Algae affecting lettuce growth in hydroponic systems

Hydroponic systems, especially those with recirculating nutrient solution, are an ideal environment for algal growth. It is not clear if this contaminant has an effect on the crop. Therefore, a greenhouse and a climate chamber experiment were carried out to monitor algal development and composition as well as their effect on crop performance. In addition, antifungal activity of algae and their aqueous extracts against root pathogens was tested. In open troughs planted with lettuce, algal density was 1.6 x 10(4) to 4 x 10(4) cells ml(-1) in the greenhouse and 1.3 x 10(6) to 3.2 x 10(6) cells ml(-1) in containers used in the climate chamber. Algal density and composition was influenced by initial density in the rainwater, temperature, and nutrient solution concentration during the experiments. The diversity of the algal community was poor. Exclusively unicellular species were observed with Chlamydomonas spp. and Scenedesmus spp. as main representatives. Lettuce fresh weight, shoot/root ratio, water and nitrogen uptake were reduced significantly in the presence of algae compared with treatments which excluded algae. Aqueous extracts prepared from algae collected from the greenhouse significantly inhibited growth of Fusarium oxysporum on slow nutrient agar but not growth of Pythium aphanidermatum. The presence of algae in containers with lettuce plants inoculated with P. aphanidermatum showed no effect on growth characteristics. It is assumed that the algae present do not release antifungal compounds. Overall, it appears that algae have more negative effects on crop yield in hydroponic systems. The adverse effect on the water supply system, which is usually cited as the main problem caused by algae, was not encountered in our experiments.