Sun Yat-sen University Wang Shanquan's team ISME Comms: Habitat-based "combination" of obligate dehalogenation bacteria and non-obligate dehalogenation bacteria

Click the blue word above to follow usFirst author: Liang Yongyi Corresponding author: Wang Shanquan A research team from the University's School of Environmental Science and Engineering published a research paper titled "Substrate-dependent competition and cooperation relationships between Geobacter and Dehalococcoides for their organohalide respiration" in the journal ISME Communications
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In this study, based on the growth rate/yield tradeoff strategy of different microbial populations, a very small amount of non-obligate dehalogenation was isolated from a mixed population dominated by obligate Dehalococcoides.
Geobacter provides a new method for the enrichment and isolation of difficult-to-cultivate dehalogenated respiratory bacteria
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Further research on the "Dehalococcoides-Geobacter" interaction relationship showed that: based on different habitats, there are three typical interaction mechanisms between obligate dehalogenation bacteria and non-obligate dehalogenation bacteria - free competition, conditional competition and mutualism
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This study revealed the typical interaction mechanism between obligate dehalogenation bacteria and non-obligate dehalogenation bacteria, and provided a new strategy for the separation of dehalogenation bacteria, which provided the basis for the development of halogenated organic bioremediation functional bacteria and the regulation of flora composition and function.
theoretical basis
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Introduction Based on the results of previous studies, we found that the cell growth of different organohalide-respiring bacteria (OHRB) conforms to the "rate/yield balance theory": non-obligate dehalogenation bacteria (non-obligate OHRB, such as Geobacter and Desulfobacter) usually grow faster than obligate OHRB (eg: Dehalogenococcus), but have lower cell yields, and the growth advantage is more significant in the case of low population density and high food utilization in the early inoculation period; In contrast, obligate dehalogenation bacteria had lower growth rates and higher cell yields (Fig.
1)
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Based on the above characteristics of dehalogenation bacteria, combined with the "rate/yield balance theory", it is possible to isolate the target dehalogenation bacteria from the microbial community where obligate dehalogenation bacteria and non-obligate dehalogenation bacteria coexist
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To confirm this hypothesis, we conducted this study
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Figure 1.
Rate/yield balance between high growth rate-low cell concentration (red line) and low growth rate-high cell concentration (blue line) populations In ethylene (PCE) dechlorination cultures, Dehalococcoides, which grows slowly but with high cell concentration, dominates (68.
87% relative abundance), while Geobacter, which grows fast and has low cell concentration, has a very low relative abundance.
(<1%)
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According to the rate/yield balance theory, Geobacter with high growth rate should benefit under the condition of low population density and high substrate availability, while the high population density and low substrate condition are more favorable for the growth of Dehalococcoides with the extension of culture time
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In the experiment, the culture period was shortened from 8 days to 3 days.
After continuous subculture, Dehalococcoides was gradually eliminated from the community, and Geobacter gradually became the dominant population (relative abundance reached 97.
4%) after 25 subcultures (Figure 2).

.

Fig.
2 The enrichment process of target dehalogenation bacteria: (A) Geobacter gradually became the dominant dehalogenation bacteria in the community; (B) growth curve of obligate dehalogenation bacteria Dehalococcoides and non-obligate dehalogenation bacteria Geobacter
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 2.
Isolation and identification of Geobacter lovleyi LYY strains Geobacter lovleyi LYY strains were finally obtained after enriching Geobacter into an absolutely dominant population, further adopting the dilution-to-extinction method and adjusting the composition of the medium
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The obtained pure bacteria were further confirmed as pure bacteria by scanning electron microscope (SEM) and analysis methods based on single nucleotide polymorphism (SNP) (Fig.
3)
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Through whole genome sequencing analysis, it was found that the LYY strain genome contains 3521 protein-coding genes, including one dehalogenase gene
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The amino acid sequence similarity between the dehalogenase gene and the perchloroethylene dehalogenase gene pceA in the G.
lovleyi SZ genome reached 99%
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Figure 3 Geobacter lovleyi LYY scanning electron microscope Figure 3.
Competition and cooperation between Geobacter and Dehalogenococcus After obtaining LYY isolates, the LYY strain (Geo) and the existing Dehalococcoides mccartyi CG1 strain (Dhc) in the laboratory were used to form Geobacter -Dhc co-culture, and set up three culture conditions: (i) pyruvate as carbon source/electron donor, PCE as electron acceptor; (ii) pyruvate as carbon source and electron donor, PCB180 as electron acceptor ; (iii) acetic acid/H2 as carbon source/electron donor and PCE as electron acceptor
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The results showed that in the PCE dechlorination experiment, when acetic acid/H2 was the carbon source/electron donor and PCE was the electron acceptor, the two dehalogenation bacteria were in a state of complete competition, competing for both the electron donor/carbon source and the electron acceptor.
Electron acceptor; when pyruvate is the carbon source/electron donor and PCB is the electron acceptor, the two dehalogenation bacteria are in a strict symbiotic relationship, Geo provides the electron donor/carbon source for Dhc, and Dhc is used in the PCB dehalogenation process In , the thermodynamic reaction conditions of Geo bacteria are promoted by consuming the metabolites of Geo; when pyruvate is the carbon source/electron donor and PCE is the electron acceptor, the two dehalogenation bacteria are in a competitive and cooperative relationship.
But Geo is in a dominant position (Figure 4)
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Figure 4.
Summary of the interaction between Geobacter lovleyi LYY and Dehalococcoides mccartyi CG1 co-cultures under different culture conditions.
Based on the "rate/yield balance theory", this study successfully isolated Geobacter lovleyi LYY, which opened up opportunities for the isolation of refractory dehalogenated respiratory bacteria.
new way
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At the same time, based on the co-culture study of non-obligate dehalogenation bacteria represented by LYY strain and obligate dehalogenation bacteria represented by CG1, as well as the relationship between the two types of functional bacteria in mixed culture, it was found that the obligate dehalogenation bacteria There are three types of interactions between halogen bacteria and non-obligate dehalogenation bacteria: free competition, conditional competition and mutualism (Figure 5)
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The discovery of this interaction model provides a theoretical basis for understanding and regulating the composition and function of different dehalogenated populations in the environment
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Figure 5 The interaction model between obligate dehalogenation bacteria and non-obligate dehalogenation bacteria Introduction of the main author First author: Liang Yongyi, a master student at the School of Environmental Science and Engineering, Sun Yat-Sen University, tutored by Professor Wang Shanquan, whose research direction is halogenation persistence Bioremediation of organic pollution
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Corresponding author: Wang Shanquan, professor and doctoral supervisor of the School of Environmental Science and Engineering, Sun Yat-Sen University; mainly engaged in the research on the mechanism and application of reductive dehalogenation of halogenated organics by microorganisms
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Among them, the obtained polychlorinated biphenyls (PCBs) dechlorination strains and their functional gene information are a major breakthrough in this research field in the past 30 years (PNAS, Wang et al.
, 2014; PNAS, Bedard, 2014)
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Relevant research results have been published in important academic journals such as PNAS, ES&T and AEM
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Contact email: wangshanquan@mail.
sysu.
edu.
cn Contribution: Wang Shanquan team of Sun Yat-sen University
.

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