Membrane transport activity and ultradian ion flux oscillations associated with cell cycle of <i>Thraustochytrium</i> sp

<p dir="ltr">Membrane transport activity associated with growth and nutritional status of a marine microheterotroph <i>Thraustochytrium</i> 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 <i>Thraustochytrium</i> 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 <i>Thraustochytrium</i> 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 <i>Thraustochytrium</i> 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.</p>