Extensive bloom of a N2-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 Amenca in autumn 1996. Each diatom cell contained the heterocystous. N2-fWng cyanobacterial endosymbiont Richeiia intracellularis. Surface Richeiia heterocyst (and filament) densities increased from <I00 to >106 heterocyst 1-' in the bloom. Total abundance ranged from 10"eterocyst m-2 outside the bloom to over 10" heterocyst m-2 within the bloom. Rates of primary production averaged 1.2 g C m-2 d-', higher than typical for oligotrophic Open ocean waters.