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2010 Annual Science Report

Arizona State University Reporting  |  SEP 2009 – AUG 2010

Stoichiometry of Life - Task 1d - Experimental Studies - the Role of Molybdenum in the Nitrogen Cycle, Past and Present

Project Summary

The element molybdenum (Mo) is critical for key processes in the cycling of nitrogen (N); for example, it is essential for the enzyme nitrogenase which bacteria use to convert gaseous N to “fixed” N that can be used in biological processes. This project seeks to understand how Mo might limit N processing in modern ecosystems (lakes and oceans) and infer its potential role in the past.

4 Institutions
3 Teams
13 Publications
1 Field Site
Field Sites

Project Progress

Mo storage protein Mop in cyanobacteria: Graduate Student Jennifer Glass, Postdoctoral Fellow Felisa Wolfe-Simon, James Elser and Ariel Anbar published research on molybdenum (Mo) requirements for nitrogen fixation and evidence of Mo storage in freshwater heterocystous cyanobacteria in early 2010 in Limnology & Oceanography (Glass et al., 2010). In order to determine the regulation of the putative Mo storage protein “Mop” in freshwater heterocystous cyanobacteria, Glass and undergraduate research assistant Eric Hughes performed more experiments in February 2010 with the model organism Nostoc sp. PCC 7120 under three treatment conditions: low Mo (1 nM), medium Mo (150 nM) and high Mo (3000 nM). All of the treatments were grown without added nitrogen so that they were forced to fix nitrogen from the atmosphere using the molybdenum-dependent enzyme nitrogenase. In short-term experiments (four transfers/17 days), we found that transcription of mop and nifD is up-regulated when Mo is low (~1 nM). However there is minimal Mop protein present in the same samples, suggesting that post-transcriptional regulation maintains low levels of Mop protein at low cellular Mo (<5ppm). It is possible that proteases preferentially degrade Mop in its apo-form. Mop was present in cultures grown at higher Mo, and in previously Mo-limited cultures 5 days after addition of 3000 nM Mo. In contrast, NifD protein levels were elevated at 1 nM Mo compared to other treatments. Our findings support Mop’s role as a Mo storage protein in heterocystous cyanobacteria and suggest that cyanobacteria maintain Mo cellular homeostasis by sensing Mo at the post-transcriptional level. These results will be submitted to the journal Applied and Environmental Microbiology.

Mo-nitrate co-limitation at Castle Lake: To explore the Mo requirements for nitrate assimilation of freshwater planktonic and microbial communities, Glass performed incubation experiments during the summers of 2008 and 2009 at the low-Mo ecosystem of Castle Lake, northern California. In situ bottle incubations with Mo and nitrate added singularly or in combination were performed at three depths (3, 15 and 25 meters). Addition of Mo stimulated nitrate assimilation in the Castle Lake hypolimnion in 2008 and in the epilimnion in 2009. Interannual and depth response differences were explained by ammonium inhibition of nitrate uptake and seasonal succession of plankton species with differing Mo requirements. Both summers a dissolved Mo minimum was observed in the Castle Lake epilimnion, which was likely a result of strong Mo draw-down by nitrogen-fixing periphyton communities in the littoral zone. Laboratory chemostat experiments with a common freshwater green alga, Scenedesmus acutus, confirmed that low Mo (1 nM) severely depressed activity of the Mo-containing enzyme nitrate reductase when nitrate was the sole nitrogen source. This study lends further ­support to the theory that low Mo can limit nitrate assimilation in freshwaters with typical low Mo levels (<5 nM) when ammonium is scarce, possibly resulting in decreased capacity of terrestrial ecosystems to serve as sinks for anthropogenic carbon dioxide emissions. A paper on this research is currently in review in the journal Biogeochemistry. Another paper on the geochemistry of Mo cycling in Castle Lake is in preparation for the journal Geochimica Cosmochimica Acta.

    James Elser James Elser
    Project Investigator
    Ariel Anbar Ariel Anbar
    Jennifer Glass Jennifer Glass
    Doctoral Student
    Eric Boyd

    Anthony Chappaz

    Felisa Wolfe-Simon

    Eric Hughes
    Undergraduate Student

    Objective 4.1
    Earth's early biosphere.

    Objective 5.1
    Environment-dependent, molecular evolution in microorganisms

    Objective 6.1
    Effects of environmental changes on microbial ecosystems