Nonrandom structures in the locomotor behavior of Halobacterium: a bifurcation route to chaos?

Halobacteria spontaneously reverse their swimming direction about every 10-15 s. They respond to light stimuli by a transient perturbation of this rhythm. During periodic stimulation the system shows features that are known from nonlinear oscillators. Increasing stimulation frequencies cause the following phenomena: (i) the frequency of reversals follows the stimulation frequency, (ii) transition to a state where a long and a short interval occur alternatingly and further transition to four interval lengths, (iii) appearance of irregular interval sequences, which, in a two-dimensional plot of successive intervals, reveal clearly discernible structures and suggest chaotic motion. A similar series of events can be induced in the absence of periodic stimulation, when a control parameter is changed to various constant levels. The data suggest that the system is governed by deterministic dynamical laws

Role of the response oscillator in inverse responses of Halobacterium halobium to weak light stimuli.

Under certain conditions Halobacterium halobium organisms respond to a weak attractant light stimulus with a repellent response and to a weak repellent stimulus with an attractant response. The appearance of inverse responses depends on the stimulus strength, on the interval length between spontaneous reversals, and on the moment of stimulation during the interval. Although the cells are absolutely refractory to repellent stimuli for 500 ms after a reversal, repellent responses can be evoked even during that period if they are inverse responses to weak attractant stimuli. Simultaneous attractant and repellent stimuli cancel each other even when one of them leads to an inverse response, indicating that normal cellular signals occur at the site of signal integration. We postulate that the inverse responses are caused by certain properties of a cellular oscillator for which we previously postulated a role in response regulation and sensory control in halobacteria (A. Schimz and E. Hildebrand, Nature [London] 317:641-643, 1985)