The tropical diazotrophic phytoplankter Trichodesmium: biological characteristics of two common species

Abstract

The 2 tropical cyanobactenal specles Trichodesmium thiebautii and 7: erythraeum had simlar photosynthetic characteristics in the southwestern Sargasso Sea and Canbbean Sea, with mean rates of hght saturated photosynthesis (uslng O2 electrode) of 42 (SD = 21.3) and 37 (SD = 18.4) mg 0, mg chl a -' h-' at 1410 FE m-2 S-', respectively over a 1300 n mlle cruise track.Rates of dark respiration were high, and the compensation point for both specles was 150 FE m-2 S-' (ca 55 m, m d d a y ) .Estimates of carbon doubllng times (using photosynthetic quotient) were from 3.0 to 3.8 d based on expected photosynthetic rates in the water column.The mean rate of nltrogenase actlvlty at 300 pE m-2 S-' by T thiebautu averaged 0.45 nmol ethylene colony-' h -' , 1.6 times that of 7: erythraeum (p < 0.01) as observed from samples collected on 3 cruises (64 paired observations).Furthermore, in a comparison of nltrogenase actlvitles, at light intensities between ca 500 and 2500 FE m-2 S-', 7: thiebautii was about t w ~c e as actlve as T erythraeum.The phycoerythrin content of 7: erythraeum averaged 260 ng colony-', 4.4 times that of T thiebautii, and the mean PE : chl a ratios were 3.2 and 1.2, respectively Other pigments: (p-carotene, zeaxanthin, myxoxanthophyll, echinenone, and trace pigments) were s i d a r between the 2 specles.The organization of subcellular inclusions was distinctly different in these 2 species.The high abundance of T thiebautii relative to T erythraeum in many tropical seas may be due to higher rates of N2 fixation and a previously reported neurotoxin in the former specles.