AGGREGATION TO COLONIES OF THE GREEN ALGA PEDIASTRUM: EXPERIMENTS AND SIMULATIONS

Abstract

We simulate the aggregation of zoospores of the green alga Pediastrum simplex into a planar colony — including radially outward cell orientation. The model assumptions are: cells are randomly kicked by flagella, an ellipsoidal boundary repells cells and flagella, overlapping cell surfaces lead to dissipative cell-cell repulsion, cell-cell attractive forces occur around arches close to the cell's surfaces, there is friction of the cells with the medium; the process ends by an extension of the attractive arches followed by the stop of flagellar propulsion and finally a change of cell shapes from spherical to nearly triangular with contact-inhibited horn-like extensions. These assumptions are directly observed or indirectly inferred from the literature, light microscopy (video recordings) and scanning electron microscopy. We perform calculations with cell numbers ranging from 4 to 64. Additional simulations permit to discard alternative models, including planarization via cell-cell attraction only, chemotaxis or cohesive intercellular gliding. Our model yields patterns agreeing well with symmetrical as well as with disordered natural aggregates.