3.1. Sequence data and OTUs analysis
After sequencing samples from different fermentation periods of Gardenblue, Powderblue, and Britewell blueberry wines, the quality sequence distribution statistics obtained are shown in
Table 1. A total of 482852 valid sequences were collected from 9 blueberry fermentation broth samples, among which the sequence lengths in the range of 420-440 accounted for the majority of the valid sequences. This experimental result indicates that the detection results are excellent and can be applied to the later experimental analysis. The dilatability curves allow the sequencing depth of the samples to be derived[
14]. From the dilution curves of the fungal ITS1-ITS2 regions of different varieties of blueberries before, during, and after fermentation under the 97% similarity condition shown in
Figure 1, it can be seen that when the number of fungal OTUs reached about 70, the dilution curves of the samples of pre-fermentation of Gardenblue and late fermentation of Britewell had begun to level off, and the sampling depth was sufficient, and the sequencing number could cover all taxa. The remaining 7 samples also had sufficient sequencing depth when the number of fungal OTUs reached about 100, and the taxa were covered completely.
The OTU-Venn diagram (
Figure 2) labels the number of OTUs in the three varieties of blueberry wine samples, with the main objects being those that are conventional in blueberry wine samples and those that are specific to blueberry wine samples. As can be seen in
Figure 2, the samples of the three blueberry varieties have 128 OTUs in common, of which only Gardenblue has 14, only Powderblue has 24, and 38 are specific to Britewell. This shows that the fungal communities of the three blueberry varieties have overlapping parts, and also have their own varieties of unique fungal communities. To better represent the OTU distribution of blueberry wine samples, a petal chart of OTU distribution (
Figure 3) shows the number of endemic and overlapping OTUs in the full sample of blueberry wine. As can be seen in
Figure 3, the total number of OTUs for all blueberry wine samples was 32, indicating the presence of 32 fungal communities that survived the entire fermentation process of blueberry wine.
From
Figure 3, the total number of OTUs in the nine samples of Gardenblue, Powderblue, and Britewell before, during, and after fermentation was 32. 133, 159, and 177 for Gardenblue, 79, 132, and 141 for Powderblue, and 135, 186, and 109 for Britewell before, during, and after fermentation, respectively. This shows that there are both similarities and differences in the bacterial flora of different varieties of blueberries in the same appellation during fermentation.
3.2. Analysis and discussion of alpha diversity data of fungal community zones during the fermentation of blueberry wine
Alpha diversity index table for fungal detection[
15], The alpha diversity index table contains six main indices, namely: chao1 index, coverage index, observed_species, PD_whole_tree, Shannon and Simpson indices, by which the experimental results can reflect the richness and diversity of samples of three different blueberry wines at different fermentation stages in the Majiang appellation of Guizhou. chao1 and observed_species indices are mainly to estimate the number of species contained in the fungal community in blueberry wine samples. Also, these two indices can represent the OUT estimate and the actual value of the sample community respectively, which can reflect the abundance of fungal communities at each stage of blueberry wine fermentation. The coverage is a test of microbial coverage in blueberry wine, and the higher the value, the lower the probability that new species are not detected in the samples of different fermentation stages of blueberry wine. The coverage index of all blueberry wine samples was 1.00, indicating that the coverage of blueberry wine samples was 100%, indicating the actual situation of true actual value bacteria in samples of different varieties of blueberry wine at different fermentation stages and reflecting the reliability of the experimental data. As can be seen from
Table 2, Chao1 reflects the richness and diversity of fungal species in the blueberry wine samples, which is the number of species in the sample, and the more species in the sample, the richer the sample. Species richness does not consider the relative abundance of each species, it gives the same weight to species with low relative abundance as those with high relative abundance. As shown in
Table 2: The largest fungal richness index Chao1 for different varieties of blueberries before, during, and after fermentation was for Britewell in the middle of fermentation, and the smallest Chao1 index was for Gardenblue in the pre-fermentation period, indicating that Britewell in the middle of fermentation was the most abundant species among these nine samples, and the abundance of Gardenblue reached the minimum value in the pre-fermentation period. Among them, the PD_whole_tree index responded to the changes of fungal communities in the samples during the fermentation of blueberry wine and the diversity, reflecting the trend and variation of fungi in blueberry wine samples. Uniformity reflects the relative abundance of different species. Diversity increases with increasing species richness and evenness. The diversity indices, Shannon and Simpson, are a composite indicator of species richness and evenness in a sample higher Shannon and Simpson values indicate higher diversity of community species
[16-18]. From
Table 2, it can be seen that Powderblue had the highest fungal community diversity among different varieties in the same fermentation stage, followed by Britewell and the lowest Gardenblue. Among different fermentation stages of the same variety, all had the highest fungal community diversity in the pre-fermentation stage, followed by the middle stage, and the lowest in the late fermentation stage.
3.4. Analysis of fungal species and abundance
Comparison of fungal communities in pre, mid, and late- fermentation fungal samples from different varieties of blueberries using visual bar graphs[
19] revealed the presence of major phyla and genera in all samples, but with differences in abundance (
Figure 5). From the figure, it can be seen that at the level of the fungus phylum,
Ascomycetes are present throughout the fermentation process of blueberry wine, with the relative abundance pairs of
Ascomycetes in the pre-fermentation fungal communities of Gardenblue, Powderblue, and Britewell being 92.35 %, 93.67 % and 92.92 %, respectively, and still increasing as fermentation progressed, and at the late fermentation stages of Gardenblue, Powderblue, and Britewell being 98.85 %, 99.06 %, 99.36 %, the percentage of
Ascomycetes in the blueberry wine samples of all three blueberry varieties showed an increasing trend as fermentation proceeded; followed by the
unidentified phylum; followed by the
Basidiomycota, the relative abundance of the
Basidiomycota in the fungal communities of Gardenblue, Powderblue and Britewell at the pre-fermentation stage were 0.28 %, 0.28 % and 0.69 %, respectively, and gradually decreased as fermentation proceeded, with the abundance of at the late stage of fermentation, only 0.22 %, 0.25 % and 0.11 % were found in the fungal communities of Gardenblue, Powderblue and Britewell, respectively.
At the genus level, the fungi in the pre-and mid-fermentation samples of Gardenblue, Powderblue, and Britewell were distributed in seven genera: Hanseniaspora, Saccharomyces, unidentified, Aureobasidium, Penicillium, Mortierella, and Colletotrichum. The relative abundance of Hanseniaspora was the highest in the pre-fermentation samples of Gardenblue, Powderblue, and Britewell, with 84.72 %, 80.26 %, and 81.50 %, respectively; followed by Saccharomyces with 10.23 %, 4.97 %, and 3.43 %, respectively; the abundance of the remaining genera was very small. The relative abundance of Saccharomyces was the highest in the mid-fermentation Gardenblue, Powderblue, and Britewell samples, with 90.40 %, 63.84 %, and 62.92 %, respectively, followed by Hanseniaspora with 2.62 %, 33.70 %, and 33.87 %, respectively, while the abundance of the remaining genera was very low. The relative abundance of the Saccharomyces was the highest in the late fermentation samples of Gardenblue, Powderblue, and Britewell with 96.18 %, 92.40 %, and 90.62 %, respectively, followed by Hanseniaspora with 1.22 %, 3.55 %, and 7.87 %, respectively, while the abundance of the remaining genera was minimal. From the relative abundance of Saccharomyces on the fourth day, it is clear that the fermentation rate of Gardenblue was faster than that of Powderblue and Britewell, and the Saccharomyces were dominant by the fourth day.
On the whole, the genus Saccharomyces began to exist at the beginning of fermentation, increased rapidly in number in the middle of fermentation, and became absolutely dominant in the middle and late stages of fermentation, while the genus Hanseniaspora was present in large numbers at the beginning of fermentation and became absolutely dominant, and its relative abundance decreased rapidly in the middle stage as fermentation progressed and was lower in the late stage. Thus, it can be seen that in the fermentation process of Majiang appellation Gardenblue, Powderblue, and Britewell blueberry wines, the Hanseniaspora genus was the first to develop in the early stage and gradually gave way to Saccharomyces in the middle and late stages.
Nine samples from different fermentation periods of three varieties of blueberry wine were analyzed using the spearman test, and the 20 fungal genera with the highest absolute abundance in the blueberry wine samples were extracted for correlation association analysis, while the analysis results were plotted against the fungal phylum as a legend. The data with p-value > 0.05 or correlation value |R|< 0.4 were plotted to obtain
Figure 6. It can be seen from the figure that the association status between
Hanseniaspora and
Saccharomyces during the fermentation of blueberry wine was negatively correlated. The correlation between
Aureobasidium and
Hanseniaspora was positive during the fermentation of blueberry wine.
3.5. Dynamic changes of fungal flora
The top 20 genera in terms of relative abundance values were selected and clustered according to their abundance information in samples at different fermentation stages, and the fungal thermogram was used to visually analyze the distribution of fungal communities of the three blueberry varieties Gardenblue, Powderblue, and Britewell at different fermentation stages and different blueberry varieties at the same fermentation stage during the fermentation of blueberry wine. The relative abundance of
Saccharomyces,
Hanseniaspora, etc, the dominant fungal community was present in all fermentation stages of the dry red blueberry wine, and its fungal flora showed similar trends in the samples, as can be seen in
Figure 7. For example, the abundance of
Hanseniaspora,
Aureobasidium, and
Penicillium in the pre-fermentation samples, Gardenblue was similar to Britewell and higher than in the mid and late-fermentation samples.
From
Table 3, it can be seen that the three groups of bacteria involved in the fermentation process of blueberry wine such as
H. uvarum,
S. cerevisiae,
A. pullulans,
M. elongata, etc.
Saccharomyces cerevisiae was the most commonly used organism species in fermentation and dominated in the pre-fermentation stage in this experiment. The main product of wild-type
S. cerevisiae is ethanol, and traditional
S. cerevisiae strains have good fermentation performance, are well tolerated under anaerobic conditions, and can ferment sugars to produce high ethanol yields and production rates[
20].
S. cerevisiae has a short growth cycle, efficient sugar conversion capacity, competes for limited nutrient capacity, is easily
S. cerevisiae, as a dominant strain in fermentation, is rich in nutrients such as proteins, amino acids, vitamins, and bioactive substances, and promotes the production of volatile aroma components such as esters, alcohols, terpenoids, and volatile thiol substances using its metabolic activities
[21, 22].
S. cerevisiae has synergistic effects with other yeasts to improve wine quality and shape-rich aromas together with other yeasts[
23]. As can be seen from
Table 3, the fungal community of blueberries in the process of fermentation into blueberry wine is constantly changing, with
S.cerevisiae becoming the dominant flora, accounting for more than 90%.
3.6. Analysis of fungal OTUs of different blueberry varieties before, during and after fermentation
The 32 shared OTUs were analyzed for abundance, resulting in the top 20 OTUs in terms of abundance, as shown in
Table 3, namely:
Saccharomyces cerevisiae,
Hanseniaspora uvarum,
Hanseniaspora osmophila, Hanseniaspora vineae,
Pseudaleuria sp,
Aureobasidium pullulans,
Mortierella alpina,
Mortierella elongata,
Stachybotrys microspora, and
Guehomyces pullulans.
Saccharomyces cerevisiae is a traditional ethanol-producing strain with good fermentation performance, good growth capacity under anaerobic conditions, the ability to ferment glucose to produce high ethanol yields and production rates, and high tolerance to ethanol and other inhibitors
[24-26].
S. cerevisiae, as the dominant flora in mid and late fermentation, also promotes the production of volatile aroma substances such as alcohols, esters, terpenoids, and volatile thiols through its own metabolic activities and the release of non-aromatic precursors[
27].
Hanseniaspora uvarum is the most abundant non-enological yeast commonly found in vineyards and winemaking environments[
28]. The glycosidases secreted by
H. uvarum mainly promote the hydrolysis of desmethyl isoprenoids and some terpene glycosides, and less so for C6 compound glycosides, thereby increasing the temperate fruit and floral characteristics of the wine[
29].
Hanseniaspora osmophila and others can ferment higher levels of acetate compounds such as phenyl acetate and isopentyl acetate[
30]. Zhang et al[
31] found that
Hanseniaspora vineae and
Saccharomyces cerevisiae mixed fermentation had a synergistic effect on the production of ethyl 2-phenylacetate. ethyl 2-phenylacetate has a peachy aroma and is present in apples, grapes, poplar berries, tea, beer, whiskey, and brandy, and is a desired aroma compound in wine. In this experiment,
Hanseniaspora had a high relative abundance (around 80%) in the pre-fermentation period;
Saccharomyces dominated in the middle and late stages, with relative abundance up to 96%, and they acted synergistically to play an important role in improving blueberry wine quality and aroma shaping, etc.
Aureobasidium pullulans, a class of yeast-like fungi[
32], is the predominant pectinolytic species (pectinolytic strain) in the grape skin flora and is present in the must and pre-fermentation. Although low-temperature maceration for wine production enhances the aroma of the wine, the color and phenolics of the wine are affected. Marin et al[
33] applied
A. pullulans to cold maceration and low-temperature fermentation before fermentation and found a significant increase in total anthocyanin content, total polyphenol index and color intensity (CI), and a higher content of polymer pigments at 9 months of storage in the bottle. This indicates that A. pullulans affects the color and stability of the wine and plays an important role in the quality of the wine. In this experiment, the relative abundance of
A. pullulans was detected at 1.11 %, 0.24 %, and 0.19 % before and during the middle of the fermentation of Gardenblue, 0.25 %, 0.01 %, and 0.001 % during the three fermentation periods of Powderblue, and 1.30 %, 0.15 %, and 0.07 % during the three fermentation periods of Britewell, respectively. The relative abundance of
A. pullulans decreased gradually during the fermentation of different blueberry wine varieties and was the lowest in the Powderblue samples.
Microbial cells usually contain only 2 % to 3 % of oil, but under specific culture conditions, certain microbial dry bodies can contain up to 80-90% oil[
34]. Some molds have a strong lipid production capacity, and the lipids produced also contain varying degrees of γ-linoleic acid (GLA), arachidonic acid, and other beneficial components for humans, so scholars at home and abroad have conducted in-depth studies on them.
Mortierella alpina has a strong lipid synthesis capacity, and its lipid accumulation can reach more than 50 % of its dry weight, where the production of Arachidonic acid can exceed 40% of the total fatty acid content[
35], which has been applied to commercial production and has good application value and development prospects. strains such as
Mortierella elongata and
Mortierella alpina contain γ-linoleic acid (GLA), and the process of GLA production by
Peridiomycetes is mature and can produce annually several hundred tons per year[
36]. It was found that
M. elongata, M. alpina, and other mycobacteria also contain EPA and DHA[
37]. Although these two species were detected in this experiment, their relative abundance was small. Among the three species,
M. alpina was the most abundant in Britewell, with abundances of 0.01 %, 0.04 %, and 0.02 % before, during, and after fermentation, respectively. The abundance of
M. elongata in Gardenblue was 0.03 %, 0.09 %, and 0.07 % before, during, and after fermentation, respectively; the relative abundance in Powderblue was 0.10 %, 0.14 %, and 0.03 % during the three fermentation periods, respectively; and 0.17 %, 0.04 % and 0.09 % during the three fermentation periods in Britewell, respectively. The relative abundance of
M. elongata was greatest during the pre-fermentation period of Britewell and decreased significantly in the middle and late stages, while it increased in the middle of the mash of Powderblue blueberries, and the specific factors influencing this need to be studied in depth. It is worth acknowledging that both bacteria have a role in enhancing the nutritional value of blueberry wine.
Stachybotrys microspora is a filamentous fungus with cellulolytic activity. The basic process of cellulose hydrolysis is the production of cellobiose and some cellooligosaccharides by the synergistic action of endoglucanase and exoglucanase, which in turn are hydrolyzed by β-glucosidase to produce glucose[
38]. By studying the wild-type strain N1 of
S. microspora and its mutant A19, Abdeljalil et al[
39]found that
Stachybotrys produced much higher yields of glucanase, β-glucanase, and protease than the current commercial lyase "zymolyase", in addition to endoglucanase and exoglucanase. In this experiment, the abundance of
S. microspora was small, accounting for 0.0027 %, 0.027 %, and 0.045 % in the three stages of fermentation for Gardenblue, 0.03 %, 0.05 %, and 0.0053 % in the three stages of fermentation for Powderblue, and 0.0053 %, 0.04 %, and 0.01 % in the three stages of fermentation for Britewell. As a whole, the abundance of this bacterium increased in the middle of fermentation. From previous studies, this bacterium has some roles in blueberry wine such as hydrolysis of cellulose to produce glucose and promotion of blueberry juice maceration to stabilize the color.
Guehomyces pullulans were found to produce pectinase even at low temperatures (8°C)[
40]. Pectinase has a very important role in juice yield, clarification, and promotion of nutrient leaching in grape juice
[41-43]. Zhang et al[
44] found
G. pullulans to be the dominant bacterium in traditional Korean rice wine distillers. This bacterium was also detected in nine samples of this experiment, but the relative abundance was low and the relative abundance did not change significantly at different fermentation stages, accounting for only about 0.01 %.