Macroalgae are the source of many harmful allelopathic compounds, which are synthesized as a defense strategy against competitors and herbivores. Therefore, it can be predicted that certain species reduce aquaculture performance. Herein, the allelopathic ability of 123 different taxa of green, red, and brown algae have been summarized based on literature reports. Research on macroalgae and their allelopathic effects on other animal organisms was conducted primarily in Australia, Mexico, and the United States. Nevertheless, there are also several scientific reports in this field from South America and Asia; the study areas in the latter continents coincide with areas where aquaculture is highly developed and widely practiced. Therefore, the allelopathic activity of macroalgae on coexisting animals is an issue that is worth careful investigation. In this work, we characterize the distribution of allelopathic macroalgae and compare them with aquaculture locations, describe the methods for the study of macroalgal allelopathy, present the taxonomic position of allelopathic macroalgae and their impact on coexisting aquatic competitors (Cnidaria) and herbivores (Annelida, Echinodermata, Arthropoda, Mollusca, and Chordata), and compile information on allelopathic compounds produced by different macroalgae species. This work gathers the current knowledge on the phenomenon of macroalgal allelopathy and their allelochemicals affecting aquatic animal (competitors and predators) worldwide and it provides future research directions for this topic.

Air pollution has been a significant problem threatening human health for years. One commonly reported air pollutant is benzo(a)pyrene, a dangerous compound with carcinogenic properties. Values which exceed normative values for benzo(a)pyrene concentration in the air are often noted in many regions of the world. Studies on the worldwide spread of COVID-19 since 2020, as well as avian flu, measles, and SARS, have proven that viruses and bacteria are more dangerous to human health when they occur in polluted air. Regarding cyanobacteria and microalgae, little is known about their relationship with benzo(a)pyrene. The question is whether these microorganisms can pose a threat when present in poor quality air. We initially assessed whether cyanobacteria and microalgae isolated from the atmosphere are sensitive to changes in PAH concentrations and whether they can accumulate or degrade PAHs. The presence of B(a)P has significantly affected both the quantity of cyanobacteria and microalgae cells as well as their chlorophyll a (chl a) content and their ability to fluorescence. For many cyanobacteria and microalgae, an increase in cell numbers was observed after the addition of B(a)P. Therefore, even slight air pollution with benzo(a)pyrene is likely to facilitate the growth of airborne cyanobacteria and microalgae. The results provided an assessment of the organisms that are most susceptible to cellular stress following exposure to benzo(a)pyrene, as well as the potential consequences for the environment. Additionally, the results indicated that green algae have the greatest potential for degrading PAHs, making their use a promising bioremediation approach. Kirchneriella sp. demonstrated the highest average degradation of B(a)P, with the above-mentioned research indicating it can even degrade up to 80% of B(a)P. The other studied green algae exhibited a lower, yet still significant, B(a)P degradation rate exceeding 50% when compared to cyanobacteria and diatoms.

Macroalgae can directly restrict the growth of various phytoplankton species by releasing allelopathic compounds; therefore, considerable attention should be paid to the allelopathic potential of these organisms against harmful and bloom-forming cyanobacteria. The main aim of this study was to demonstrate for the first time the allelopathic activity of Ulva intestinalis on the growth, the fluorescence parameters: the maximum PSII quantum efficiency (F_v/F_m) and the effective quantum yield of PSII photochemistry (ΦPSII), the chlorophyll a (Chl a) and carotenoid (Car) content, and the microcystin-LR (MC-LR) and phenol content of three bloom-forming cyanobacteria, Aphanizomenon sp., Nodularia spumigena, and Nostoc sp. We found both negative and positive allelopathic effects of U. intestinalis on tested cyanobacteria. The study clearly showed that the addition of the filtrate of U. intestinalis significantly inhibited growth, decreased pigment content and F_v/F_m and ΦPSII values of N. spumigena and Nostoc sp., and stimulated Aphanizomenon sp. The addition of different concentrations of aqueous extract also stimulated the cyanobacterial growth. It was also shown that the addition of extract obtained from U. intestinalis caused a significant decrease in the MC-LR content in Nostoc sp. cells. Moreover, it the phenol content in N. spumigena cells was increased. On the other hand, the cell-specific phenol content for Aphanizomenon sp. decreased due to the addition of the filtrate. In this work, we demonstrated that the allelopathic effect of U. intestinalis depends on the target species’ identity as well as the type of allelopathic method used. The study of the allelopathic Baltic macroalgae may help to identify their possible role as a significant biological factor influencing harmful cyanobacterial blooms in brackish ecosystems.

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.