MEPS 185:273-283 (1999)  -  doi:10.3354/meps185273

Extensive bloom of a N2-fixing diatom/cyanobacterial association in the tropical Atlantic Ocean

Edward J. Carpenter1, Joseph P. Montoya2, James Burns3,*, Margaret R. Mulholland3,**, Ajit Subramaniam3,*, Douglas G. Capone3,*,***

1Marine Sciences Research Center, State University of New York, Stony Brook, New York 11794-5000, USA
2School of Biology, Georgia Institute of Technology, 310 Ferst Drive, Atlanta, Georgia 30332-0230, USA
3Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, Solomons, Maryland 20688, USA
Present addresses:
*Wrigley Institute for Environmental Studies and Department of Biological Sciences, University of Southern California, Los Angeles, California 90089, USA
**Marine Sciences Research Center, State University of New York, Stony Brook, New York 11794-5000, USA
***Addressee for correspondence. E-mail:

ABSTRACT: We encountered an extensive bloom of the colonial diatom Hemiaulus hauckii along a 2500 km cruise track off the NE coast of South America in autumn 1996. Each diatom cell contained the heterocystous, N2-fixing cyanobacterial endosymbiont Richelia intracellularis. Surface Richelia heterocyst (and filament) densities increased from <100 to >106 heterocyst l-1 in the bloom. Total abundance ranged from 106 heterocyst m-2 outside the bloom to over 1010 heterocyst m-2 within the bloom. Rates of primary production averaged 1.2 g C m-2 d-1, higher than typical for oligotrophic open ocean waters. N2 fixation during the bloom by the Richelia/Hemiaulus association added an average of 45 mg N m-2 d-1 to the water column. The relative importance of NH4+ uptake over the course of the bloom increased from 0 to 42% of total N uptake by the Hemiaulus/Richelia association. N2 fixation by Richelia exceeded estimates of 'new' N flux via NO3 diffusion from deep water and, together with additional N2 fixation by the cyanobacterium Trichodesmium, could supply about 25% of the total N demand through the water column during the bloom. Suspended particles and zooplankton collected within the bloom were depleted in 15N, reflecting the dominant contribution of N2 fixation to the planktonic N budget. The bloom was spatially extensive, as revealed by satellite imagery, and is calculated to have contributed about 0.5 Tg N to the euphotic zone. Such blooms may represent an important and previously unrecognized source of new N to support primary production in nutrient-poor tropical waters. Furthermore, this bloom demonstrates that heterocystous cyanobacteria can also make quantitatively important contributions of N in oceanic water column environments.

KEY WORDS: N2 fixation · Cyanobacteria · Richelia · Symbiont · Diazotroph · N cycling · Hemiaulus

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