HR: 16:45h
AN: B24A-04 INVITED     [Abstracts]
TI: Chemolithoautotrophy and its Relation to Magnetism and Biomineralization in Marine Magnetotactic Bacteria
AU: * Bazylinski, D A
EM: dbazylin@iastate.edu
AF: Iowa State University, Department of BBMB 207 Science I, Ames, IA 50011 United States
AU: Williams, T J
EM: tijawi@iastate.edu
AF: Iowa State University, Department of BBMB 207 Science I, Ames, IA 50011 United States
AU: Zhang, C L
EM: Zhang@srel.edu
AF: University of Georgia, Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802 United States
AU: Scott, J H
EM: j.scott@gl.ciw.edu
AF: Carnegie Inst. of Washington, Geophysical Laboratory 5251 Broad Branch Rd. NW, Washington, DC 20015 United States
AB: All cultured, marine, magnetite-producing, magnetotactic bacteria (MB) are capable of chemolithoautotrophy and use a number of electron donors to support this mode of growth including reduced sulfur compounds. Several vibrioid strains are known to rely on the Calvin-Benson-Bassham (CBB) cycle for autotrophy. An obligately microaerophilic, magnetite-producing, coccoid strain (MC-1) grew with sulfide and thiosulfate as electron donors and ¹⁴C-labelling experiments showed that virtually all cell C was derived from H¹⁴CO₃^-/¹⁴CO₂ confirming autotrophy in this strain. Cell-free extracts of strain MC-1 did not exhibit ribulose-1,5-bisphosphate carboxylase-oxygenase (RubisCO) activity and nor were RubisCO genes found in the draft genome of the organism. Cell extracts also did not exhibit carbon monoxide dehydrogenase activity indicating that the acetyl-CoA pathway also does not function in strain MC-1. The ¹³C content of whole cells of strain MC-1 relative to the ¹³C content of the H¹⁴CO₃^-/¹⁴CO₂ used for growth (Δδ¹³C) was -11.4 ppt. Cellular fatty acids showed enrichment of ¹³C relative to biomass. Activities for three key enzymes of the reverse or reductive tricarboxylic acid (rTCA) cycle were demonstrated for MC-1: fumarate reductase, pyruvate: acceptor oxidoreductase and 2-oxoglutarate: acceptor oxidoreductase. Although ATP citrate lyase (another key enzyme of the rTCA cycle) activity was not detected in cell-free extracts of strain MC-1 using commonly used assays for this enzyme, cell-free extract was found to rapidly cleave citrate, and the reaction was dependent upon the presence of ATP, coenzyme A and NADH. Thus, we infer the presence of an ATP-dependent citrate-cleaving enzyme or enzymes. The Δδ¹³C value and results from enzyme studies are consistent with the operation of the rTCA cycle for autotrophy in strain MC-1. Strain MC-1 appears to be the first known member of the alpha-Proteobacteria to assimilate CO₂ during autotrophic growth using the rTCA cycle. Based on the type of chemolithoautotrophy described above, it is clear why marine magnetite-producing MB occupy a precise location, the oxic-anoxic interface, in vertical chemical gradients within chemically-stratified coastal environments: they must have an electron donor, sulfide and perhaps others, and an electron acceptor, O₂. The presumed function of magnetosomes is that the magnetic dipole resulting from the magnetosomes aids the cell in locating and maintaining an optimal position within vertical chemical gradients. MB process large amounts of Fe in the biomineralization of magnetosomes: cells consist of 1-3% Fe (dry wt). Because of this, and the fact that many chemolithoautotrophic, non-magnetotactic bacteria occupy a similar niche, we have been investigating possible physiological reasons for the production of magnetosomes and the processing of such large amounts of Fe. We have found that some marine vibrioid strains grow in O₂-gradient medium with Fe(II) as the electron donor. Cells appear to oxidize the Fe(II) and produce a layer of Fe oxyhydroxides within the gradient suggesting that cells obtain energy from the oxidation of Fe(II).
DE: 0419 Biomineralization
DE: 0428 Carbon cycling (4806)
DE: 0463 Microbe/mineral interactions
DE: 0465 Microbiology: ecology, physiology and genomics (4840)
DE: 0471 Oxidation/reduction reactions (4851)
SC: Biogeosciences [B]
MN: Fall Meeting 2005