GRAPPLING WITH CONFLICT AMONG INFORMATION SOURCES IN RECONSTRUCTION OF THE EARLY EVOLUTION OF LAND PLANTS

In an assay addressing the toxicity of copper to Dictyosphaerium pulchellum , a 48 hours' exposure of axenic cultures of the alga to metal concentrations ranging from 0 to 300 mg L⁻¹ caused a significant decrease in the growth rate and cell chlorophyll content. Extracellular mucilage remotion by controlled sonication before exposure to copper emphasized chlorophyll content reduction, but not cellular density diminution. Because mucilage acts as a cation interchanger, these results suggested that, during at least the first 48 hours, division rates might be affected by lower Cu⁺² concentrations than chlorophyll content. Ultrastructural observations showed that the normal organization of the chloroplasts was altered; they presented dilated lamellae, irregularly arranged with practically no stacking. Copper precipitates were mainly observed outside the plasmalemma or within the extracellular mucilage. Electron probe microanalysis confirmed these observations, showing only minor precipitates inside the cells.