ENDOGENOUS AND PHOTOPERIODIC DIURNAL RHYTHMS OF IN VIVO LIGHT ABSORPTION AND SCATTERING IN THE GREEN ALGA ULVA LACTUCA L

¹ McCollum Pratt Institute and Department of Biology, The Johns Hopkins University, Baltimore, Maryland 21218

Techniques of in vivo spectroscopy were employed to demonstrate the presence of rhythms of light absorption and scattering in the green thalloid alga Ulva lactuca L. maintained in artificial nutrient medium under constant photoperiod. The absorbance during photophase at 682 nm, the chlorophyll a in vivo absorption maximum, was typically two to three times that during scotophase. Prephased endogenous rhythms (in continued darkness) were observed for a time period equal to three photoperiods. The absorbance rhythm did not correlate with changes in extractable chlorophyll a or b concentration. No changes in linear dichroism were observed, indicating the rhythm of light absorption could not be explained on the basis of orientation of individual absorber molecules. In vivo light microscopy did not reveal differences in chloroplast orientation. Alternative mechanisms are discussed.