Rhythmic settling induced by temperature cycles in continuously-stirred autotrophic cultures of Euglena gracilis (Z strain).

Temperature cycles which produced synchrony of cell division in autotrophic Euglena (preceding paper, Terry and Edmunds, 1970) usually also evoked a cellular settling rhythm. The rhythm was expressed as a recurrent cycle of cell attachment to culture-vessel surfaces, with nearly the same phase angle to each of the different temperature cycles. Attachment occurred in cultures stirred rapidly enough for thorough mixing of the cell suspension, but it was possible to prevent attachment by increased agitation. The settling rhythm was entrained by temperature cycles with periods shorter than the minimum period length required to phase cell division, but the rhythm also persisted with a circadian period at constant temperature in continuous light following entrainment by 12,12 hr temperature cycles. The rhythm appeared in stationary-phase cultures exposed to either cold-synchronizing or heat-synchronizing temperature cycles, and also in growth-phase cultures exposed to cold-synchronizing cycles. The settling rhythm was generally not observable in heat-synchronized growth-phase cultures although it often appeared in the same cultures as they approached the stationary phase.