Environmental factors such as light, temperature, nutrient salts, pH and oxygen are associated with akinete germination and resting cell awakening [44, 45]. Each species of cyanobacteria has unique optimal environmental conditions for germination and growth [30, 46]. Water temperature is a key environmental factor that promotes cyanobacteria blooms, and many species of cyanobacteria show a preference for high water temperatures [37]. For instance, Microcystis aeruginosa, M. wesenbergii, Dolichospermum spiroides, Raphidiopsis raciborskii, Nodularia spumigena and Aphanizomenon flos-aquae show high emergence mainly in summer with high water temperature [47—49]. The increased temperature of the waterbody and sediment layer in spring and early summer induces cyanobacteria akinete germination and resting cell awakening, while facilitating the metabolic activities of germinating cells toward exponential growth [50]. Thus, water temperature is an important factor that facilitates the metabolic activities of cyanobacteria [37]. This study demonstrated that the germination of D. circinale akinetes was induced at the water temperature of 25°C–30°C and that the growth of M. aeruginosa resting cells rapidly increased at 30℃, suggesting that the rates of germination and growth were high as the high water temperature was maintained. However, at ≤ 15℃, the two cyanobacteria species showed low rates of germination and growth, and in the conditions of 5℃ and 10℃, no resting cell growth was detected. Certain species of cyanobacteria such as Anabaena cylindrica and Aphanizomenon flos-aquae grow at low water temperatures [51, 52]; and D. mendotae akinetes showed a markedly high rate of germination at low temperatures ≤ 10℃ [32]. This suggested that, despite a comparatively low rate of germination at low water temperatures, the level of germination could still contribute to the proliferation of trophocytes, which coincided with the results in other previous studies. Dolichospermum mendotae, Nodularia spumigena and Raphidiopsis raciborskii showed the highest akinete germination rates in the range of 20°C–25℃ (> 40%), while the rate decreased at low temperatures ≤ 10℃ (< 15%) [30, 53]. M. aeruginosa and M. wesenbergii showed a rapid population increase with a sudden rise in temperature to ≥ 25℃, which was attributed to the growth within the sediment layer [54]. D. mucosum akinetes at low water temperatures (5°C–8℃) showed a limited rate of germination ≤ 10%, whereas the rate increased by ≥ 60% in an environment of relatively high water temperatures (14°C–23℃) [55]. Hence, D. circinale germination and M. aeruginosa awakening could be induced at water temperatures of ≥ 25℃, which would have a direct influence on cell proliferation. Consequently, in the stepwise process from akinete germination to resting cell awakening, the temperature acts as a trigger, and the water temperature of a specific range (25°C–30℃) can increase the physiological activities of akinetes and resting cells. This is presumed to influence the response time from triggering that causes re-entry to germination and optimal growth so as to determine the range of temperature for germination and growth [56, 57]. Light is another crucial environmental factor that influences cyanobacterial germination and growth [58, 59]. For instance, the germination rate of Nodularia spumigena akinetes was the highest at the light intensity of 15–50 µmol m2 s1 but no germination occurred in the dark condition without light [30]. For many biological species, light induces the circadian or seasonal lifecycle, and induces the germination and growth of cyanobacteria, acting as a trigger [30, 60, 44, 61]. The akinetes of certain cyanobacteria species have been reported to germinate in conditions without light [62]; however, light is an essential condition for the germination and growth of most cyanobacteria akinetes and resting cells [63]. In general, high light intensity is required for cyanobacteria germination and growth, which may still present adequate levels, even at low intensity [30, 44]. The two isolated cyanobacteria species in this study demonstrated no germination or growth in the condition without light, but the germination of D. circinale could be induced in the condition of lowest light intensity of 5 µmol m2 s1, and the onset of growth of M. aeruginosa was observed from 15 µmol m2 s1, despite the absence of growth in the condition of minimal light intensity of 5 µmol m2 s1. The growth of M. aeruginosa was shown to increase as the light intensity increased, whereas the germination of D. circinale rapidly decreased from the light intensity ≥ 50 µmol m2 s1, which indicated a variation in the growth pattern between the two species. As can be seen, an increase in light intensity or a condition of minimal light intensity could promote the growth of akinetes and resting cells, and the rate of growth could increase in a specific range of light intensity [64—66]. Another known environmental factor that is critical for cyanobacteria growth and akinete germination is the pH [67, 68]. In this study, pH ≤ 6 and pH ≥ 11 led to low rates of D. circinale germination ≤ 10%, and pH 7–10 led to high germination rates (≥ 40%) with optimal growth. For M. aeruginosa, pH ≤ 6 and pH ≥ 10 led to growth rates < 50%, and the conditions of pH 7–10 led to high growth rates ≥ 300%. At pH 8, in particular, the rate of cell growth approached 800%. These results coincided with those of previous studies. Other species such as Microcystis aeruginosa, Anabaena vaginicola, Anabaena cylindrica, Westiellopsis prolifica, Anabaenopsis arnoldii and Nostochopsis lobatus also showed the highest rates of germination and growth at pH 7–8 [67, 69, 70]. In an acidic condition, phaeophytin is degraded [71] with a potential reduction of cell viability or damage on the akinete cell wall [63]. Meanwhile, high pH could alter the permeability of cell membrane [72], and the reduced absorption of anions at pH ≥ 11 could inhibit the cellular physiological activities and cause insolubility of carbon dioxide to prevent photosynthesis [73]. Consequently, we suggest that each cyanobacteria species has an optimal pH condition for adequate growth and that the rates of germination and growth could be optimized by maintaining such optimal conditions [63, 67]. For the germination and growth of cyanobacteria, different nutrient salts may play different roles [39], and notably, N and P are known to have an effect on the germination and growth of cyanobacteria [73, 30, 44, 67]. As an example, the akinetes of different cyanobacteria species; Anabaena fertilissima, Anabaena variabilis, and Nostoc linckia, showed different rates of germination and growth in previous studies testing the conditions of P and N depletion or addition [69, 17]. The population increase of Microcystis in the water layer could increase the concentration of P, and the relatively high concentration of N in the sediment layer could contribute to the formation of colonies of Microcystis resting cells in the sediment [45, 74]. In this study, the rate of D. circinale akinete germination was 10% in the condition without N and P but the rate was high, ≥ 50%, in the condition with both N and P. For M. aeruginosa, no growth was observed in the condition without N and P, and the rate of growth was the highest in the condition with both N and P. Notably, the germination and growth rates were higher in the condition without P than in the condition without N for both species, which suggested a slightly higher influence of N than P on the growth of the two cyanobacteria species. For the two isolated species in this study, the N from NH4N and P from phosphate were shown to be the critical factors that contribute to the germination and growth. As a limiting element of algal biosynthesis, N allows algal growth via its metabolism. The N from NH4N is one of the most essential elements primarily required for algae; the N dissolved in the waterbody is reduced to ammonia and used in the synthesis of amino acids [75]. The germination rate of Westiellopsis prolifica akinetes showed an over 5-fold increase in the condition with N or P compared to the condition without N or P [76]. The germination rate of Nodularia spumigena akinetes also increased by ≥ 68% as the concentration of P increased [30]. In contrast, the addition or depletion of N did not have a significant effect on the germination of Anabaena cylindrica and Nostoc sp. akinetes [64, 77], whereas Huber [30] reported an increase in germination rate by 70–80% for Nodularia spumigena akinetes in the condition of N addition. In this study, D. circinale akinetes showed a high rate of germination even in the condition of low phosphate salt concentration, based on which phosphate salt was presumed not to be an essential factor for germination. The results implied that, to control the eutrophication of freshwater ecosystems, the management of N in addition to P was required. The results in this study regarding the rates of cyanobacteria germination and growth in varying environmental conditions collectively suggested that the optimal conditions to allow D. circinale germination were water temperature 25°C–30°C, light intensity 5–30 µmol m2 s1, pH 7–10, and adequate levels of N and P, while the optimal growth conditions for M. aeruginosa were water temperature 30℃, light intensity 50–100 µmol m2 s1, pH 8–9, and adequate levels of N and P. The investigation of the germination and growth of two cyanobacteria species isolated from the sediment layers of the Han River and Nakdong River water systems revealed that it is necessary to conduct in-depth analyses of the life history of cyanobacteria regarding germination and growth in sediment layers and furthermore to predict algal blooms in future and develop appropriate response measures. In addition, studies to deepen the understanding of the distribution and development of cyanobacteria akinetes and resting cells should be conducted with a focus on specific water bodies, and respective management policies should be developed.