association in the tropical Atlantic Ocean

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, N-2-fixing cyanobacterial endosymbiont Richelia intracellularis. Surface Richelia heterocyst (and filament) densities increased from 10(6) heterocyst l(-1) in the bloom. Total abundance ranged from 10(6) heterocyst m(-2) outside the bloom to over 10(10) 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. N-2 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. N-2 fixation by Richelia exceeded estimates of 'new' N flux via NO3 diffusion from deep water and, together with additional N-2 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 N-15, reflecting the dominant contribution of N-2 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.