DAILY RHYTHM OF O-2-EVOLUTION IN THE CYANOBACTERIUM TRICHODESMIUM-THIEBAUTII UNDER NATURAL AND CONSTANT CONDITIONS

The rate of oxygen evolution by the tropical marine cyanobacterium Trichodesmium thiebautii was recorded at different times during the day in samples collected in 1992 from the Bahama Islands and the NE Caribbean Sea. This cyanobacterium is unique in that it is the only non-heterocystous diazotroph capable of N-2- fixation in daylight. Oxygen evolution was measured under conditions of natural day/night (LD, N = 50), constant light (LL, N = 14), and constant dark (DD, N = 2 x 14). Photosynthesis vs intensity (P-I) relationships were calculated at various times of day, and the following parameters were used for further evaluation: photosynthesic capacity (P-max, 66 to 91 mg O-2 mg chi a(-1) h(-1)), initial slope of the P-I curve (alpha, 0.23 to 0.27), dark respiration (R, 12 to 27 mg O-2 mg chl a(-1) h(-1)), and the intensity at which O-2 consumption is compensated by O-2 production (I-c, 78 to 160 mu Em(-2) s(-1)). All means showed large standard deviations (for some parameters more than 200 %). In some cases, these variations could be explained with a sinusoidal 24-h time course, but only the compensation point showed a significant daily variation (p less than or equal to 0.001) in both LD and DD. The fact that the time course of I-c typical for natural conditions remains rhythmic under constant dark conditions strongly suggests a circadian regulation. Few circadian rhythms have been observed in prokaryotes, and this appears to be the first observation of such a rhythm in a cyanobacterium which fixes N-2 in daytime.