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Molybdenum (Mo) is a key trace nutrient for biological assimilation of nitrogen, either as nitrogen gas (N2) or nitrate (NO3-). Although Mo is the most abundant metal in seawater (105 nM), its concentration is low (<5 nM) in most freshwaters today, and it was scarce in the ocean before 600 million years ago. The use of Mo for nitrogen assimilation can be understood in terms of the changing Mo availability through time; for instance, the higher Mo content of eukaryotic vs. prokaryotic nitrate reductase may have stalled proliferation of eukaryotes in low-Mo Proterozoic oceans. Field and laboratory experiments were performed to study Mo requirements for NO3- assimilation and N2 fixation, respectively. Molybdenum-nitrate addition experiments at Castle Lake, California revealed interannual and depth variability in plankton community response, perhaps resulting from differences in species composition and/or ammonium availability. Furthermore, lake sediments were elevated in Mo compared to soils and bedrock in the watershed. Box modeling suggested that the largest source of Mo to the lake was particulate matter from the watershed. Month-long laboratory experiments with heterocystous cyanobacteria (HC) showed that <1 nM Mo led to low N2 fixation rates, while 10 nM Mo was sufficient for optimal rates. At 1500 nM Mo, freshwater HC hyperaccumulated Mo intercellularly, whereas coastal HC did not. These differences in storage capacity were likely due to the presence in freshwater HC of the small molybdate-binding protein, Mop, and its absence in coastal and marine cyanobacterial species. Expression of the mop gene was regulated by Mo availability in the freshwater HC species Nostoc sp. PCC 7120. Under low Mo (<1 nM) conditions, mop gene expression was up-regulated compared to higher Mo (150 and 3000 nM) treatments, but the subunit composition of the Mop protein changed, suggesting that Mop does not bind Mo in the same manner at <1 nM Mo that it can at higher Mo concentrations. These findings support a role for Mop as a Mo storage protein in HC and suggest that freshwater HC control Mo cellular homeostasis at the post-translational level. Mop's widespread distribution in prokaryotes lends support to the theory that it may be an ancient protein inherited from low-Mo Precambrian oceans.
ContributorsGlass, Jennifer (Author) / Anbar, Ariel D (Thesis advisor) / Shock, Everett L (Committee member) / Jones, Anne K (Committee member) / Hartnett, Hilairy E (Committee member) / Elser, James J (Committee member) / Fromme, Petra (Committee member) / Arizona State University (Publisher)
Created2011
Description
Hydrothermal environments are important locales for carbon cycling on Earth and elsewhere in the Universe. Below its maximum temperature (~73 °C), microbial photosynthesis drives primary productivity in terrestrial hydrothermal ecosystems, which is thought to be performed by bacterial phototrophs in alkaline systems and eukaryotic algae in acidic systems, yet has received little attention at pH values intermediate to these extremes. Sequencing of 16S and 18S rRNA genes was performed at 12 hot springs with pH values 2.9-5.6 and revealed that cyanobacteria affiliated with the genus Chlorogloeopsis and algae of the order Cyanidiales coexisted at 10 of the sites. Cyanobacteria were present at pH values as low as 2.9, which challenges the paradigm of cyanobacteria being excluded below pH 4. Presence of the carotenoid β-cryptoxanthin in only 2 sites and quantitative PCR data suggest that algae were inactive at many of the sites when sampled. Spatial, but perhaps not temporal, overlap in the habitat ranges of bacterial and eukaryal microbial phototrophs indicates that the notion of a sharp transition between these lineages with respect to pH is untenable.
In sedimentary basins, biosphere-derived organic carbon is subjected to abiotic transformations under hydrothermal conditions. Benzaldehyde was experimentally evaluated as a model to assess the chemistry of aldehydes under these conditions. It was first demonstrated that gold, a traditional vessel material for hydrothermal experiments, caused catalysis of benzaldehyde degradation. Experiments in silica tubes were performed at 250, 300, and 350 °C yielding time-dependent data at several starting concentrations, which confirmed second-order kinetics. Therefore, disproportionation was expected as a major reaction pathway, but unequal yields of benzoic acid and benzyl alcohol were inconsistent with that mechanism. Consideration of other products led to development of a putative reaction scheme and the time dependencies of these products were subjected to kinetic modeling. The model was able to reproduce the observed yields of benzoic acid and benzyl alcohol, indicating that secondary reactions were responsible for the observed ratios of these products. Aldehyde disproportionation could be an unappreciated step in the formation of carboxylic acids, which along with hydrocarbons are the most common organic compounds present in natural systems.
In sedimentary basins, biosphere-derived organic carbon is subjected to abiotic transformations under hydrothermal conditions. Benzaldehyde was experimentally evaluated as a model to assess the chemistry of aldehydes under these conditions. It was first demonstrated that gold, a traditional vessel material for hydrothermal experiments, caused catalysis of benzaldehyde degradation. Experiments in silica tubes were performed at 250, 300, and 350 °C yielding time-dependent data at several starting concentrations, which confirmed second-order kinetics. Therefore, disproportionation was expected as a major reaction pathway, but unequal yields of benzoic acid and benzyl alcohol were inconsistent with that mechanism. Consideration of other products led to development of a putative reaction scheme and the time dependencies of these products were subjected to kinetic modeling. The model was able to reproduce the observed yields of benzoic acid and benzyl alcohol, indicating that secondary reactions were responsible for the observed ratios of these products. Aldehyde disproportionation could be an unappreciated step in the formation of carboxylic acids, which along with hydrocarbons are the most common organic compounds present in natural systems.
ContributorsFecteau, Kristopher Michael, 1986- (Author) / Shock, Everett L (Thesis advisor) / Gould, Ian R (Committee member) / Hartnett, Hilairy E (Committee member) / Arizona State University (Publisher)
Created2016