1999 Articles

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

Carpenter, Edward J.; Montoya, Joseph P.; Burns, James; Mulholland, Margaret R.; Subramaniam, Ajit; Capone, Douglas G.

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 Richeiia intracellularis. Surface Richeiia heterocyst (and filament) densities increased from <I00 to >10⁶ heterocyst 1⁻¹ in the bloom. Total abundance ranged from 10⁶ eterocyst 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 Hemiauluslficheiia association. N₂ fixation by Richelia exceeded estimates of 'new' N flux via NO₃ diffusion from deep water and, together with additional N, 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.