CIRCADIAN RHYTHMS - Cyanobacteria - page 305

I. EARLY STUDIES
Gallon et al . (1974) reported that nitrogen fi xation and photosynthesis in cultures of Gloeocapsa
depended on the age of cultures and there was a rise and fall in their activities in continuous light.
However, these variations have not been correlated to daily cyclic variation. Nitrogen fi xation by
Gloeocapsa grown in 12:12 light and dark cycles was observed only during dark period (Millineaux
et al ., 1981). However, in case of Anabaena cylindrica where nitrogenase is restricted to heterocysts
and PSII activity is confi ned to vegetative cells, nitrogen fi xation preferentially occurred during light
period. The circadian control of nitrogen fi xation in Gloeocapsa was apparent only when the cultures
were subjected to 16:8 light and dark cycles where nitrogenase activity was maximal after 8 h of the
onset of darkness. Likewise, a temporal separation of nitrogen fi xation and oxygen production in
Plectonema boryanum was also reported (Weare and Benemann, 1994).
Stal and Krumbein (1985) demonstrated that nitrogenase activity peaked during darkness
when the cells of Oscillatoria sp. were subjected to LD cycle of 16:8 h. Besides, they observed that
nitrogen fi xation as well as photosynthetic activity reciprocally reached maximum activities that
persisted even after the cells adapted to LD cycle are shifted to LL condition. Likewise, Mitsui et
al . (1986) reported a temporal separation of nitrogen fi xation and photosynthesis in two species of
Synechococcus (Miami BG 43511 and 43522) where the pattern (peaking of the activities once daily)
was established after the cells were transferred to LL for 20 h.
Though these studies have not been correlated with the circadian rhythms but unequivocally
pointed towards a temporal separation of nitrogen fi xation (occurring at night) from photosynthesis
(taking place in the day) while at the same time dependent on LD cycles. The fi rst report on the
existence of circadian rhythms in cyanobacteria was made in Synechococcus sp. strain RF-1 that fi xed
nitrogen only during dark phase while the fi xation varied at different levels in continuous light
(Grobbelaar et al ., 1986; Huang and Chow, 1986). These workers identifi ed that the pattern seen in LL
after an adaptive period in LD to be an endogenous rhythm. Chen et al . (1991) also reported rhythmic
amino acid uptake by Synechococcus RF-1. In a series of communications, Huang and coworkers
have confi rmed the existence of circadian clock in cyanobacteria. This was substantiated by the
isolation of mutants of Synechococcus RF-1 that showed alteration in the rhythm as well (Huang et
al ., 1990, 1993, 1994; Huang and Chou, 1991; Huang and Grobbelaar, 1995; Chen et al ., 1993,1998).
Sweeney and Borgese (1989) reported the operation of temperature compensated cell division cycle
in Synechooccus sp. strain WH 7803 in a 24 h period.
The above studies thus dispelled the dogma regarding the absence of circadian rhythms in
prokaryotes. Huang and Grobbelaar (1995) studied several aspects of endogenous rhythm in
Synechococcus RF-1 for nitrogenase activity and suggested that: (i) the rhythms were temperature-
compensated showing approximate 24 h periods in constant conditions between 22ºC and 33ºC
(Huang et al ., 1990); (ii) the endogenous nitrogenase rhythm in RF-1 is independent of nitrogen fi xation
per se as evidenced by the accumulation of nifH transcripts under only nitrogen-fi xing conditions;
(iii) RF-1 also showed rhythmic uptake of eight amino acids. Furthermore, Schneegurt et al . (1994)
demonstrated the persistent 24 h alteration in photosynthesis and nitrogen fi xation in Cyanothece
sp. strain ATCC 51142 with a rhythmic accumulation of stored carbohydrate.
II. CHOICE OF EXPERIMENTAL ORGANISM
Convincing evidences for the existence of circadian rhythms in cyanobacteria have been provided
with the studies on Synechococcus sp. strain PCC 7942 (identifi ed initially as Anacystis nidulans and