Cyanobacteria synthesize various harmful materials, including off-flavor substances and toxins, that are regarded as potential socio-economic and environmental hazards in freshwater systems, however, their production is still not well understood. In this study, we investigated the potential and properties of harmful materials produced by cyanobacteria, depending on temperature, and undertook a phylogenetic analysis of cyanobacteria present in the North Han River (South Korea). Production potentials were evaluated using gene-specific probes, and the harmful material production properties of strains showing positive potentials were further characterized at different temperatures in the range 15 to 30 °C. We identified six cyanobacterial strains based on 16S rDNA analysis: two morphological types (coiled and straight type) of Dolichospermum circinale, Aphanizomenon flos-aquae, Oscillatoria limosa, Planktothricoides raciborskii, Pseudanabaena mucicola, and Microcystis aeruginosa. We confirmed that cyanobacterial strains showing harmful material production potential produced the corresponding harmful material, and their production properties varied with temperature. Total harmful material production was maximal at 20~25 °C, a temperature range optimal for cell growth. However, harmful material productivity was highest at 15 °C. These results indicate that the expression of genes related to synthesis of harmful materials can vary depending on environmental conditions, resulting in variable harmful material production, even within the same cyanobacterial strains.

Aquatic organisms are exposed to a wide range of salinity, which could critically affect their survival and growth. However, their survival and growth response to salinity stress remain unclear. This study evaluates the growth response and intracellular proline accumulation of green algae, Scenedesmus quadricauda, isolated from brackish water, against dissolved salts stress with N and P enrichment. We tested a hypothesis that nutrient enrichment can relieve the dissolved salts stress of algae by accumulating intracellular proline, thereby improving survival and growth. Four levels of salinity (0, 3, 6, 12 psu) were experimentally manipulated with four levels of nutrient stoichiometry (N:P ratio = 2, 5, 10, 20) at constant N (1 mgN/L) or P levels (0.05 and 0.5 mgP/L). In each set of experiments, growth rate and intracellular proline content were measured in triplicate. The highest level of salinity inhibited the growth rate of S. quadricauda, regardless of the nutrient levels. However, with nutrient enrichment, the alga showed tolerance to dissolved salts, reflecting intracellular proline synthesis. Proline accumulation was most prominent at the highest salinity level, and its maximum value appeared at the highest N:P ratio (i.e., highest N level) in all salinity treatments, regardless of P levels. Therefore, the effects of P and N on algal response to salt stress differ.

Eutrophic freshwater ecosystems are vulnerable to toxin-producing cyanobacteria growth or harmful algal blooms. Cyanobacteria belonging to the Nostocales order form akinetes that are similar to the seeds of vascular plants, which are resting cells surrounded by a thick membrane. They overwinter in sediment and germinate when conditions become favorable, eventually developing into vegetative cells and causing blooms. This review covers the cyanobacterial akinete of the Nostocales order and summarizes the environmental triggers and cellular responses involved in akinete germination and formation based on data from the literature. It also emphasizes the intimate and dynamic relationship that exists between the germination and formation of akinete in the annual life cycle of cyanobacteria. After comparing many published data, it is found that the tolerance ranges for factors affecting both akinete germination and formation do not differ significantly and are broadly consistent with the tolerance ranges for vegetative cell growth. However, the optimal range varies with different species and strains of cyanobacteria. The life cycle of cyanobacteria, as a result of akinete germination and formation, has a seasonal periodicity and spatial connectivity between the water column and the sediment. However, during the summer growing season, intimate coupling between akinete formation and germination can occur in the water column, and this can contribute to high population densities being maintained in the water column. During this time, shallow sediment could also provide suitable conditions for akinete germination, thereby contributing to the establishment of water column populations. The information summarized in this review is expected to help improve our shared understanding of the life cycle of the Nostocales cyanobacteria while also providing insights into the monitoring and management of harmful algal blooms.

Climate change has increased the frequency and intensity of extreme weather events worldwide. In South Korea, annual precipitation in 2014–2015 was only 50% of the long-term average, resulting in severe drought conditions. This drought extended water residence time in dam reservoirs, enhancing internal nutrient recycling, degrading water quality, and promoting harmful cyanophyta blooms in downstream reservoirs. Using the Standardized Precipitation Index—for drought assessment, and monthly water sampling—for environmental factors and phytoplankton analyses, this study examined the impacts of drought on water quality and phytoplankton communities in a series of interconnected dam reservoirs (Uiam, Cheongpyeong, Sambong-ri, and Paldang Lakes) within the Bukhan River system from 2013 to 2016. The prolonged residence time during drought facilitated nutrient accumulation and recycling within the reservoirs, intensifying eutrophication and water quality deterioration, alongside a pronounced cyanobacterial dominance and harmful algal blooms. These findings suggest that changes in upstream dam discharges directly influence water quality and ecosystem health in downstream reservoirs and that diverse hydrological changes associated with drought pose a significant threat to water source management. These findings may inform the development of integrated water management strategies for maintaining water quality and protecting water sources during droughts and extreme climatic events.

Harmful cyanobacteria and their metabolites often contaminate drinking water resources, and effective control remains challenging. Here, we developed a physical algal pre-treatment method, the vertical weir curtain (VWC), to mitigate cyanobacteria and some of their metabolites (geosmin, 2-methylisoborneol (2-MIB), and microcystins) in situ and evaluated its performance in a raw water reservoir used for drinking water supply. The VWC was manufactured with two fibrous polypropylene mats (0% and 92% porosity) which were mounted to maintain a constant underwater depth. We installed the VWC to cover the entire epilimnion of the drinking water intake zone and monitored its efficiency during an algal bloom period (July–October 2015). Reduction rates were 40–59% for total algae, 60–75% for cyanobacteria, 23–55% for geosmin, 30–51% for 2-MIB, and 47–89% for microcystin-LR during the study period. Significant reductions were observed in the shallow layer of the water column (1–3 m water depth), particularly during August, when cyanobacterial density was the highest. The results indicate that the VWC can effectively mitigate harmful cyanobacteria and their metabolites when suitably applied, serving as a valuable reference for the algal reduction in raw drinking water resources.

Cyanobacteria from the orders Nostocales and certain Stigonematales form akinetes, spore-like dormant cells that allow them to survive adverse environmental conditions. Temperature is known to be one of the key factors affecting akinete formation, but there is currently little known about akinete formation during cell growth over a wide range of temperature conditions and its relation to the overall survival strategy of cyanobacteria. Therefore, in the current study, we conducted a temperature-controlled experiment to analyze the akinete formation of a harmful cyanobacterium Dolichospurmum circinale using a growth chamber. We measured the concentration and size of both vegetative cells and different types of akinetes (free, attached, and empty type) under varying temperatures (5-25 degrees C). We also analyzed the buoyant ability and vertical migration velocity of trichomes along with changes in the volume of vegetative cells and akinetes. The total akinete concentration and ratio (number of akinetes to total number of cells) were both found to be higher at high temperatures (20-25 degrees C) than they were at low temperatures (5-10 degrees C) (p<0.05). Meanwhile, the rate of formation of akinetes (both free and attached akinetes) was highest at low temperature (10 degrees C) and decreased with increasing temperature. The rate of empty akinete formation increased with increasing temperature and was highest at 25 degrees C, indicating that most of the akinetes produced under high temperature conditions germinated. The change in vegetative cell size was proportional to the increase in the growth rate in response to increasing temperature (p<0.05). At high temperature, vegetative cells exhibited positive buoyancy and higher vertical migration velocity, while at low temperature, they exhibited negative buoyancy and relatively low migration velocity. Akinete size was larger at low temperature than it was at high temperature. These findings suggest that akinetes play an important role in maintaining populations in the water column, with a link between akinete formation and germination during summer cyanobacteria blooms. This information is expected to contribute to a deeper understanding of the D. circinale life cycle.