Membrane transport activity and ultradian ion flux oscillations associated with cell cycle of Thraustochytrium sp.

Abstract

Membrane transport activity associated with growth and nutritional status of a marine microheterotroph Thraustochytrium sp. was studied using non-invasive ion-selective slowly vibrating microelectrodes (the MIFE technique). Net fluxes of H ⁺ , Ca ²⁺ and Na ⁺ underwent regular changes as the cell progressed from the zoospore to sporangium stages of development. The most pronounced change was a decrease in the net H ⁺ influx, which we suggest could be associated with the changes in cytoskeletal organization required for cell cleavage and zoospore release. As cell development progressed from the zoospore stage towards maturity, non-damping endogenous ultradian oscillations (period range of several minutes) became evident. At the sporangium stage, as many as 85% of cells possessed oscillatory membrane transport activity. It is suggested that ultradian ion flux oscillations in Thraustochytrium sp. may be causally linked with cell developmental processes. Discrete Fourier transform and cross-correlation analysis revealed a close association between oscillatory patterns of H ⁺ and Na ⁺ fluxes. The possibility that these oscillations result from the rhythmical activity of a Na ⁺ /H ⁺ co-transporter located at the plasma membrane of Thraustochytrium sp. is considered. Oscillations in net Ca ²⁺ flux were apparently not linked to those in H⁺ and Na ⁺ , and are believed to be due to some other physiological processes. Periods of net H ⁺ and Na ⁺ flux oscillations were strongly dependent on the external Na ⁺ concentrations in the bathing medium. As sodium is considered to be an essential element in Thraustochytrium sp., it is suggested that the functional role of such ultradian oscillations may be their involvement in the frequency-encoding mechanism that provides developing cells with information about environment, and nutritional status in particular.