Satellite captures trichodesmium blooms in the southwestern tropical Pacific

Obtaining a true estimate of nitrogen fixation by cyanobacteria in the oceans, mainly Trichodesmium , is an important step toward understanding the entire nitrogen cycle in the tropical ocean. This strictly anaerobic process, which has a high Fe requirement, could regulate atmospheric CO 2 over geological time. For example, during interglacial periods, N 2 fixation would be too low (low Fe) to balance denitrification and the ocean would lose its fixed nitrogen [ Falkowski , 1997]. Has the level of marine nitrogen fixation been underestimated until now? High N 2 fixation rates measured on Trichodesmium spp. communities have led to an upward revision of this marine flux [ Capone et al , 1997]. Recent modeling studies and observations predict that N 2 fixation could regulate the long‐term N:P equilibrium in the oceans and balance denitrification [ Tyrell , 1999; J L. Sarmiento and N. Gruber , manuscript in preparation, 1999].The major nitrogen fixer, Trichodesmium spp., which are filamentous, nonheterocystous N 2 ‐fixing cyanobacteria, has a nearly ubiquitous distribution in the euphotic zone of tropical and subtropical seas and could play a major role in bringing new N to these oligotrophic systems. Satellite images from Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFs), the recently launched ocean color sensor, and data from a recent cruise, provide further evidence of the importance of Trichodesmium in the southwestern tropical Pacific Ocean.