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Project Progress

Overview. During the four expeditions of 2001-2002 (our second year of funding), we carried out extensive integrated sampling and experimental studies at a number of field study sites in the Cuatro Cienegas Basin, Central Mexico. In addition, we processed samples from previous expeditions and prepared data for presentations at several scientific meetings and for publication. Key results for each project component are briefly described below.

Stoichiometry. We continued to characterize nutrient supply conditions in various habitats in the basin, concentrating our efforts in the Churince drainage and in the Rio Mezquites. Field sampling indicated that PO₄ is in short supply (relative to inorganic N and other potential chemical limiting factors) and indeed algal mats and stromatolite biomass in the basin appear to have extremely high C:P and N:P ratios. Consistent with this potentially poor food quality, a 3-week P-fertilization experiment was successful in lowering the C:P ratio of microbial biomass in Rio Mezquites stromatolites. This lowering of microbial C:P stimulated snail P content and ribonucleic acid (RNA):DNA ratio, indicating that snail growth in this system is limited by stoichiometric food quality (low dietary P-content).

Snail Morphometry (Tang/Rooparnine). We completed intensive quantitative morphometric analysis of Mexipyrgus from various localities in the basin. These results indicate an extraordinary degree of morphometric differentiation in this taxon. Preliminary findings from this work have been submitted to Astrobiology. Additional samples are being processed and we are now preparing DNA for analysis so that we can determine if this morphometric diversity is associated with genetic differentiation.

Pupfish (Dowling). We have completed foundational work for this study by performing basin-level, population genetic analyses of mitochondrial and nuclear DNA in Cyprinodon bifasciatus and C. atrorus (Carson and Dowling 2002, In Preparation), and phylogenetic analysis of Cyprinodon mitochondrial DNA (Echelle et al 2001, Submitted). Currently we are assessing whether physiological differences between C. bifasciatus and C. atrorus have important influences on local adaptation of introgressed Cyprinodon within the environmental gradient between these species.

Cyanobacteria (Garcia-Pichel). We completed (and published) a study related to the buoyancy regulation of calcite-producing colonial cyanobacteria (“waterwarts”) in Posa Escobedo. In addition, we characterized cyanobacteria communitry structure along longitudinal transects in Rio Mezquites using molecular techniques (denaturing gradient gel electrophoresis (DGGE)) and in the process developed a new procedure for DNA extraction from carbonate-dominated samples. We also completed detailed studies on metabolic processes, including calcification, in Rio Mezquites stromatolites using microelectrode techniques.

Archaea and Eubacteria (Souza, Eguiarte). Field samples from several dozen more localities were obtained from around the basin. We were successful in establishing >2500 cultures of Eubacteria in the laboratory as well as a variety of isolates of Archaea. Preliminary genetic analysis by Restriction Fragment Length Polymorphism (RFLP) indicates that all of the >2500 Eubacteria isolates are genetically distinct.

Stromatolite Morphogenesis and Microbial Taphonomy (Farmer). This year, we began studies of the microbial paleontology (taphonomy) and morphogenesis of oncolitic stromatolites (spherical stromatolites formed by rolling). Long-term in situ experiments were deployed at three sites in the Rio Mezquitas to determine the average accretion rate and transport history for oncoids. Samples were collected and fixed in the field. Characterization of these samples by scanning electron micrograph (SEM) and light microscopy showed that oncoids possess distinctly zoned communities consisting of a surficial assemblage of larger (>10 micron diameter) filamentous cyanobacteria and diatoms overlying a subsurface community (1-2 mm depth) dominated by finely-filamentous and coccoidal (<2 micron diameter) bacteria. Pervasive carbonate precipitation and lithification of oncoids is associated with activities of this deeper community. Fossil microbiotas captured during carbonate mineralization are mostly preserved as permineralized cyanobaterial sheaths and filament molds. Microbial fossil assemblages are dominated by the finely filamentous component of the subsurface community. Carbonate precipitation appears to be mediated by the metabolic activities of this subsurface community, while oncoid morphogenesis and microfabric development appear to be controlled by rapid, high angle branching growth of the finely (2 micron diameter) filamentous bacterial species that dominates that community.

Modeling (Fagan, Odell). Little additional progress in ecosystem modeling was made during the past year, and the subcontract with the University of Washington was terminated.