Nitrogen-fixation by the cyanobacterial symbiont of the diatom genus Hemiaulus

Abstract

Nitrogen-fixation by cyanobacterial symbionts in the oceanic diatoms Hemiaulus mernbranaceus, H. hauckii, and H. sinensis was documented in the Southwestern Atlantic Ocean. All Herniaulus cells with diatom chlorophyll autofluorescence contained fluorescent symbionts undetectable by standard light microscopy. Average cell-specific ethylene reduction rates (2.3 to 5.3 X 10-l3 M ethylene cell h-') were 2.2 to 5.2 tlmes lower than calculated rates from Rhizosolenia-Richelia blooms in the central Pacific gyre and 26 times lower than results from fichelid-containing Rhizosolenia cultures. Calculations suggest that the N2 fixation associated with Herniaulus is 21 to 45 times greater than that from the Rhizosolenia-Richelia association in the southwestern Atlantic Ocean.