Inter-Research
Marine Ecology Progress Series

ABSTRACT: 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 I_k (irradiance saturation parameter) values ranging from 142 to 295 umol m^-2 s^-1 photon flux and a compensation irradiance of between 96 and 134 umol m^-2 s^-1. 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 Trichodesmium spp. have gas vesicles, they are buoyant, and the population maxima are typically found at 15 m, approximately 50% of I₀ (incident surface irradiance). Thus for most of the day they almost always receive saturating irradiances. The mean P_max values were 44.7 (January-February 1992), 8.29 (September 1992), and 6.45 (September-October 1992) mg O₂ mg^-1 chl a h^-1. A calibration with ¹⁴C gave a very low photosynthetic quotient (PQ) which averaged 0.46 and 0.61 on 2 cruises. This could have been because of O₂ 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.

KEY WORDS: Trichodesmium photosynthesis . Primary productivity . Caribbean Sea phytoplankton . Marine planktonic cyanobacteria . Marine diazotrophs . N₂ fixation . Photosynthetic quotient

Published in MEPS Vol. 118 (1995) on March 9
Print ISSN: 0171-8630; Online ISSN: 1616-1599. Copyright © Inter-Research, Oldendorf/Luhe, 1995