Diversity and Co-Occurrence Patterns of Fungal and Bacterial Communities from Alkaline Sediments and Water of Julong High-Altitude Hot Springs at Tianchi Volcano, Northeast China

Abstract

The Julong high-altitude volcanic hot springs in northeast China are of undeniable interest for microbiological studies due to their unique, extreme environmental conditions. The objective of this study was to provide a comprehensive analysis of the unexplored fungal and bacterial community composition, structure and networks in sediments and water from the Julong hot springs using a combination of culture-based methods and metabarcoding. A total of 65 fungal and 21 bacterial strains were isolated. Fungal genera Trichoderma and Cladosporium were dominant in sediments, while the most abundant fungi in hot spring water were Aspergillus and Alternaria. Bacterial communities in sediments and water were dominated by the genera Chryseobacterium and Pseudomonas, respectively. Metabarcoding analysis revealed significant differences in the microorganism communities from the two hot springs. Results suggested a strong influence of pH on the analyzed microbial diversity, at least when the environmental conditions became clearly alkaline. Our analyses indicated that mutualistic interactions may play an essential role in shaping stable microbial networks in the studied hot springs. The much more complicated bacterial than fungal networks described in our study may suggest that the more flexible trophic strategies of bacteria are beneficial for their survival and fitness under extreme conditions.

Keywords: Illumina sequencing; bacteria; extreme environments; fungi; hot springs; microbial community; microbial network analysis; microorganism diversity; morphology.

Figure 1

(A) Location of Tianchi Volcano in China and vertical view; (B) detailed location of the sampling area on the northern side of Tianchi Volcanic cone; (C) the main pond system: Pond A; (D) the smaller spring area: Pond B. The maps were generated from ArcGIS Online (https://maps.arcgis.com/index.html, accessed on 31 March 2021).

Figure 2

pH values of water and sediment from Pond A and Pond B of the Julong hot springs.

Figure 3

Sediment fungal and bacterial alpha diversity (OTU richness and Shannon index) in two ponds of the Julong hot springs. Difference between Pond A and Pond B as evaluated by a Wilcoxon rank-sum test is indicated as: ** p < 0.01. (A) Fungal OTU richness; (B) Fungal Shannon index; (C) Bacterial OTU richness; (D) Bacterial Shannon index.

Figure 4

Phylum-level relative abundance of fungi (A) and bacteria (B) from the two analyzed ponds of the Julong hot springs. The bacterial phyla with very low relative abundances (<1%) were merged as “others” in the bar plot. In the taxa names, “d” = domain and “k” = kingdom.

Figure 5

Genus-level relative abundance of fungi (A) and bacteria (B) from all samples. The relative abundance of fungal and bacterial genera <10% were merged as “others” in the bar plots. Samples A-S1–A-S11 were from Pond A and B-S1–B-S9 from Pond B. In the taxa names, “d” = domain, “k” = kingdom, “p” = phylum, “c” = class, “o” = order, and “f” = family.

Figure 6

Heatmap of the top 20 abundant fungal (A) and bacterial (B) genera. Block color represents the abundance of different microbial genera. In the taxa names, “d” = domain, “k” = kingdom, “p” = phylum, “c” = class, “o” = order, and “f” = family.

Figure 7

Comparison of the dominant genera of fungal (A) and bacterial (B) communities in the two analyzed ponds. Differences in the genera between both ponds as evaluated by Wilcoxon rank-sum test is indicated as: * p < 0.05, ** p < 0.01, *** p < 0.001. In the taxa names, “d” = domain, “k” = kingdom, “p” = phylum, “c” = class, “o” = order, and “f” = family.

Figure 8

Principal coordinate analysis plots of fungal (A) and bacterial (B) communities in two hot spring ponds based on weighted Unifrac distance. The r and p-values of the analysis of similarity were shown respectively in each figure (p < 0.01).

Figure 9

Linear regression between pH value and fungal OTU richness (A), bacterial OTU richness (B), and richness of OTU belonging to the fungal genus Emericellopsis (C).

Figure 10

Sediment OTUs network analysis of the Julong hot springs (Fruchterman–Reingold layout). (A) Network of fungal community; (B) network of bacterial community. Each node represents an OTU indicating a single species. Color codes for nodes belonging to different dominant phyla. The node size is proportional to the degree (degree: number of direct correlations to a node). Positive interactions are displayed as red edges and negative interactions are displayed as blue edges.

Figure 11

Comparison of culture-dependent and culture-independent analysis methods of the fungal community at the phylum (A) and genus (B) levels, and the bacterial community at the phylum (C) and genus (D) levels. In the taxa names, “d” = domain, “k” = kingdom, “p” = phylum, “c” = class, “o” = order, and “f” = family.