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Aquatic Microbial Ecology

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AME 42:243-253 (2006)  -  doi:10.3354/ame042243

Plasticity of N:P ratios in laboratory and field populations of Trichodesmium spp.

Jamie M. Krauk1,5, Tracy A. Villareal2, Jill A. Sohm3, Joseph P. Montoya4, Douglas G. Capone3,*

1Chesapeake Biological Laboratory, University of Maryland Center for Environmental Sciences, Solomons, Maryland 20615, USA
2Marine Science Institute, The University of Texas at Austin, 750 Channel View Dr., Port Aransas, Texas 78373, USA
3Wrigley Institute for Environmental Studies and Department of Biological Sciences University of Southern California, Los Angeles, California 90089, USA
4School of Biology, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
5Present address: National Sea Grant Office, Department of Commerce, National Oceanic and Atmospheric Administration, 1315 East West Highway, R/SG, SSMC 3 room 11853, Silver Spring, Maryland 20910, USA
*Corresponding author. Email:

ABSTRACT: We followed changes in N:P ratios in batch cultures of the planktonic marine cyanobacterium Trichodesmium (IMS 101) grown in 2 different media and in field populations from 4 different oceanic regions. Cultures grown on low P media showed a rapid rise in N:P ratio upon depletion of phosphate. Ratios exceeding 125 were reached in 1 experiment before attaining stationary phase. A transect across the North Atlantic Ocean along 32°N showed a monotonic decrease in the N:P ratio of field collected colonies, dropping from about 60:1 on the western side of the basin to about 30:1 on the eastern side. A second cruise sampled colonies and surface slicks in waters along the north coast of Australia, where ratios of N:P were generally lower than in the North Atlantic, ranging from 11:1 to 47:1 with an average of 22:1. A comparison of rising and sinking colonies collected at 8 stations in the Gulf of Mexico shows a higher mean N:P ratio among sinking colonies than floating colonies. Overall, the average N:P in the Gulf of Mexico was about 68:1. N:P ratios of Trichodesmium around the Hawaiian Islands were very consistent between 2 consecutive years of sampling, with an average colony N:P for both years of about 38:1. Our research demonstrates high variability in the cellular N:P in Trichodesmium both in the laboratory and in the field. Trichodesmium N:P ratio may provide an index to the relative severity of P limitation in these diazotrophs. Geochemical and ecological modeling efforts which rely on using the N:P ratio of diazotrophs in deriving nitrogen fixation rates should account for the variability of these ratios in situ.

KEY WORDS: Trichodesmium · N:P ratio · Nitrogen fixation · Diazotroph · Cyanobacteria

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