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Applied and Environmental Microbiology, May 2006, p. 3217-3227, Vol. 72, No. 5
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.5.3217-3227.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Modeling the Dynamic Regulation of Nitrogen Fixation in the Cyanobacterium Trichodesmium sp.

Sophie Rabouille,* Marc Staal, Lucas J. Stal, and Karline Soetaert

Centre for Estuarine and Marine Ecology, Netherlands Institute of Ecology, KNAW, Yerseke, The Netherlands

Received 28 December 2005/ Accepted 21 February 2006

A physiological, unbalanced model is presented that explicitly describes growth of the marine cyanobacterium Trichodesmium sp. at the expense of N2 (diazotrophy). The model involves the dynamics of intracellular reserves of carbon and nitrogen and allows the uncoupling of the metabolism of these elements. The results show the transient dynamics of N2 fixation when combined nitrogen (NO3–, NH4+) is available and the increased rate of N2 fixation when combined nitrogen is insufficient to cover the demand. The daily N2 fixation pattern that emerges from the model agrees with measurements of rates of nitrogenase activity in laboratory cultures of Trichodesmium sp. Model simulations explored the influence of irradiance levels and the length of the light period on fixation activity and cellular carbon and nitrogen stoichiometry. Changes in the cellular C/N ratio resulted from allocations of carbon to different cell compartments as demanded by the growth of the organism. The model shows that carbon availability is a simple and efficient mechanism to regulate the balance of carbon and nitrogen fixed (C/N ratio) in filaments of cells. The lowest C/N ratios were obtained when the light regime closely matched nitrogenase dynamics.

* Corresponding author. Present address: Ocean Sciences Department, University of California—Santa Cruz, 1156 High Street, Santa Cruz, CA 95064. Phone: (831) 459-5152. Fax: (831) 459-4882. E-mail: srabouille{at}pmc.ucsc.edu.

Applied and Environmental Microbiology, May 2006, p. 3217-3227, Vol. 72, No. 5
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.5.3217-3227.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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