Selective effect of the herbicide DCMU on unicellular algae - A potential tool to maintain monoalgal mass culture of Nannochloropsis

The selective effect of DCMU on photosynthetic activity and growth rate was examined in several marine unicellular algae: Nannochloropsis sp. (Eustigmatohyceae), Dunaliella salina (Chlorophyceae) Isochrysis galbana (Prymnesiophyceae) and Chaetoceros sp. (Bacillariophyceae). DCMU at 10^-7 M caused an immediate decrease in the photosynthetic rate of Dunaliella and Isochrysis (about 50% inhibition), whereas 10^-6 M imposed 80% inhibition in the photosynthetic rate of Chaetoceros. In Nannochloropsis the rate was affected only when DCMU concentration exceeded 10^-6 M. Cellular growth rate of all studied algae was affected by DCMU in a similar manner to photosynthesis. The differential effect of DCMU was further examined in mixed cultures in which Nannochloropsis was cultivated together with an additional species simulating a contamination situation of a Nannochloropsis culture. When DCMU was added at concentrations higher than 10^-7 M, the growth of the competing algae significantly decreased, while Nannochloropsis maintained a relatively high growth rate. Consequently, after a growth period of 4 to 7 days a clear domination of Nannochloropsis was observed. These results demonstrate that DCMU and probably other herbicides of similar characteristics can be used effectively as a selective tool to suppress contaminating unicellular algae in open ponds in order to maintain a monoculture of Nannochloropsis.