2010 Annual Science Report

Arizona State University Reporting  |  SEP 2009 – AUG 2010

Stoichiometry of Life - Task 1c - Laboratory Studies - the Role of Arsenic in Microbial Physiology, Ecology, and Evolution

Project Summary

It has previously been assumed that arsenic (or its more common form, arsenate) acts only as a poison for living things. However, As is very close to the nutrient element phosphorus (P; phosphate) in the Periodic Table suggesting that possibility that under some conditions organisms might use arsenate instead of phosphate in key molecules. This study examines microbes isolated from a high arsenate, low phosphate environment (Mono Lake, CA) to see whether or not they can grow on arsenate in the absence of phosphate and if As is incorporated into major molecules.

4 Institutions
3 Teams
11 Publications
1 Field Site
Field Sites

Project Progress

“Arsenolife”. Current research by Postdoc Felisa Wolfe-Simon, working with collaborator Ron Oremland (USGS ā€“ Menlo Park) has led to the isolation of a microbe from Mono Lake, CA that maintains growth under high arsenate, no added phoshpate (+As/-P condition). This microbe, identified as GFAJ-1 and a member of the Gammaproteobacteria, is maintained in the lab at 40 mM arsenate with no added phosphate but with a complement of other required cellular nutrients including nitrogen and vitamins. The cells grow well with phosphate (-As/+P) but change morphologically and enlarge significantly when grown +As/-P with large vacuolar regions evident (scanning and transmission electron microscopy). We showed that the cells accumulate intracellular arsenic (ICP-MS analyses) and this arsenic is distributed into the major repositories of the cell including nucleic acids, proteins and small molecular weight compounds (radiolabeled 73AsO4). To characterize the intracellular arsenic further, we determined that the arsenic was indeed present as arsenate bound to carbon in a structurally similar bonding environment that P would have in biomolecules like DNA but also ATP, NADH and glucose-6-phosphate (synchrotron studies). We then showed that arsenic is present in gel purified DNA extracted from +As/-P grown cells, moreover the intracellular distribution of As in +As/-P cells compared as we would expect to P in -As/+P grown cells (nanoSIMS). Taken together our findings suggest this microbe can substitute As for P in the major biomolecules of life. We currently have a paper in revision at Science. Wolfe-Simon is also engaged in the next set of experiments with a planned manuscript for submission to Nature Cell Biology.

Facultative anoxygenic photosynthesis. The rise in atmospheric oxygen (O2) over geologic time is attributed to the evolution and widespread proliferation of oxygenic photosynthesis in cyanobacteria. However, cyanobacteria maintain a metabolic flexibility that may not always result in O2 release. In the environment, cyanobacteria may use a variety of alternative electron donors rather than water known to be used by other anoxygentic phototrophs (eg. purple sulfur bacteria) including reduced forms of sulfur, iron, nitrogen, and arsenic. Recent evidence suggests cyanobacteria do actively take advantage of at least a few of these alternatives. Wolfe-Simon used a classical Winogradsky approach to enrich for cyanobacteria from the high salinity, elevated pH and arsenic-enriched waters of Mono Lake (CA). Experiments, optimized for cyanobacteria, revealed light-dependent, anaerobic arsenite-oxidation in sub-cultured slurries dominated by a filamentous cyanobacteria. We identified and isolated the dominant member of this enrichment to be a member of the Oscillatoriales by 16S ribosomal gene sequence. Addition of 1 mM arsenite induced facultative anoxygenic photosynthesis under continuous and circadian light. This isolate also oxidized sulfide under the same light-based conditions. Aerobic conditions elicited no arsenite oxidation in the light or dark and the isolate grew as a typical cyanobacterium using oxygenic photosynthesis. Under near-infrared light (700 nm) there was a direct correlation of enhanced growth to an increase in the rate arsenite or sulfide oxidation suggesting the use of photosystem I. Additionally, to test the wide spread nature of this metabolism in the Oscillatoriales, we have data which suggested arsenite- and sulfide-driven facultative anoxygenic photosynthesis as well as nitrogen fixation in the axenic isolate Oscillatoria sp. CCMP 1731. The wide-spread potential for this metabolism is of interest as well as the geobiological implications related to nitrogen-fixation and the evolution of O2 on Earth. We have a manuscript in preparation for submission to Applied and Environmental Microbiology.

Ron Oremland and Felisa Wolfe-Simon sample sediments at Mono Lake, CA. Wolfe-Simon isolated microorganisms from this location that apparently substitute As for P in biomolecules.