investigating functional linkages between microbes, minerals, and the environment and for addressing fundamental questions such as "what is the role of biodiversity in biogeochemical processes and what are the effects of geochemical processes on species richness ?how do microorganisms modify and adapt to their abiotic environment? what constrains a seemingly 'equilibrium 'state and what are the forces that produce ecosystem transitions?"
The proposed research will use diverse but highly complementary methodologies to investigate the complexity of the stromatolite ecosystem over spatial scales ranging from single cells (m) to kilometers and temporal scales ranging from minutes to seasonal and multi-annual. The program adopts a powerful approach to the investigation of linkages between 'populations processes and products' through integration of field studies of natural systems, experimental studies, and quantitative modeling. Our studies of microbial populations will focus on composition, abundance, and distribution of species in four main functional groups (cyanobacteria, aerobic heterotrophs, sulfate reducers, and sulfide oxidizers) using a variety of techniques including 16S rDNA, Terminal Restriction Fragment Length Polymorphism analysis, and fluorescent in situ hybridization coupled to confocal microscopy. A combination of field measurements and flume experiments will determine the response of populations, metabolic rates, and chemical gradients to variations in hydrodynamic conditions, sedimentation and light; chemical gradients will be measured in situ using microelectrodes. As exopolymeric secretions (EPS) are likely to regulate mass transport and so the exchange within the stromatolites, EPS structure and composition will be measured using a variety of techniques, including Fourier-Transform Infrared and Raman spectroscopies, atomic force microscopy, and GC-MS; solute flux will be measured using benthic chambers. Effects of EPS properties and elemental cycling on mineral deposition will be determined through construction of C, O and S budgets, complemented by studies of Ca 2+ binding, microscopy analyses to define micrometer-scale relationships between activities and microstructure, and culture studies of precipitation by species. Flume and field studies will determine critical erosion thresholds and erosion rates of emergent macrostructures. Finally, we will look for 'fingerprints'of microbial and environmental processes preserved in the rock record through analyses of stable isotopes, morphometric parameters, and intracrystalline organic matter. Ultimately, a full understanding of the stromatolite system will rely on development of quantitative models.
Three separate modeling approaches, which relate to different aspects of the stromatolite ecosystem, will be developed -(1)a diagenetic model, which will quantify elemental cycles;(2) a simulation model, which will quantify the interactions between elemental cycles, populations of microbes and community status in the stromatolite and (3) a stage-based matrix model, which will attempt to relate environmental components to the distribution of, and transitions among, the microbial assemblages at the level of the ecosystem. These models will serve as critical tools for understanding the reciprocal interactions between elemental cycles, microbial populations, and sedimentation in past,present, and future environments on earth and beyond.
Educational activities that foster the integration of education and multidisciplinary research will include (1) active participation of undergraduates, graduate students, and postdoctoral associates in field and lab studies, and an international student exchange program (2) expansion and maintenance of the RIBS public web site; and (3)a field guide to Bahamian stromatolites for the general public. Participation by four European scientists will establish global networks for continuing collaborative studies in the emerging field of geomicrobiology.
