Title:
Extensive bloom of a N₂-fixing diatom/cyanobacterial association in the tropical Atlantic Ocean
Extensive bloom of a N₂-fixing diatom/cyanobacterial association in the tropical Atlantic Ocean
Author(s)
Carpenter, Edward J.
Montoya, Joseph P.
Burns, James
Mulholland, Margaret R.
Subramaniam, Ajit
Capone, Douglas G.
Montoya, Joseph P.
Burns, James
Mulholland, Margaret R.
Subramaniam, Ajit
Capone, Douglas G.
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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.
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1999-08-20
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