Extensive bloom of a N-2-fixing diatom/cyanobacterial association in the tropical Atlantic Ocean

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 19 96. Each diatom cell contained the heterocystous, N-2-fixing cyanobacterial endosymbiont Richelia intracellularis. Surface Richelia heterocyst (and fi lament) densities increased from <100 to >10(6) heterocyst l(-1) in the blo om. 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 oc ean waters. N-2 fixation during the bloom by the Richelia/Hemiaulus associa tion added an average of 45 mg N m(-2) d(-1) to the water column. The relat ive 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 fixati on by Richelia exceeded estimates of 'new' N flux via NO3 diffusion from de ep water and, together with additional N-2 fixation by the cyanobacterium T richodesmium, could supply about 25% of the total N demand through the wate r column during the bloom. Suspended particles and zooplankton collected wi thin the bloom were depleted in N-15, reflecting the dominant contribution of N-2 fixation to the planktonic N budget. The bloom was spatially extensi ve, 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 importan t and previously unrecognized source of new N to support primary production in nutrient-poor tropical waters. Furthermore, this bloom demonstrates tha t heterocystous cyanobacteria can also make quantitatively important contri butions of N in oceanic water column environments.