Allelopathic effect on a nutrient-limited phytoplankton species

For aquatic systems, studies on allelopathic interactions among phytoplankton have increased over recent years, with the main focus on the role of the donor organism. In this study, we report on the response of a target organism to allelochemicals and whether this response was affected by stress conditions (nutrient limitation). We exposed the diatom Thalassiosira weissflogii, grown under different nitrogen (N) and phosphorus (P) conditions (NP, –N, or –P), to single or daily additions of a cell-free filtrate of Prymnesium parvum (grown with no nutrient limitation). When we exposed T. weissflogii to a single addition of filtrate, all 3 treatments were inhibited by P. parvum. However, T. weissflogii NP was the most resistant, while T. weissflogii –N showed the highest sensitivity to P. parvum filtrate, followed by T. weissflogii –P. When T. weissflogii was exposed to daily additions of P. parvum, the degree of inhibition of all T. weissflogii treatments was higher than when only 1 initial addition was made. In this case, even the treatment that had the highest resistance (T. weissflogii NP) was not only inhibited by the filtrate, but also showed a decrease in cell numbers. Nevertheless, T. weissflogii –N was still more sensitive than the other treatments. Therefore, nutrient-limiting conditions may increase allelopathic effects, by making the target more susceptive to allelopathic compounds. Under these conditions, allelopathy may play a strong role in phytoplankton competition, especially in natural environments where the allelochemicals are continuously released and, thus, the target species do not have time to recover.