Coherent domains in the streaming cytoplasm of a giant algal cell

Abstract

Giant internodal cells of the charophyte Lamprothamnium respond to hypotonic shock with an extended action potential and transient cessation of cytoplasmic streaming. The macro-structure of streaming cytoplasm was analysed before, during, and after hypotonic shock. Streaming cytoplasm contains coherent, cloud-like macroscopic domains, whose perimeter varies from hundreds to many thousands of micrometres. Some domains avidly associate with the fluorochrome 6-carboxyfluorescein (6CF), and others do not. The 6CF-labelled domains are recognisable through many cycles of streaming, despite constantly changing irregular edges. Domain perimeters were described by a fractal dimension of 4/3, the exponent of a power law fitted to a log-log plot of domain perimeter-Area. Following hypotonic shock, the stable pattern of coherent domains enters an unstable phase of coalescence, and discrete domains subsequently amalgamate into stable, extended domains. Instability is associated with Ca2+ influx and Cl- efflux, and a large increase in cell conductance. The electrophysiological K+ state, with greatly reduced conductance, is associated with the new, amalgamated stable state. The results support a concept of cytoplasm as a sponge-like percolation cluster, undergoing transition from discrete to extended domains. Results are discussed in terms of published theories concerning co-operative behaviour of supramolecular water-ion-protein complexes © 2006 Springer.