Anaerobic ATP Levels in the Blue-Green Alga Anabaena: Dark-Light Transients and Effects of Light Intensity

An enzymatic method was used to determine ATP extracted from anaerobically incubated samples of the blue-green alga Anabaena variabilis. The dark-light-dark transients of the ATP level in the whole cells were studied in the presence and in the absence of 3 (p-chlorophenyl)-l.l-dimethylurea (CMU). When O₂ evolution was completely inhibited by CMU, there was no significant difference between the transient under illumination of a low intensity and that of fourfold intensity. On the other hand in the absence of CMU, the ATP level decreased more rapidly after high intensity illumination than after low intensity one. The increase in ATP content during the first 5 s illumination was smaller in the absence of CMU than in the presence of CMU. With or without CMU the increase was saturated at the vicinity of the intensity at which photosynthetic O₂ evolution compensates for respiratory O₂ consumption. The calculated quantum requirement of cyclic photophosphorylation in vivo was 2.2 quanta per ATP.