ALTERATIONS IN GROWTH-RATE ASSOCIATED WITH A NORMALLY PERSISTING CIRCADIAN-RHYTHM DURING SPACEFLIGHT

Background: Biological clocks time many physiological parameters with periodicities close to 24 h; those which persist in the absence of env ironmental cues are circadian. An earlier shuttle experiment (STS-9) e xamined circadian pacemaker function and growth rate of Neurospora cra ssa and demonstrated damped rhythm amplitudes, increased variability i n period lengths and altered growth rates. Hypothesis: Postflight stud ies suggested that accelerative forces of launch could have induced rh ythm alterations. Differences in growth rate may have been due to an a lteration of metabolic rate. Methods: Race tubes inoculated with bd or csp strains were flown aboard STS-32, exposed to ambient mid-deck tem peratures throughout flight, and exposed to light only during marking procedures. Period, rhythm amplitude, and growth rate were determined and compared to orbital environmental controls (OES) and 25 degrees C ground controls (GC). Results: Unlike the previous flight experiment, the rhythm persisted quite normally. bd flight and OES cultures each d isplayed lengthened periods of a similar magnitude when compared to GC . The lengthened periods of csp flight cultures while longer than GC, were shorter than OES. Shuttle temperatures were relatively warm, howe ver the increased period length in space was greater than predicted by the known Q(10). Growth rates also increased substantially during fli ght, which could not be accounted for by thermal mechanisms alone. Con clusion: It is likely that some of the cultures may have entrained to the unexpected diurnal temperature variations; however, other cultures did not entrain, yet retained rhythmicity with increased periods. The results also suggest an increased metabolic rate during spaceflight.