Homoacetogenic Bacteria & Environmental Biotechnologies - Biology Assignment Help

Assignment Task

Homoacetogenic bacteria: a review
Homoacetogenic bacteria and their application in environmental biotechnologies: a review
1. The manuscript had summarized the researches on homoacetogenic bacteria from several aspects such as discovery and diversity, identification, metabolism, significance, and industrial application. However, most of the aspects had not been expounded in depth.
2. The manuscript should not try to cover everything about homoacetogenic bacteria, but should run a common thread through the aspects. What is the common thread of the manuscript? The purpose and significance of the manuscript should be clear.
3. The manuscript, as well as each of the sections (or aspects), should collect the most valuable information based on the purpose and significance, and be constructed logically (a common thread should be run through the aspects).
4. The manuscript should focus on homoacetogenic bacteria and their application in environmental biotechnologies.
Abstract
Homoacetogens are obligative anaerobe, a characteristic feature of homoacetogenic bacteria is their ability to use carbon dioxide as a pervasive and readily available electron pool and reduce to acetate through carbon monoxide dehydrogenase production as their conventional fermentation product. They are vital microbes in frequent anaerobic conditions like sewage sludge, sediments, intestinal tracts of animals and human. Homoacetogens are involved in Methane reduction can be provided through treatment strategies in existing animals. Moreover, the potential industrial use of homoacetogens has providing motivations for research on the diversity of these microbes. After several scientific reports, the metabolic diversity of isolated strains of homoacetogenic bacteria has increased significantly, and there is no doubt that homoacetogens are the most adaptable physiological group of known anaerobic bacteria. (What is the purpose of this review? What is the revelation of this review?)
Key wards: Homoacetogens, Wood-Ljungdahl pathway, formyltetrahydrofolate synthetase, fhs gene, Gut microbes (these key wards should come into the abstract)
Introduction (As an introduction, this section should not be too long)
1. “Therefore, in this review focused the habitat, physiological pathways and importance of homoacetogens in various fields” is the issues to be posed. Thus, Section Introduction should be constructed around the issues. In other words, Section Introduction should explain why the review would focus on the issues.
2. Hence, the difference between homoacetogens and acetogens should not be the emphasis in Introduction.
3. However, the difference between homoacetogens and acetogens is also important for readers to understand homoacetogens. A separate topic out of Introduction is suggested to state the difference between homoacetogens and acetogens.
Homoacetogens are obligative anaerobes under eubacteria that are categorized by the production of acetate from CO2 in their energy metabolism (Muller 2003). They can utilize different substrates like alcohol, carbon monoxide, hexose, hydrogen, formate, methoxylated compounds and for their energy source. The homoacetogens is generating only acetate as a byproduct; the term is coined based on types of energy source and end products (the meaning of this sentence is the same as the first sentence?). The first reference to this process goes to Fischer, Liske and Winsor (1932), a sewage sludge sample acetic acid fertilizer under an oxygen-free hydrogen atmosphere in the presence of bicarbonate. Some microorganisms produced butyrate from CO2 using same acetate pathway these are called homobutarogens. The other terms used for homoacetogens are CO2 reducing acetogens, reducing acetogens, or merely acetogens. Later these terminologies are ambiguous because they produce some other products with acetate, then they focused with the difference between the metabolism of Acetogenesis and homoacetogenesis.
Acetogenesis states to the synthesis of acetate by reducing CO2 and organic acids, it must be eminent from homoacetogenesis. Acetogenesis is the production of acetate by heterotrophic microorganisms by oxidation of organic acids for example butyric, probionic, Valeric acids and alcohol (Saady, 2013). Homoacetogenesis, conversely, is the production of acetate through the reduction of CO2 with H2 by autotrophic acetogenic microorganisms. Although both acetogenesis and homoacetogenesis produce acetate, the former produces H2 and CO2, while using H2 and CO2 (Fu et al., 2019). Homoacetogens are acetogens that convert their metabolism to spontaneous growth in H2 / CO2 under stressful conditions or after a reduced organic substrate is depleted. However, it should be noted that not all acetogens are homoacetogens. The difference between acetogens and homoacetogens is a tropic or metabolic level rather than being phylogenetic. Homoacetogens share the ability to tolerate a wide range of environmental conditions with acetogens (Saady, 2013; Wang, 2013). However, homoacetogens grow faster in H2 / CO2 than in acetogens in the organic substrate. Furthermore, homoacetogens are not dependent on the activity of methanogens because they are not inhibited by high PH2. These traits are more beneficial to homoacetogens than acetogens under stress conditions. The fact that not all acetogens are homoacetogens should be the focus of research to find possible ways to control the latter while enhancing the former during H2 fermentation. Homoacetogenesis are reported in mesophilic, thermophilic, and hyperthermophilic fermentative H2 production in dark conditions (Montiel-Corona et al., 2020; Verhaart, 2010).
Acetogenic species are divided into non-proton-producing, hydrogen-producing species and species that are forced to reduce protons to hydrogen during acetogenesis. The first group is broad, consisting of homoacetogens and species that guide their metabolism in the presence of an efficient hydrogen removal system for proton reduction. They are hardy anaerobic bacteria, whose main mechanism is to reduce the acetyl-CoA pathway (i) CO2 to acetyl-CoA, (ii) the terminal electron-adopting process, and (iii) the synthesis of cell carbon from CO2 (Ragsdale and Pierce, 2008). In homoacetogenesis, this reaction is generally considered to be tropically unfavorable, resulting in low H2 fractional pressure in the total aquifer, which is common in many anaerobic habitats due to H2 consumption by microorganisms such as sulfate and iron reducers (Camacho,2009). Methanogens and homoacetogens are the major H2 consumers, and methanogens are generally thought to dominate (Ye et al., 2014). Homoacetogenic bacteria compete with methanogens for molecules such as hydrogen, formate and methanol (Angelidaki et al., 2011). Nevertheless, homoacetogens are said to be higher than methanogens at lower temperatures (Kotsyurbenko et al., 2001).
Homoacetogenic species are known as Acetobacterium, Acetoanerobium, Acetogenium, Butyribacterium, Clostridium, Eubacterium and Pelobacter. In mesophilic wastes, there are about 105 homoacetogenes per milliliter that make acetate from H2 and CO2. The competitiveness of these homoacetogens for H2 in mixed cultures has not yet been clearly established, but they can form stable interactions with H2-producing bacteria and isolate multiple thermophilic interaction cultures (Siriwongrungson et al., 2007). They are found to grow in slightly acidic conditions. In efficient H2 immersion environments, such as anaerobic digestion, many acidic organisms direct their metabolism to acetogenesis. This facial change in metabolism has been demonstrated in defined methanogenic co-cultures that degrade alcohol, lactate, pyruvate, cellophane, glucose, fructose and cellulose (Lynd et al., 2003).However, its size, magnitude, stimulus, and control have not been adequately explored. Homoacetogenic and acetogenic bacteria have received less attention than amylogenes and methanogens (Molenaar et al., 2017), especially in H2 production studies. Many studies have not focused on the metabolism of homoacetogens during H2 fermentation (Godwin et al., 2014). Almost all published studies have focused on diverting electron flux from CH4 to H2 (from H2 to CH4?); Now the research goal is to prevent homoacetogens from ingesting a pool of H2 (Conrad, 1999). Although H2 may occur all the time during dark fermentation, the results reported in the literatures indicate that homoacetogenesis is underestimated. Therefore, in this review focused the habitat, physiological pathways and importance of homoacetogens in various fields.
Discovery of homoacetogens and their diversity:
1. The bacteria (homoacetogens) should be clarified in chronological sequence along with classification.
2. The year in which a certain species was found should come to the first. Then, the characteristics and classification. And then, the follow-up discovered species in the same genus.
3. If the information in Table 1 is comprehensive, the text should be compared with the information in the table.
Homoacetogenic bacteria do not form homogeneous taxonomic unit. Maximum of the homoacetogens are coming under eubacteria with homoacetogenic energy. However, the enzymes required for this metabolism are in the kingdom. In 1932, the acetate producing organisms, it utilizes CO2 and H2 as reported by Fisher (In what environment or sample(s) were the organisms found or isolated?). The first homoacetogenic bacterium was properly described as Clostridium aceticum by Wiringa in 1936. Further, Clostridium thermoaceticum (which year?), latterly called Morella thermosetica (which year?) was cultured from horse manure, is a homoacetogen coming under Thermonorobacteriaceae family; it has the ability to produced three moles of acetic acid from a glucose molecule (therefore, is it a homoacetogen? Does Morella thermosetica form acetic acid from CO2 and H2?) (Collins et al. 1994) (is it consistent with the information in Table 1?). Advanced, several homoacetogens have been isolated from diverse sources that can metabolize hydrogen/glucose to acetate, the source of the name “homoacetogen”. Based on the physiological analysis the homoacetogenic bacteria come under Gram-positive and Gram-negative, spore forming, and non-spore-forming eubacteria. It is taxonomically diverse, totally twelve main genera coming under the homoacetogenic group based on their physiological and biochemical characteristics (Table-1). Further studies focused that the role of homoacetogens in rumen of the animals. Homoacetogens are rich in many ruminants such as deer, buffalos, cattle and sheep for example Acetitomaculum ruminis was first isolated from cattle rumen (Greening & Leedle 1989). Most of the homoacetogens are produce only acetate but certain conditions like surplus CO or H2 or acetic pH, some of the homoacetogens are utilize single carbon compounds like CO2 with H2, CO, methanol or formate to produce butyrate (Rogers & Gottschalk 1993, Drake 1994, Drake et al. 2008). For instance, Acetonema sp. is a homoacetogenic eubacterium produce butyrate along with acetate from CO2 and H2 (Schiel-Bengelsdorf & Dürre 2012).
Since the classification of the homoacetogenic bacteria is not very clear in the early stage, people tend to ignore the homoacetogenic characteristics of the strain. According to the existing reports, as early as 1993 ( Dong Xiuzhu et al 1993) purified a strict anaerobic bacterium from activated sludge with the homoacetogenic characteristics(Ilyobacter Hydrogenotrophicus Sp.Nov.), which was not taken seriously at the time, may be the first type of acetogen bacteria isolated in China. In 2013, Li Jianzheng et al isolated the anaerobic strain CA3 from the reactor activated sludge that can utilize glucose, fructose, arabinose, etc., and can also use CO2 (Li Jianzheng et al 2013) . Chong Liu et al recorded the metabolic characteristics of a homoacetogenic bacterium Blautia coccoides GA-1 with H2/CO2 and/or glucose as carbon source. The results showed that the strain was self-supported only when CO2 was used as the carbon source in the culture system. Metabolic properties are attenuated from generation to generation. When glucose is used as a carbon source, strain GA-1 exhibits opposite metabolic activity. At the same time, he also found that autotrophic metabolism is severely inhibited when autotrophic and heterotrophic coexist (Chong Liu et al 2015). To date, more than 100 acetogens (refer to homoacetogens?) have been isolated from different habitats such as soil, sediment, sludge, and the intestinal tract of many animals.
Table: 1 List of homoacetogens based on morphological and physiological characteristics (Drake et al. 2008)
Bacteria Habitat Cell shape Spore production Gram stain GC content Optimum temp? Carbon and energy sources Reduced products Ref
Acetobacterium Rods - + 38-44 27-30 Acetate Acetobacterium sp. AmMan1 freshwater sediment Mandelate,
CO2+H2 acetate Dorner & Schink 1991
Acetobacterium sp. B10 waste water pond CO2+H2 acetate Sembiring & Winter 1990
Acetobacterium sp. HA1 anoxic sewage sludge CO2+H2 acetate Schramm & Schink 1991
Acetobacterium sp. HP4 Lake sediment CO2+H2 acetate Conrad et al. 1989
Acetobacterium sp. KoB58 sewage sludge CO2+H2 acetate Wagener & Schink 1988
Acetobacterium sp. LuPhet1 sewage sludge CO2+H2
phenoxyethanol Acetate Frings & Schink 1994
 

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