The marine planktonic cyanobacteria Trichodesmium spp.: photosynthetic rate measurements in the SW Atlantic Ocean

Rates of photosynthesis of the diazotrophic marine cyanobacteria Trichodesmium spp.were measured with an O2 electrode on 4 cruises to the Bahama Islands and the eastern Caribbean Sea.These phytoplankters had a relatively high light requirement, with Ik (irradiance saturation parameter) values ranging from 142 to 295 pm01 m-2 S-' photon flux and a compensation irradiance of between 96 and 134 pm01 m-? S-' Thus the compensation depth for Trichodesmium spp. is usually between about 70 and 85 m in the study area.Trichodesmium spp.had a high dark respiration rate, and this is the major cause of the high compensation irradiance.Because Tn'chodesmium spp, have gas ves~cles, they are buoyant, and the population maxima are typically found at 15 m, approximately 50 % of I, (incident surface irradiance).Thus for most of the day they almost always receive saturating irradiances.The mean P, , , , , values were 44.7 (January-February 1992), 8.29 (September 1992), and 6.45 (September-October 1992) mg O2 m g -' chl a h-'.A calibration with 14C gave a very low photosynthetlc quotlent (PQ) whlch averaged 0.46 and 0.61 on 2 cruises.This could have been because of 0, consumption from high RUBISCO oxygenase activity, and this may play a role in protection of nitrogenase from oxygen deactivation.Based on a mean PQ of 0.53, Trichodesmium spp.carbon turnover times for the 3 cruises were 0.93, 7.8, and 6.13 d, thus indicating relatively rapid growth rates for the population.