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Project Progress

Research Summary:

Creation of mobile field laboratory for the in situ measurement of biogenic gas fluxes. A transportable laboratory was created to 1) concentrate gases evolving from the surface of a microbial mat, 2) measure the physical and chemical changes that occur at the mat surface over a diurnal cycle, and 3) quanitatively and sensitively measure a large variety of gases in real time. A gas-tight plexiglass chamber was created that could be placed on the mat surface to concentrate evolved gases. Gas-tight ports were positioned on top of the chamber to drop and hold instruments at the mat surface for measuring changes in pH, temperature, and conductivity. A light meter was attached to the top of the chamber. Two battery-operated fans on the inside of the chamber insured mixing of the gases, and a teflon-lined septum was attached to the side of the chamber to collect samples. A gas chromatograph (GC) was configured with three columns and four detectors for the quantitative measurement of a large variety of fixed gases, sulfur gases, and gases containing organic carbon. The GC was calibrated with standard samples of CO2, CH4, NO2, H2S, DMS, and SO2 in the laboratory. A system for collecting gas in the field was tested in the laboratory using evacuated vials and sterile syringes. Once in the field, all of the instrumentation was powered by a small gas generator.

Collection of data at field site. The field site was located in Baja California, Mexico approximately 100 mi south of Ensenada. The microbial mat communities were only a few millimeters thick due to their destruction during the last two El Niño events. The flux chamber was placed over a section of mat that had four visible layers; a saline crust on top, a dark green layer dominated by cyanobacteria, a brown layer containing a mixture of anoxygenic photosynthesizers and anaerobic bacteria, and a black sulfidic mud layer that was at least a meter deep. Three field trips were made, one in September of 1999, one in November of 1999, and one in February of 2000. In September, the field equipment was tested, temperature, light and pH profiles were collected over an 8 h period, and measurements of two sulfur gases were made over a 5 h period. In November, we collected data over two consecutive days for 13 h time periods, 9 h in daylight, and 4 h in darkness. We observed the flux of 5 gases (relative to ambient concentrations) that correlated with temperature inside of the flux chamber. CO2, two sulfur-containing gases, and two organic carbon-containing gases were monitored. One of the sulfur gases was likely H2S based on the retention time of H2S standards. The identities of the other gases are unknown. Light, temperature, and pH profiles were also collected. The conductivity of fluid at the mat surface was too high for our instrument to measure, and the pH remained circum-neutral throughout the diurnal cycle, with slight fluctuations towards alkalinity during high light-flux periods. In February, we attempted to repeat the experiments from November, but experienced difficulties with our instrumentation. The data from November are currently being transformed into measurements of gas flux per unit area per unit time for use in atmospheric modeling studies.

PROPOSED RESEARCH

During the first half of this project, we have accomplished our first stated goal from the grant – to monitor gas fluxes over the surfaces of microbial mats in situ. During the second half of the granting period, we will focus on collecting more gas flux measurements at the site in Baja California, Mexico, and also expand our data collection to other salt-flat environments that harbor microbial mats, such as those at Mono Lake, California. By monitoring gas flux at both sites, we will have a better understanding of the types and quantities of gases that evolve from different and geographically separated microbial communities. All of the data collected will accomplish the fourth stated goal from the grant, which is to model the impact of biogenic gas emissions on the atmosphere.

To accomplish the second stated goal of the grant, monitoring gas concentration as a function of depth in the mat, we will use voltammetric micro-electrodes which are sensitive to O2, H2S, Fe(II) and Mn(II). Vertical profiles of oxidized and reduced regions of the mat and the cycling of iron and manganese will show how different metabolisms occurring in each mat layer interact. Once we have a better understanding of how microbial mat communities behave over a diurnal cycle, both in the expulsion of gas and in the vertical cycling of nutrients, then we can begin the microbiological and molecular analysis of specific communities. Towards this end, samples will be collected, documented, and preserved for future work.