Direct examination of growing filaments to determine phosphate growth kinetics in Cladophora glomerata (L.) Kütz and Stigeoclonium tenue (Agardh) Kütz

Abstract

Semi-continuous (petri-plate) cultures were used for assessing growth kinetics in attached filaments of (Lake-Ontario derived) Cladophora glomerata and Stigeoclonium tenue relative to limiting levels of inorganic phosphate. Under enriched conditions, the filaments were allowed to produce zoospores which then attached to the base of the plastic petri-plates to produce germlings. Bottom attachment was achieved by exploiting positive phototaxis in providing bottom illumination. Six phosphate treatments, ranging from 0 to 2 µM PO4, were run for 15 days with daily replacement of medium. Cell counts were carried out on the same filaments each time, every two days, using an inverted light microscope. Specific growth rate was calculated as the doubling rate (of the number of cells per filament) per day. The Monod kinetics model was used to determine the growth kinetic coefficients. The results reinforce previous findings, derived using the Droop and Kilham variable internal stores model, that Stigeoclonium tenue outgrows Cladophora glomerata under conditions of limiting phosphate. This finding provides further evidence that the dominance of Cladophora glomerata in the North American Great Lakes may not be related strictly to the response to phosphate.