THE TROPICAL DIAZOTROPHIC PHYTOPLANKTER TRICHODESMIUM - BIOLOGICAL CHARACTERISTICS OF 2 COMMON SPECIES

The 2 tropical cyanobacterial species Trichodesmium thiebautii and T. erythraeum had similar photosynthetic characteristics in the southwest ern Sargasso Sea and Caribbean Sea, with mean rates of light saturated photosynthesis (using O2 electrode) of 42 (SD = 21.3) and 37 (SD = 18 .4) Mg O2 mg chl a-1 h-1 at 1410 muE m-2 s-1, respectively over a 1300 n mile cruise track. Rates of dark respiration were high, and the com pensation point for both species was 150 muE m-2 s-1 (ca 55 m, midday) . Estimates of carbon doubling times (using photosynthetic quotient) w ere from 3.0 to 3.8 d based on expected photosynthetic rates in the wa ter column. The mean rate of nitrogenase activity at 300 muE m-2 s-1 b y T. thiebautii averaged 0.45 nmol ethylene colony-1 h-1, 1.6 times th at of T. erythraeum (p < 0.01) as observed from samples collected on 3 cruises (64 paired observations). Furthermore, in a comparison of nit rogenase activities, at light intensities between ca 500 and 2500 muE m-2 s-1, T. thiebautii was about twice as active as T. erythraeum. The phycoerythrin content of T. erythraeum averaged 260 ng, colony-1, 4.4 times that of T. thiebautii, and the mean PE: chl a ratios were 3.2 a nd 1.2, respectively. Other pigments: (beta-carotene, zeaxanthin, myxo xanthophyll, echinenone, and trace pigments) were similar between the 2 species. The organization of subcellular inclusions was distinctly d ifferent in these 2 species. The high abundance of T. thiebautii relat ive to T. erythraeum in many tropical seas may be due to higher rates of N2 fixation and a previously reported neurotoxin in the former spec ies.