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Picocyanobacteria are extremely important organisms in the world's oceans and freshwater ecosystems. They play an essential role in primary production and their domination in phytoplankton biomass is common in both oligotrophic and eutrophic waters. Their role is expected to become even more relevant with the effect of climate change. However, this group of photoautotrophic organisms still remains insufficiently recognized. Only a few works have focused in detail on the occurrence of massive blooms of picocyanobacteria, their toxicity and allelopathic activity. Filling the gap in our knowledge about the mechanisms involved in the proliferation of these organisms could provide a better understanding of aquatic environments. In this review, we gathered and described recent information about allelopathic activity of picocyanobacteria and occurrence of their massive blooms in many aquatic ecosystems. We also examined the relationships between climate change and representative picocyanobacterial genera from freshwater, brackish and marine ecosystems. This work emphasizes the importance of studying the smallest picoplanktonic fractions of cyanobacteria.Keywords: allelopathy; allelochemicals; climate change; cyanotoxins; picocyanobacteria; picoplankton; blooms; secondary metabolites Key Contribution: We have updated a review of the literature dealing with allelopathic activity of picocyanobacteria; their toxicity; occurrence of their massive blooms and the relationships between climate change and representative picocyanobacterial genera from aquatic ecosystems.

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The impact of environmental factors and mode of action of allelopathic compounds produced by picocyanobacteria on microalgae remains unknown. Therefore, it is essential to characterize allelopathic interactions under different experimental conditions. In this study, the influence of allelopathic activity on the diatom Navicula perminuta was investigated by the addition of cell-free filtrate of picocyanobacterium Synechococcus sp. cultures grown under different light, temperature, and salinity conditions. Temperature affected the most the donor cyanobacterium by increasing its allelopathic activity. The highest decline in growth, fluorescence parameter F v /F m and maximal photosynthesis P m were observed after the addition of cell-free filtrate obtained from Synechococcus sp. grown at 190 mmol photons Á m À2 Á s À1 , 25°C and 8 PSU. These studies indicate for the first time that the picocyanobacterium Synechococcus sp. affect coexisting diatom N. perminuta negatively. It was examined that the Synechococcus sp. reveals allelopathic activity on the photosynthesis and chlorophyll fluorescence, which results in the inhibition of growth of analyzed microalgae. These findings indicate that the production of allelopathic substances by picocyanobacterium is regulated by the availability of environmental factors such as light, temperature, and salinity. Moreover, these results demonstrated that variation in light intensity, water temperature, and salinity should be considered when estimating the potential effects of cyanobacterial allelopathy in aquatic environments.

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Airborne cyanobacteria and microalgae are commonly found in the atmosphere and may pose a serious human health risk. This study presents an innovative investigation of the washout efficiency of airborne cyanobacteria and microalgae in the Gulf of Gdańsk (southern Baltic Sea). For the first time, the number and type of cyanobacteria and microalgae were determined in rainwater samples and in air before and after rainfall events. The number of cyanobacteria and microalgae cells in the rainwater samples ranged, depending on, e.g., weather conditions, from 100 cells L–1 to 342.2 × 103 cells L–1. Several harmful taxa, such as Chlorococcum sp., Oocystis sp., Anabaena sp., Leptolyngbya sp., Nodularia sp., Pseudanabaena sp., Synechococcus sp., Synechocystis sp., and Gymnodinium sp., were noted in our study. Washing out by rain is extremely relevant to human health and decreases the chance that people inhale these species and their toxic metabolic products. The greatest diversity of airborne microalgae and cyanobacteria was recorded in July 2019, despite this being the period with the lowest number of cells in rainwater samples. Research conducted in the southern Baltic Sea region confirmed the relationship between the occurrence of cyanobacteria and microalgae in the air and blooms in the sea. It is worth emphasizing that the number of microalgae and cyanobacteria cells decreased by up to 87% after a rainfall event relative to that before the rainfall event. The obtained results significantly increase the level of knowledge about cyanobacteria and microalgae present in the air. By demonstrating the washout efficiencies of cyanobacteria and microalgae, the results indicate the potential of individual taxa to be removed from the atmosphere with rainfall. The findings of this study are helpful for further research on airborne microorganisms and air quality.

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