Elemental flux between algae and duckweeds (Lemna gibba) during competition

Abstract

Axenic Lemna gibba fronds were co-cultured with five green algal, three diatom and three cyanobacterial species and were grown separately and in mixed combination on autoclaved communal wastewater. These species were obtained from a local minipond containing wastewater covered by duckweeds. The impact of selected algal species on bioproduction of Lemna cultures was measured. The flux of elements during competition for nutrients between algae and duckweeds was examined. When the duckweed cover was incomplete, duckweeds had no impact on the multiplication of algae. Under this condition, algae showed a much stronger competitive effect for nutrients against Lemna, because algal nutrient removal was much more intensive than the nutrient removal of the fronds. The elemental composition of the water was mainly determined by algae. In Lemna-alga cocultures, the elemental concentration of the water was lowered to below 0.01 mg/l for phosphorus and for iron due to the algal activity. They were below the minimal concentrations for growth of duckweeds. Algae increased the pH of the water to above 10, which resulted in 5.5 mg/l NH 3 -N in the medium. The elemental concentration of the algal treated fronds decreased by 87 % for phosphorus and by 90 % for iron. Results suggested that during competition with algae, iron and phosphorus became a potential minimum factor for duckweeds.