The sample status after fermentation of G. lucidum
(This position for Fig. 3)
As shown in Fig. 3. The observed phenomenon indicates stratification within the fermentation broth. The upper mycelium exhibits a high degree of density and possesses commendable hardness, whilst the lower mycelium assumes a gelatinous nature. It is possible that the upper mycelium is more prone to air circulation, resulting in a certain level of desiccation of the upper mycelium. The compact structure of the upper mycelium leads to limited evaporation of water from the lower mycelium, hence maintaining high water content in the lower mycelium.
Analysis of biomass and ganoderic acid content of G. lucidum when dissolved oxygen is regulated during the oscillatory culture
(This position for Fig. 4)
Glucose utilization is a valid indicator of the growth of G. lucidum (Hsu et al., 2021). Nevertheless, due to the presence of stratification in the fermentation broth and its lack of homogeneity, the assessment of G. lucidum growth was limited to biomass evaluation. Figure 4 illustrates that the biomass of G. lucidum after fermentation could be significantly affected by regulating the dissolved oxygen in the shock incubation stage, which was the highest among the biomass in the A4 group, reaching 13.1 g/L, which was 16.9% higher than that of the control. A2 had been in a constant state of closed air, its biomass was significantly lower than others. It's worth noting that the A3 and A4 appear to be significantly different. Based on the observed development curve of G. lucidum fermentation, it is plausible to believe that G. lucidum undergoes an acclimation phase during the pre-fermentation period, characterized by a sluggish growth rate and a limited reliance on oxygen. Following a fermentation period of 3.5 days, G. lucidum enters the logarithmic growth phase characterized by accelerated development and heightened metabolic activity. Consequently, the organism exhibits an increased oxygen requirement. The oxygen conditions given by A4 are highly conducive to the growth of G. lucidum. Consequently, the biomass of A4 will experience a substantial increase.
(This position for Fig. 5)
According to data in Fig. 5, the content of total triterpenes in different groups has minor elevation compared with the control(A1). Among them, the total triterpene content of A2 was higher, reaching 61.56 mg/g, which was 38.33% higher than the control. Currently, there are relevant reports suggesting that the low oxygen environment can increase the triterpene content of G. lucidum (Milovanovic et al., 2023; Zhou et al., 2019). Ganoderma triterpenes are lanosterane derivatives that undergo significant oxidation, while G. lucidum contains ergosterol and yeast sterols (Hashim et al., 2016). The colorimetric approach used for detecting the triterpene concentration of G. lucidum is susceptible to significant interference. Hence, it is imperative to do additional qualitative and quantitative study of ganoderma triterpenes using High-Performance Liquid Chromatography (HPLC) (Keypour et al., 2009).
(This position for Fig. 6)
Ganoderic acid, a triterpenoid compound, exhibits notable anti-tumor properties and is comparatively more challenging to procure than other triterpenoids, specifically sterols. Hence, the primary aim of optimizing the fermentation process was to enhance the production yield of diverse ganoderic acids (Fang et al., 2002). The Fig. 6 has shown that ganoderic acid were detected in the upper layer of the mycelium of all samples and the peak times were close to each other, while no obvious ganoderic acid were detected in the lower layer of mycelium. Based on the qualitative analysis of the standards, it was found that the components with peak times around 56.5 and 66.5 min were ganoderic acid S and R, respectively. The findings of this study demonstrate that the upper mycelium exhibits a significant enrichment of ganoderic acids S and R, whereas the lower mycelium shows negligible levels of these ganoderic acids. Additionally, the majority of sterols discovered by the colorimetric approach are present in the upper mycelium. Hence, solely the quantification of ganoderic acid S and R was conducted in the upper mycelium.
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Table 1
The results of HPLC fingerprints of ganoderic acid S and R in the oscillation phase with regulated dissolved oxygen
Sample | Ganoderic acid S(mg/g) | Ganoderic acid R(mg/g) | Total (mg/g) |
A1 | 8.7595 ± 0.2536c | 1.9346 ± 0.0407b | 10.6941 ± 0.2132c |
A2 | 9.7170 ± 0.1220b | 1.7713 ± 0.0371c | 12.1852 ± 0.1332ab |
A3 | 9.6746 ± 0.0959b | 1.9945 ± 0.0320b | 11.4459 ± 0.2304b |
A4 | 10.6314 ± 0.2853a | 2.4682 ± 0.0176a | 12.6259 ± 0.1741a |
The data in Table 1 has shown that the upper mycelium of all experimental groups was enriched in ganoderic acids S and R, and the total content was slightly higher than that of the control. Among them, the A4 had the most prominent content of ganoderic acid S and R, reaching 10.6314 mg/g and 2.4682 mg/g, which were 21.37% and 27.58% higher than the control, respectively. The findings of this study indicate that the manipulation of dissolved oxygen levels during the oscillating culture phase has a minor impact on the metabolism of ganoderic acid, while exerting a notable favorable influence on the biomass of G. lucidum. Additionally, it has been demonstrated that the presence of upper mycelium in the fermentation medium of G. lucidum has a beneficial impact on the production of S and R ganoderic acids. Currently, the primary factor contributing to the development of the top mycelium appears to be water evaporation. However, subsequent treatment of the lower mycelium under comparable conditions did not result in a significant rise in ganoderic acid content. Hence, it is plausible that factors other than mere water evaporation may contribute to the development of the upper mycelium layer in G. lucidum. There is speculation on the potential influence of a low oxygen environment on the regulation of key enzymes within the ganoderic acid metabolic pathway. This speculation suggests that the regulation of dissolved oxygen levels may contribute to the expression of a specific functional gene (Shiao & Lee, 2005).
Analysis of biomass and ganoderic acid content of G. lucidum when dissolved oxygen is regulated during the stationary culture
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According to the data presented in Fig. 7, the issolved oxygen was regulated during the stationary incubation phase and the biomass of all three experimental groups was not significantly different from the control. The potential reason for this occurrence could be attributed to the fact that G. lucidum had attained a state of stability and achieved its peak biomass during the oscillation culture phase. Hence, the predominant mycelium growth seen throughout the fermentation process of G. lucidum occurred mostly in the oscillation stage. Consequently, the modulation of dissolved oxygen levels in the stationary stage would not exert a significant influence on the biomass production of G. lucidum. The synthesis of ganoderic acid is hypothesized to predominantly occur during the stationary phase.
(This position for Fig. 8)
It has been shown that G. lucidum is more favorable for the synthesis of ganoderic acid when fermented in a stationary state (Zhang & Zhong, 2010). The data shows in Fig. 8 indicates that there was a slight elevation of triterpenoids in all groups of samples after modulation of dissolved oxygen during the stationary incubation phase compared to the control. This result is similar to that of the oscillatory stage modulated dissolved oxygen, which may also be due to the fact that the colorimetric method is susceptible to the interference of sterols. Therefore, the results of colorimetric method are only for reference, and further detection and analysis by HPLC is needed to determine the type and content of ganoderic acid in mycelium.
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Figure 9 illustrates that the upper mycelium of each group had distinct peaks of ganoderic acid S and R, while the lower mycelium contained almost no ganoderic acids S and R. This result is similar to that of the oscillatory phase modulation of dissolved oxygen, which still needs to be calculated by the regression equation for its specific content.
(This position for Table 2)
Table 2
The results of HPLC fingerprints of ganoderic acid S and R in the stationary phase with regulated dissolved oxygen
Sample | Ganoderic acid S(mg/g) | Ganoderic acid R(mg/g) | Total (mg/g) |
B1 | 7.8978 ± 0.3451c | 4.3023 ± 0.3030b | 12.2001 ± 0.2292c |
B2 | 12.0469 ± 0.3556b | 4.3861 ± 0.2459b | 16.6587 ± 0.3673b |
B3 | 11.9594 ± 0.3945b | 5.6324 ± 0.3081a | 16.3454 ± 0.5989b |
B4 | 19.5959 ± 0.3172a | 4.6118 ± 0.3376ab | 25.2283 ± 0.5452a |
Based on the regression equation, the content of ganoderic acid S and R was calculated as shown in Table 2. The contents of ganoderic acid S and R in all experimental groups were more significantly increased, with the highest total amount of ganoderic acid S and R in B4 reaching 25.2283 mg/g, which was 106.79% higher than that of the control. The findings of this study demonstrate that the manipulation of dissolved oxygen levels during the oscillation stage can have a substantial impact on the synthesis of ganoderic acid. Additionally, these results provide evidence that the production of ganoderic acid occurs mostly during the stationary culture stage. The hypothesis posits that the utilization of the stationary culture method may exert a positive regulatory effect on the key enzymes involved in the metabolic pathway of ganoderic acid. However, due to the current lack of understanding regarding the specific metabolic pathway of ganoderic acid, it remains challenging to ascertain the precise key enzyme that experiences a positive influence. Consequently, additional research endeavors are warranted to shed light on this matter.
(This position for Fig. 10)
It has been reported in the literature that ganoderic acids S and R have good anti-tumor activity (Yang et al., 2005; Liu et al., 2018). Because of ganoderic acid inhibits topoisomerase activity and inhibits DNA synthesis to the extent that it contributes to cancer cell death (Li et al., 2005), and ganoderic acid has been found to improve the functionality of the immune system in the human body, specifically by enhancing the activity of natural killer cells (NK cells) and promoting the activity of lymphocytes (Wang et al., 2007). Hence, the efficacy of ganoderic acid in terms of its bioactivity was confirmed through the application of the ethanol extract of the upper mycelium in the intervention of two distinct tumor cell lines, namely K562 and L1210. The ethanol extracts of the samples had significant inhibitory effects on both K562 and L1210 cell lines, demonstrating a clear correlation with concentration. At a dosage of 6.25 µg/mL, the inhibition rate for both tumor cells was seen to be approximately 90%. The findings of this study suggest that there is a greater similarity in the types of ganoderic acid present in the two groups of samples. Additionally, the results provide evidence that the ethanolic extract exhibits a high concentration of ganoderic acid.