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

摘要: We encountered an extensive bloom of the colonial diatom Hemiaulus hauckii along a 2500 km cruise track off NE coast South America in autumn 1996. Each cell contained heterocystous, N 2 -fixing cyanobacterial endosymbiont Richelia intracellularis. Surface heterocyst (and filament) densities increased from 10 6 1 -1 bloom. Total abundance ranged m -2 outside to over within Rates primary production averaged 1.2 g C d , higher than typical for oligotrophic open ocean waters. fixation during by Richelia/Hemiaulus association added average 45 mg water column. The relative importance NH 4 + uptake course 0 42% total Hemiaulus/Richelia association. exceeded estimates 'new' flux via NO 3 diffusion deep and, together with additional cyanobacterium Trichodesmium, could supply about 25 % demand through column Suspended particles and zooplankton collected were depleted 15 N, reflecting dominant contribution planktonic budget. was spatially extensive, as revealed satellite imagery, is calculated have contributed 0.5 Tg euphotic zone. Such blooms may represent important previously unrecognized source new support nutrient-poor tropical Furthermore, this demonstrates that heterocystous cyanobacteria can also make quantitatively contributions oceanic environments.