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Chapter category: Circadian Rhythms
Circadian Organization in the Algal Flagellate Euglena
Chapter authors:
Leland N. Edmunds
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The temporal organization of Euglena gracilis Klebs (strain Z) has been studied extensively
over the past 40 years (reviewed in Refs. 1-5). This algal flagellate can be grown on a
variety of defined media, either photoautotrophically in the presence of CO2 and vitamins
B1 and B12, or organotrophically in the light or dark on carbon sources ranging from
acetate and ethanol to lactic, glycolic, glutamic and malic acids over a wide pH range. This
versatility in growth mode, in conjunction with the fact that cell division can easily be synchronized
by 24-h (and other) lighting schedules6,7 and temperature cycles,8 has made Euglena an
important experimental organism for physiological and biochemical investigations.9-11
A number of persisting circadian rhythms (CRs) have been reported (Table 1). These studies
have been aided by the fact that Euglena can be maintained in the “stationary” growth phase
(infradian growth mode) for days or weeks with little or no net change in cell titer; circadian
output can be monitored while divorced from the driving force of the cell division cycle (CDC).
The fact that a number of photosynthetic mutants (even completely bleached strains devoid of
their chloroplast genomes) have been isolated that still exhibit light-entrainable CRs effectively
has eliminated the problem of the dual use of imposed light spans and signals—as an energy
source for growth and as a timing cue for the underlying clock.12-23
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