Allelopathic effect of Ulva intestinalis thallus on selected Baltic cyanobacteria

Macroalgae have been found to produce active allelochemicals that inhibit of growth other organisms that compete with them for light and space. However, their allelopathic activity on Baltic cyanobacteria is still insufficiently recognised. Therefore, this study aimed to demonstrate the allelopathic effects of Baltic macroalga thallus (Ulva intestinalis) on the growth and photosynthetic activity of three bloom-forming cyanobacteria: Aphanizomenon sp., Nodularia spumigena, and Nostoc sp. This study investigated the cell count of the analysed cyanobacteria (N 105 mL-1), the maximum quantum yield of the second photosystem (PSII) in the dark (Fv/Fm), and the real quantum yield of PSII in the light (ΦPSII) (in the control and the experiments). After 7 days of exposure, the following were added: 0.01, 0.05, and 0.1 g mL-1 of U. intestinalis fresh thallus. It was found that thallus obtained from U. intestinalis had no statistically significant effect on the number of cells of the cyanobacterium Aphanizomenon sp. (at 0.05 and 0.1 g mL-1) and Nostoc sp. (at concentrations of 0.01 and 0.05 g mL−1). On the other hand, it was examined a stimulating effect of 0.01 g mL-1 of the fresh thallus on the number of Aphanizomenon sp. cells which constituted 168%, relative to the control. It was shown that the fresh thallus addition resulted in a decrease in the number of N. spumigena cells (45%, 27%, and 46% after addition of 0.01, 0.05, and 0.1 g wet weight mL−1 of fresh thallus, respectively). In experiments with Nostoc sp., the addition of U. intestinalis thallus has been a negative effect on cyanobacterial growth at 0.1 g mL−1 and constituted 97% of control. It was also found, that U. intestinalis had no allelopathic effect on fluorescence parameters of N. spumigena. All tested concentrations of thallus U. intestinalis (0.01, 0.05, and 0.1 g wet weight mL-1) stimulated the values of Fv/Fm or ΦPSII of cyanobacteria Aphanizomenon sp. and Nostoc sp. compared to the control. These studies help define the role of U. intestinalis allelopathy as a biological factor in the distribution of bloom-forming cyanobacteria in the coastal Baltic Sea region.