81 Annales Universitatis Paedagogicae Cracoviensis Studia Naturae, 6: 81–94, 2021, ISSN 2543-8832 DOI: 10.24917/25438832.6.5 Gracjana Budzałek, Sylwia Śliwińska-Wilczewska* Institute of Oceanography, University of Gdansk, Gdynia, Poland, *ocessl@ug.edu.pl Allelopathic effect of macroalgae Fucus vesiculosus (Ochrophyta) and Coccotylus brodiei (Rhodophyta) on the growth and photosynthesis performance of Baltic cyanobacteria Introduction �e term “allelopathy” was introduced to science by Hans Molisch (Molisch, 1937), who used this appellation to describe the chemical interaction between plants. With the de- velopment of allelopathic research, this de�nition includes negative and positive e�ects of compounds secreted by various plant and animal organisms (Rice, 1984). Inderjit and Dakshini (1994) have also shown allelopathic activity in aquatic environments be- tween cyanobacteria (Aphanizomenon gracile (Lemmerm.) Lemmerm.) and microalgae (Pediastrum boryanum (Turpin) Menegh., Cosmarium lundellii Delp., Micrasterias sp.) (Guiry, Guiry, 2021). With the end of the 20th century, the de�nition of allelopathy was standardised by the International Allelopathy Society (IAS, 1996), describing any process in which bioactive metabolites (so-called allelochemicals or allelopathic compounds) se- creted by organisms a�ect the development of other plant and animal species (Legrand et al., 2003). Currently, allelopathy is considered to be a unique strategy to deter or eliminate competitors and predators coexisting in the same ecosystem (Granéli et al., 2008; Gomes et al., 2017; Śliwińska-Wilczewska et al., 2021). In aquatic ecosystems, the allelopathy of donor organisms depends on the pro- duction and secretion of active allelopathic compounds and their e�cient dispersal to target organisms a�ected by these compounds (Śliwińska-Wilczewska et al., 2021). For photoautotrophic organisms, the production of active allelopathic compounds becomes a crucial adaptation in achieving a competitive advantage over other primary producers (Legrand et al., 2003). Red and brown macroalgae are the important source of many biologically active metabolites (El Gamal, 2010). For instance, the tichocarpols A and B isolated from the red alga Tichocarpus crinitus (Gmel.) Rupr. exhibit antifeedant activity against the sea urchin (Ishii et al., 2004). On the other hand, dictyterepenoids A and G ra cj an a B ud za łe k, S yl w ia Ś liw iń sk a- W ilc ze w sk a 82 B isolated from the brown algae Dilophus okamurae E.Y.Dawson display antifeedant activity against the abalone (Suzuki et al., 2002). In benthic communities, organisms are o�en located at shorter distances from each other. �us, direct contact between donor and target cells may occur. �is also leads to constant competition for space, nutrients, and light. Competition for resources may then be crucial in the occurrence of allelopathic interactions (Gross, 2003). It has been found that target organisms can be inhibited, resisted, or even stimulated by allelopathic compounds present in water (Suikkanen et al., 2004; Śliwińska-Wilczewska et al., 2021). Consequently, allelopathic interactions may contribute to changes in phytoplankton and phytobenthic structure in aquatic ecosystems. Compared to the intensive and extensive research on allelopathic interactions among terrestrial plants, knowledge of allelopathy in aquatic plant communities is still not fully demonstrated. �is is partly because it is di�cult to provide direct evidence for allelopathic interactions in water ecosystems under natural conditions. It is challenging to study allelopathic e�ects among aquatic organisms under natural conditions because factors such as competition for nutrients and light, temperature, and pH change can mask allelopathic e�ects (Legrand et al., 2003). �erefore, it is important that attempts to identify allelopathic interactions among aquatic organisms be conducted in a con- trolled system, such as by conducting a series of laboratory experiments. �erefore, in the present study, the allelopathic e�ect of aqueous extracts from the Baltic red alga Coccotylus brodiei (Turner) Kütz. and the brown alga Fucus vesiculosus L. (Guiry, Guiry, 2021) on the growth and chlorophyll �uorescence of two bloom-forming cyanobacteria from the genus Aphanizomenon and Nostoc were investigated. Material and methods Material and place of sampling �e material used in the experiments consisted of strains of Baltic cyanobacteria Apha- nizomenon sp. (CCBA-69) and Nostoc sp. (CCBA-81) (Fig. 1). Strains of cyanobacteria were isolated from the natural phytoplankton communities of the coastal waters of the Gulf of Gdansk (southern Baltic Sea: 54°30’53.7”N 18°54’23.5”E). Currently, these strains are maintained as monocultures in the Culture Collection of Baltic Algae (CCBA) at the Laboratory of Marine Plant Ecophysiology at the University of Gdańsk (Latała et al., 2006). �e macroalgae used in the study were collected from the coastal zones of the Gulf of Gdańsk (54°30’08.7”N 18°33’32.3”E). Determination of macroalgae based on the examination of morphological features. �e herbarium sheets were deposited at the Institute of Oceanography, University of Gdansk (Poland) and are available on the website (https://zielnik.ug.edu.pl/en/home/). 83 Fig. 1. Photographs of the cyanobacterial culture in 25-mL Erlenmeyer �asks (a) of strain: CCBA-69 Aph- anizomenon sp. (Aa) and CCBA-81 Nostoc sp. (Ba); light microscope photographs (b) of cyanobacterial strain; scale = 10 μm, (Photo. S. Śliwińska-Wilczewska) Cyanobacteria cultivation �e studied cyanobacteria were cultured on sterile mineral medium f/2 (Guillard, 1975) prepared with Baltic Sea water �ltered through glass �ber �lters (Whatman GF/C) and autoclaved. �e salinity was 8 PSU as measured with a salinometer (inoLab Cond Level 1, Weilheim in Oberbayern, Germany). �e cyanobacterial strains used in the experi- ments was maintained in 25mL glass Erlenmeyer �asks. Cyanobacteria were cultured at a PAR intensity of 10 μmol photons m–2s–1 (16:8 h light: dark cycle) and a temperature of 18°C. Photosynthetically active irradiance (PAR) was measured using a  quantum meter (LI-COR, Nebraska, USA). �e light sources used in the experiment were lamps (Cool White 40W, Sylvania, USA). �e culture was acclimated to these conditions for 7 days, and these growth conditions were used for the experiments. A llelopathic effect of m acroalgae Fucus vesiculosus (O chrophyta) and Coccotylus brodiei (R hodophyta) on the grow th and photosynthesis perform ance of B altic cyanobacteria G ra cj an a B ud za łe k, S yl w ia Ś liw iń sk a- W ilc ze w sk a 84 Determination of the allelopathic e�ect of an aqueous extract obtained from macroalgae �e allelopathic e�ect of Baltic macroalgae was investigated by adding di�erent concen- trations of aqueous extract to the target cyanobacteria monocultures. �e allelopathic e�ect was examined according to the method proposed by Złoch et al. (2018) with modi�cations. First, dried macroalgae material was mechanically ground to powder in a mortar. In the next step, 50 g of dry matter was weighed and �ltered in 50 mL of culture medium. Finally, the extract was �ltered with a glass �bre �lter (Whatman GF/C) using a vacuum pump (400 mbar) to remove suspended particles. �e concentration of major nutrients in the control and experimental samples were adjusted to the same level as in the f/2 standard. �erefore, the in�uence of nutrients, micronutrients and vitamins on the experimental result can be excluded. Target cyanobacterial strains were maintained in 25-mL Erlenmeyer �asks. In all experiments, the starting concentration of chlorophyll a in cyanobacterial cultures was 0.4 μg chl a mL–1. �e experiment was conducted by adding 100, 500, and 1000 µL of macroalgae extract to Erlenmeyer �asks containing 20 mL of the target cyanobacteria. �us, the �nal extract concentrations were: 5, 25, and 50 µL mL–1. Control cultures were prepared analogously, but f/2 medium was added instead of extract at 100, 500, and 1000 µL. A�er 7 days of the exposure, the cells concentration as well as chlorophyll a �uo- rescence parameters were determined. Each experiment was performed in triplicate. Determination of cyanobacterial number of cells �e number of cells (N) in Aphanizomenon sp. and Nostoc sp. cultures was estimated with previously determined linear correlations between cell abundance (N mL–1) and optical density (OD). N was counted using a Bürker chamber (48 squares per count) and light microscope following a procedure according to Guillard and Sieracki (2005), and the OD was measured spectrophotometrically at 750 nm with a Multiskan GO UV- VIS spectrophotometer (�ermo Scienti�c, Massachusetts, USA). �e linear correlation between N and OD for Aphanizomenon sp. was described by Śliwińska-Wilczewska et al. (2017) whereas for Nostoc sp. was examined by Budzałek et al. (2018). OD meas- urements were performed on the 7th days of the experiment and control and converted to cyanobacteria cells. Determination of the chlorophyll a �uorescence In the conducted experiments, the Pulsed Amplitude Modulation (PAM) method was used to measure the chlorophyll a  �uorescence (FMS1, Hansatech). �is method is widely used to measure chlorophyll a �uorescence in cyanobacteria, both in the lab- oratory and in the natural environment (Schreiber et al., 1995; Campbell et al., 1998). Samples were taken for chlorophyll �uorescence analysis a�er the 7th days of the ex- 85 periment. About 5 mL of target cyanobacteria were �ltered through 13-mm glass �ber �lters (Whatman GF/C). In the next step, the �lters were placed in holders. �e samples were kept in the dark for 5 min before measurement. In this study, the maximum PSII quantum e�ciency (Fv/Fm) and the e�ective quantum yield of PSII photochemistry (ΦPSII) were determined (Campbell et al., 1998). Statistical analysis To con�rm the e�ect of the extracts obtained from macroalgae on the number of cells and chlorophyll a  �uorescence parameters of target cyanobacteria, a  t-test was performed at three levels of signi�cance: *p < 0.05, **p < 0.01, ***p < 0.001. Data are reported as the means ± standard deviations (SD). �e statistical analyses were per- formed using Statistica® 13.1 so�ware. Results Fig. 2. �e number of cells (N 105 mL–1) of Aphanizomenon sp. (A) and Nostoc sp. (B) in controls and cultures to which were added: 5 (a), 25 (b), and 50 (c) (µL mL–1) of aqueous extract obtained from Fucus vesiculosus L. a�er 7 days of the experiment. �e values shown are mean (n = 3, mean ± SD). * indicates statistically signi�cant di�erences compared to control based on t-test at: * p < 0.05; ** p < 0.01; *** p < 0.001 Allelopathic e�ect of aqueous extract on cyanobacterial abundances In this work, the number of cells (N 105 mL–1) of Aphanizomenon sp. and Nostoc sp. in controls and cultures to which were added: 5, 25, and 50 (µL mL–1) of aqueous extract obtained from Fucus vesiculosus and Coccotylus brodiei a�er 7 days of the experiment were determined. It was found that aqueous extracts obtained from F. vesiculosus had no statistically signi�cant e�ect on the number of cells of the cyanobacteria Aphanizomenon sp. and Nostoc sp. (Fig. 2A–B). On the other hand, it was examined a stimulating e�ect of 5, 25, and 50 µL mL–1 of the aqueous extract obtained from C. brodiei on the number of A llelopathic effect of m acroalgae Fucus vesiculosus (O chrophyta) and Coccotylus brodiei (R hodophyta) on the grow th and photosynthesis perform ance of B altic cyanobacteria G ra cj an a B ud za łe k, S yl w ia Ś liw iń sk a- W ilc ze w sk a 86 Nostoc sp. cells which constituted 108% (p < 0.01), 140% (p < 0.01), and 147% (p < 0.001), respectively, relative to the control treatment (Fig. 3 Ba-Bc). Moreover, the C. brodiei extracts had no signi�cant e�ect on the growth of Aphanizomenon sp. (Fig. 3Aa–Ac). Fig. 3. �e number of cells (N 105 mL–1) of Aphanizomenon sp. (A) and Nostoc sp. (B) in controls and cultures to which were added: 5 (a), 25 (b), and 50 (c) (µL mL–1) of aqueous extract obtained from Coccotylus brodiei (Turner) Kütz. a�er 7 days of the experiment. �e values shown are mean (n = 3, mean ± SD). * indicates statistically signi�cant di�erences compared to control based on t-test at: * p < 0.05; ** p < 0.01; *** p < 0.001 Allelopathic e�ect of an aqueous extract on �uorescence parameters �e values of the �uorescence parameter Fv/Fm (the maximum PSII quantum e�ciency) and ΦPSII (the e�ective quantum yield of PSII photochemistry) for Aphanizomenon sp. and Nostoc sp. in control and cultures to which were added a di�erent concentrations (5, 25, and 50 µL mL–1) of aqueous extract obtained from F. vesiculosus and C. brodiei a�er 7 days of experiment were examined. A stimulating e�ect of concentrations of 5 and 25 µL mL–1 of F. vesiculosus extract on the values of �uorescence parameters of cyanobacteria Aphanizomenon sp. were observed and in this conditions these parameters constituted 119% (t-test, p < 0.01) and 113% (t-test, p < 0.05), respectively, for Fv/Fm and 122% (t-test, p < 0.001) and 118% (p < 0.001), respectively, for ΦPSII, relative to the control culture (Fig. 4Aa–Ab). Moreo- ver, a stimulatory e�ect of 25 and 50 µL mL–1 of F. vesiculosus extract on �uorescence parameters Fv/Fm and ΦPSII of tested cyanobacterium Nostoc sp. was observed. It was found that the values of Fv/Fm was 109% (t-test, p < 0.01) at 25 µL mL –1 and 109% (t-test, p < 0.05) at 50 µL mL–1, relative to the control culture (Fig. 4Bb–Bc). Furthermore, in the same experiment conditions, the values of ΦPSII were 111% (t-test, p < 0.01) and 87 116% (t-test, p < 0.001), respectively, compared to control (Fig. 4Bb–Bc). On the other hand, an inhibitory e�ect of 50 µL mL–1 of F. vesiculosus extract on Fv/Fm was noted for cyanobacteria Aphanizomenon sp. which was 92% (t-test, p < 0.01) relative to the control (Fig. 4Ac). Similarly, an inhibitory e�ect of this brown alga extract at a concentration of 5 µL mL–1 on Nostoc sp. was observed. In this condition, the Fv/Fm constituted 92% (t-test, p < 0.01) relative to the control (Fig. 4Ba). Fig. 4. �e values of the �uorescence parameter Fv/Fm and ΦPSII for Aphanizomenon sp. (A) and Nostoc sp. (B) in control and cultures to which were added: 5 (a), 25 (b), and 50 (c) (µL mL–1) of aqueous extract obtained from Fucus vesiculosus L. a�er 7 days of the experiment. �e values shown are mean (n = 3, mean ± SD). * indicates statistically signi�cant di�erences compared to control based on t-test at: * p < 0.05; ** p < 0.01; *** p < 0.001 It was found a stimulating e�ect of concentrations of 5 µL mL–1 of C. brodiei ex- tract on the Fv/Fm and ΦPSII parameters of cyanobacteria Aphanizomenon sp. In these conditions these parameters constituted 133% (t-test, p < 0.001) and 130% (t-test, p < 0.001), respectively, relative to the control culture (Fig. 5Aa). A stimulatory e�ect of 5 and 25 µL mL–1 of C. brodiei extract on �uorescence parameter Fv/Fm of Nostoc sp. was also examined. It was found that the values of Fv/Fm was 147% (t-test, p < 0.001) and 106% (t-test, p < 0.01), respectively (Fig. 5Ba–Bb). Furthermore, a stimulating e�ect of concentrations of 5, 25, and 50 µL mL–1 of C. brodiei extract on the ΦPSII values of cyanobacteria Nostoc sp. was observed and in these conditions these parameters constituted 164% (t-test, p < 0.001), 121% (t-test, p < 0.001), and 107% (t-test, p < 0.001), respectively, relative to the control culture (Fig. 5Ba–Bc). On the other hand, an inhibitory e�ect of 25 and 50 µL mL–1 of C. brodiei extract on Fv/Fm was demon- strated for Aphanizomenon sp. which was 72% (t-test, p < 0.001) and 93% (t-test, p < 0.01), respectively, of the control (Fig. 5Ab–Ac). Furthermore, an inhibitory e�ect of the highest concentration of the C. brodiei extract (50 µL mL–1) on the ΦPSII values of the cyanobacterium Aphanizomenon sp. was noted and constituted 88% (p < 0.001) of control (Fig. 5Ac). A llelopathic effect of m acroalgae Fucus vesiculosus (O chrophyta) and Coccotylus brodiei (R hodophyta) on the grow th and photosynthesis perform ance of B altic cyanobacteria G ra cj an a B ud za łe k, S yl w ia Ś liw iń sk a- W ilc ze w sk a 88 Fig. 5. �e values of the �uorescence parameter Fv/Fm and ΦPSII for Aphanizomenon sp. (A) and Nostoc sp. (B) in control and cultures to which were added: 5 (a), 25 (b), and 50 (c) (µL mL–1) of aqueous extract obtained from Coccotylus brodiei (Turner) Kütz. a�er 7 days of the experiment. �e values shown are mean (n = 3, mean ± SD). * indicates statistically signi�cant di�erences compared to control based on t-test at: * p < 0.05; ** p < 0.01; *** p < 0.001 Discussion Allelopathic activity of red and brown algae on cyanobacteria growth In the present study, the allelopathic e�ect of Baltic red alga Coccotylus brodiei and brown alga Fucus vesiculosus extract on growth and chlorophyll �uorescence of two cyanobacteria Aphanizomenon sp. and Nostoc sp. was investigated. It was noted that aqueous extracts obtained from F. vesiculosus had no e�ect on the change in cell concentrations of the cyanobacteria Aphanizomenon sp. and Nostoc sp. Additionally, there was no signi�cant e�ect of extract from C. brodiei on the cell abundance of cy- anobacteria Aphanizomenon sp. Only a stimulating e�ect of each of the three tested concentrations (5, 25, and 50 µL mL–1) of the aqueous extract of C. brodiei on the cell abundance of Nostoc sp. was observed. It is worth noting here that the slightest e�ect was observed when the allelopathic e�ect was tested at the lowest extract concentration (5 µL mL–1), while the ,most substantial e�ect was observed at the highest tested con- centration of 50 µL mL–1. It may be due to the fact that the extract obtained from the dry thallus of tested macroalgae contained certain mineral compounds that stimulated the growth of Baltic cyanobacteria. It should be emphasized here that there are very few studies on the allelopathic activity of brown and red algae on associated cyanobacteria and microalgae species. Kakisawa et al. (1988) showed inhibition or no signi�cant e�ect of the brown alga Cladosiphon okamuranus Tokida which depended on the target species (Cyanobacte- ria: Microcyctis wesenbergii (Komárek) Komárek ex Komárek, Oscillatoria raciborskii Woloszynska; Rhodophyta: Cyanidium caldarium (Tilden) Geitler; Bacillariophyta: Aulacosira ambiqua (Grunow) Sim., Chaetoceros debilis P.T. Cleve, Coscinodiscus granii 89 L.F. Gough, Skeletonema costatum (Greville) P.T. Cleve, Tabellaria �occulosa (Roth) Kütz.; Miozoa: Gymnodinium nagasakiense H. Takayama & M. Adachi, G. sanguineum K. Hirasaka, Heterocapsa triquetra (Ehrenb.) F. Stein, Prorocentrum micans Ehrenb.; Ochrophyta: Chattonella antiqua (Y. Hada) C. Ono, C. marina (Subrahmanyan) Y. Hara & M. Chihara, Olisthodiscus luteus N. Carter, Heterosigma akashiwo (Y. Hada) Y. Hada ex Y. Hara & M. Chihara; Haptophyta: Cricosphaera rosco�ensis (P.A. Dangeard) Gayral & Fresnel; Cryptophyta: Cryptomonas sp.; Chlorophyta: Hafniomonas reticulata (Korshikov) Ettl & Moestrup, Nephroselmis sp., Pterosperma cristatum Schiller, Plym- nesium parvum N. Carter, Pyramimonas sp., Tetraselmis cordiformis (H.J. Carter) F. Stein, Tetraselmis chui Butcher, Chlamydomonas augustae Skuja, Chlamydomonas sp., Chlorella pyrenoidosa H. Chick, Chlorosarcinopsis sp., Chlorosarcinopsis delicata Shin Watanabe, Haematococcus lacustris (Girod-Chantrans) Rosta�nski, Oltmannsiella sp., Scenedesmus quadricauda (Turpin) Bréb. in Bréb. & Godey, Volvox aureus Ehrenb., Closterium acerosum Ehrenb. ex Ralfs, and also Euglenozoa: Euglena gracilis G.A. Klebs, Eutreptia sp.) (Guiry, Guiry, 2021). On the other hand, Nagayama et al. (2003) investigated the e�ect of the extract of brown alga Ecklonia kurome Okamura on bloom-forming dino�agellates Karenia mikimotoi (Miyake & Kominami ex Oda) Gert Hansen & Moestrup, Cochlodinium polykrikoides Margalef, and Chattonella antiqua. �e authors showed a very high inhi- bition of dino�agellate growth, morphological changes, and even complete cell death. In turn, Suzuki (1998) studied the inhibitory e�ect of extract obtained from red alga Lithophyllum sp. on dino�agellate Heterosigma akashiwo. Wang et al. (2007) investigated the inhibitory e�ects of extract, �ltrate, and live thallus of the brown alga Sargassum thunbergii (Mertens ex Roth) Kuntze and the red alga Corallina pilulifera Postels & Ruprecht on the growth of two dino�agellates Heterosigma akashiwo and Alexandrium tamarense (Lebour) Balech. Live material from both macroalgae caused lysis of H. akashiwo cells. Growth of A. tamarense was inhibited, but cells were not lysed. �e �ltrate caused an overall decrease in both macroalgae, but only the �ltrate from Corallina caused lysis of H. akashiwo cells. �e extract caused the least e�ect on the cell concentration of the tested dino�agellates. An exception was an experiment where the in�uence of S. thunbergii �ltrate on H. akashiwo was tested where cells were lysed a�er the �rst day of culture. Later, Lu et al. (2011) demonstrated both inhibitory and stimulatory e�ects of the extract obtained from red alga Gracilaria lemaneiformis (Bory) Greville on the diatom Skeletonema costatum. To the best of the author’s knowledge, there are no more papers con�rming the allelopathic activity of brown and red algae on phytoplankton representatives. �e results obtained in this work constitute an important contribution to the knowledge on the allelopathic activity of Baltic red and brown algae on certain bloom-forming species of �lamentous cyanobacteria. A llelopathic effect of m acroalgae Fucus vesiculosus (O chrophyta) and Coccotylus brodiei (R hodophyta) on the grow th and photosynthesis perform ance of B altic cyanobacteria G ra cj an a B ud za łe k, S yl w ia Ś liw iń sk a- W ilc ze w sk a 90 E�ects of allelopathic compounds produced by Baltic macroalgae on photosynthesis performance Analysis of chlorophyll a �uorescence parameters allows for the assessment of photo- synthetic activity (Machado et al., 2015; Song et al., 2017). Fluorescence measurements are a useful tool in physiological studies and are a highly sensitive method of studying photosynthetic reactions in cyanobacteria (Campbell et al., 1998). In this study, we investigated the e�ect of the allelopathic e�ect of macroalgal extracts on the maximum PSII quantum e�ciency (Fv/Fm) and the e�ective quantum yield of PSII photochemistry (ΦPSII). It was found that Fucus vesiculosus and Coccotylus brodiei had the allelopathic e�ect on �uorescence parameters of tested cyanobacteria. �e low and medium concentrations of aqueous extract (5 and 25 µL mL–1) from F. vesiculo- sus stimulated the values of Fv/Fm and ΦPSII of cyanobacterium Aphanizomenon sp. compared to the control. �e inhibitory e�ect of the highest concentration of aqueous extract (50 µL mL–1) on the Fv/Fm parameter of Aphanizomenon sp. was demonstrated. Moreover, a stimulating e�ect of medium and high extracts concentrations of F. vesic- ulosus on cyanobacterium Nostoc sp. on the �uorescence parameters Fv/Fm and ΦPSII was observed. In addition, the inhibitory e�ect of this brown alga at the lowest extract concentration on Fv/Fm of Nostoc sp. was demonstrated. On the other hand, the stimu- latory e�ect of the lowest extract concentration (5 µL mL–1) obtained from C. brodiei on the �uorescence parameter Fv/Fm and ΦPSII of cyanobacterium Aphanizomenon sp. was noted. In addition, the inhibitory e�ect of 25 and 50 µL mL–1 extracts on �uorescence parameters of this cyanobacterium was demonstrated. It was also shown that all tested extract concentrations obtained from C. brodiei stimulated the ΦPSII parameter of Nostoc sp. In our study it has been demonstrated that brown alga F. vesiculosus and red alga C. brodiei can release some allelopathic substances which a�ect the �uorescence parameters of cyanobacteria Nostoc sp. and Aphanizomenon sp. Similarly, as in the case of cyanobacterial growth, the stimulation of the �uorescence parameters may be caused by additional mineral compounds contained in the macroalgal extracts. Moreover, the high values of the �uorescence parameters indicate a relatively high potential e�cien- cy of photosystem II in the studied cyanobacteria. �e low level of these parameters indicates certain disturbances in the photosynthesis process. �ree years earlier, Budzałek et al. (2018) demonstrated that the dry powder of Ulva intestinalis L. contains water-soluble allelochemical(s) is capable of restricting the �uorescence parameter Fv/Fm of cyanobacterium Nostoc sp. �e authors noted that the highest decrease in Fv/Fm for Nostoc sp. was observed a�er the �rst, third and seventh day of the experiment, a�er the addition of 100 µL mL–1 extracts obtained from U. inestinalis with a magnitude of 69%, 59%, and 49% respectively, compared to the control treatment. To the best of the author’s knowledge, no more works con�rm the allelopathic e�ects of macroalgal extracts on the �uorescence parameter Fv/Fm of cyanobacteria. Moreover, 91 this work is the �rst to indicate the allelopathic e�ect of macroalgal aqueous extracts on the e�ective quantum yield of PSII photochemistry (ΦPSII) of Baltic cyanobacteria. �erefore, these studies indicate the need to study the photosynthesis performance in target organisms that are exposed to contact with Baltic macroalgae in more detail. �e characterisation of the allelopathic compounds is a time-consuming task. Some studies have described novel secondary metabolites, produced by marine red (Rhodo- phyta) and brown (Ochrophyta) macroalgae, which have signi�cant biological activity on target organisms (El Gamal, 2010) however, to the best of the authors knowledge, there is no information about the allelopathic compounds produced by F. vesiculosus and C. brodiei. 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In addition, macroalgae have been found to pro- duce active metabolites that a�ect other organisms that compete with them for nutrients. However, the allelopathic activity of Baltic red and brown macroalgae on �lamentous cyanobacteria is still insu�ciently understood. �erefore, the main objective of this study was to demonstrate and compare the allelopathic e�ects of macroalgae Fucus vesiculosus L. and Coccotylus brodiei (Turner) Kütz. on the growth and photo- synthetic activity of two Baltic cyanobacteria Aphanizomenon sp. and Nostoc sp. It was found a stimulating e�ect of di�erent concentrations (5, 25, and 50 µL mL–1) of the aqueous extract obtained from C. brodiei on the number of cells of Nostoc sp. which constituted 108%, 140%, and 147%, respectively, relative to the control treatment. On the other hand, extracts obtained from F. vesiculosus had no statistically signi�- cant e�ect on the number of cells of the cyanobacteria Aphanizomenon sp. and Nostoc sp. Moreover, the C. brodiei extracts had no signi�cant impact on the growth of Aphanizomenon sp. Furthermore, Baltic macroalgae F. vesiculosus and C. brodiei was able to exert allelopathic e�ects on photosynthesis performance of Nostoc sp. and Aphanizomenon sp. and compounds produced by them had inhibitory, stimulatory, or no signi�cant e�ect on the maximum PSII quantum e�ciency (Fv/Fm) and the e�ective quantum yield of PSII photochemistry (ΦPSII). �e results obtained in this work constitute an important contribution to the knowledge on the allelopathic activity of Baltic red and brown algae on certain bloom-forming species of �lamentous cyanobacteria. Key words: allelopathy, aqueous extract, brown algae, cyanobacteria, �uorescence, growth, macroalgae, red algae Received: [2021.04.29] Accepted: [2021.07.06] A llelopathic effect of m acroalgae Fucus vesiculosus (O chrophyta) and Coccotylus brodiei (R hodophyta) on the grow th and photosynthesis perform ance of B altic cyanobacteria G ra cj an a B ud za łe k, S yl w ia Ś liw iń sk a- W ilc ze w sk a 94 Oddziaływanie allelopatyczne Fucus vesiculosus (brunatnica) i Coccotylus brodiei (krasnorost) na wzrost oraz aktywność fotosyntetyczną bałtyckich sinic Streszczenie Aktywność allelopatyczna w ekosystemach wodnych zależy od produkcji i uwalniania związków allelo- patycznych oraz ich skutecznego rozprzestrzeniania się w środowisku. Stwierdzono, że makroglony wy- twarzają aktywne metabolity, które wpływają na inne organizmy, konkurując z nimi o światło i składniki odżywcze. Jednak aktywność allelopatyczna bałtyckich krasnorostów i brunatnic na nitkowate sinice jest nadal niedostatecznie poznana. Dlatego głównym celem niniejszej pracy było wykazanie i porównanie ak- tywności allelopatycznej makroglonów Fucus vesiculosus L. (brunatnica) i Coccotylus brodiei (Turner) Kütz. (krasnorost) na wzrost i aktywność fotosyntetyczną dwóch bałtyckich sinic Aphanizomenon sp. i Nostoc sp. W pracy stwierdzono stymulujący wpływ różnych stężeń (5, 25 i 50 µL mL–1) wodnego ekstraktu otrzy- manego z C. brodiei na liczebność komórek Nostoc sp., które wynosiły odpowiednio: 108%, 140% i 147%, w stosunku do grupy kontrolnej. Z drugiej strony ekstrakty uzyskane z  F. vesiculosus nie miały istotnego statystycznie wpływu na liczebność komórek sinic Aphanizomenon sp. i Nostoc sp. Wykazano także, że eks- trakty z C. brodiei nie miały istotnego wpływu na wzrost Aphanizomenon sp. Ponadto bałtyckie makroglony F. vesiculosus i C. brodiei wpływały allelopatycznie na aktywność fotosyntetyczną u Nostoc sp. i Aphanizomenon sp., a wydzielane przez nie związki wykazywały hamujący, stymulujący lub brak wpływu na maksymalną wydajność kwantową drugiego fotosystemu (PSII) w ciemności (Fv/Fm) oraz na rzeczywistą wydajność kwantową PSII w świetle (ΦPSII). Wyniki uzyskane w niniejszej pracy stanowią ważny wkład w stan wiedzy na temat aktywności allelopatycznej bałtyckich krasnorostów i brunatnic na wybrane gatunki nitkowatych sinic, zdolnych do tworzenia masowych zakwitów. Słowa kluczowe: allelopatia, brunatnice, ekstrakt wodny, �uorescencja, krasnorosty, makroglony, sinice, wzrost Information on the authors Gracjana Budzałek �e �eld of her interest is allelopathic interactions between macroalgae and cyanobacteria. In her resear- ch She is focusing mostly on Baltic species. She is investigating the in�uence of allelopathic compounds produced by macroalgae on bloom-forming cyanobacteria. Sylwia Śliwińska-Wilczewska https://orcid.org/0000-0002-3147-6605 She is interested in allelopathy of cyanobacteria and microalgae; in particular of picocyanobacteria Syne- chococcus sp. Allelopathy plays an important role in interspeci�c competition and contributes to cyano- bacterial bloom maintenance. In her study, the in�uence of allelochemicals on the growth, chlorophyll �uorescence and photosynthesis irradiance curves of di�erent phytoplankton species was investigated. She is also investigating the in�uences of environmental factors on produced allelopathic compounds on algae and cyanobacteria.