Effect of Geobacillus toebii GT-02 addition on composition transformations and microbial community during thermophilic fermentation of bean dregs

Abstract

Bean dregs can be prepared into organic fertilizer by microbial fermentation. Geobacillus toebii GT-02, which has promoting effect on bean dregs fermentation, was isolated from horse dung and it grows within a range of 40-75 °C and pH 6.50-9.50. The effectiveness of GT-02 addition on composition transformations and the microbial community in bean dregs thermophilic fermentation at 70 °C for 5 days was investigated (T1). Fermentation of bean dregs without GT-02 served as control (CK). The results showed that T1 (the germination index (GI) = 95.06%) and CK (GI = 86.42%) reached maturity (defined by GI ≥ 85%) on day 3 and day 5, respectively. In addition, the total nitrogen loss of T1 (18.46%) on day 3 was lower than that in CK (24.12%). After thermophilic fermentation, the total organic carbon and dry matter loss of T1 (53.51% and 54.16%) was higher than that in CK (41.72% and 42.82%). The mean microbial number in T1 was 4.94 × 10⁷ CFUs/g dry matter, which was 5.37 times higher than that in CK. 16S rDNA sequencing identified Bacillus, Geobacillus and Thermobacillus as dominant in CK, while Bacillus, Ammoniibacillus and Geobacillus were dominant in T1. A canonical correspondence analysis showed that Geobacillus and Ammoniibacillus were positively correlated with the GI. Thus, thermophilic fermentation with GT-02 can promote the maturity of bean dregs, which indicated the potential application value of GT-02 in thermophilic fermentation.

Figure 1

Fermentation system.

Figure 2

(a) Phylogenetic tree based on 16S rDNA gene sequences from related species of the genus Geobacillus constructed using the neighbour-joining method with 1000 bootstrap replicates. Branch length is indicated at each node. (b) The growth curve of strain GT-02 with temperature. (c) The growth curve of strain GT-02 with pH.

Figure 3

Profiles of GI (a), TOC (b), TN (c), pH (d) and EC (e) during the fermentation process of CK and T1. The data represent the means ± standard deviations from three measurements.

Figure 4

Profiles of NH₄⁺-N (a) and NO₂^–N (b) during the fermentation process of CK and T1. The data represent the means ± standard deviations from three measurements.

Figure 5

Profiles of dry matter loss (a), protein loss (b), fat loss (c) and fibre loss (d) in CK and T1 during fermentation process. The data represent the means ± standard deviations from three measurements.

Figure 6

(a) Fluorescence excitation-emission matrix spectra of dissolved organic matter in CK and T1. Em (nm): emission wavelength (nm); Ex (nm): excitation wavelength (nm). Regions I and II: simple aromatic proteins; Region III: fulvic acid-like substances; Region IV: soluble microbial byproducts; and Region V: humic acid-like substances. (b) The 4000-to-600-cm⁻¹ regions of the Fourier transform infrared spectra of CK and T1.

Figure 7

(a) Profiles of CFUs in CK and T1 during fermentation process. (b) Taxonomic classification at the genus level of predominant 16S rDNA gene sequences (relative abundance, top 15) in CK and T1. The data represent the means ± standard deviations from three measurements.

Figure 8

CCA between the composition and microbial community of CK and T1.