Validity of N2 fixation rate measurements in marine Oscillatoria (Trichodesmium)

No measurable differences in Trichodesmium nitrogenase activitywere observed between colonies collected by diving and incubatedunder ultra-clean conditions compared with those collected andincubated using standard techniques. Measurements were madein the northeastern Caribbean Sea, near the Bahama Islands andin the Sargasso Sea. Surprisingly, mean rates of ethylene productionwere high relative to most previous in situ measurements onTrichodesmium. The calculated cellular N doubling times (viaN₂ fixation) ranged from 1.13 days in the northeastern CaribbeanSea, 1.48 days in the Sargasso Sea to 1.8 days near the BahamaIslands. A comparison of these doubling times with those inthe literature illustrates the high variability in rate of N₂fixation by Trichodesmium. From this study, we conclude thatthe often observed slow rates of N₂ fixation are valid. Populationsof Trichodesmium can probably remain within the water columnat low growth rates via gas vesicles, which keep the colonysuspended, and low grazing rates by herbivores.