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Biodegradation-based remediation – overview and case studies
Published in Katalin Gruiz, Tamás Meggyes, Éva Fenyvesi, Engineering Tools for Environmental Risk Management – 4, 2019
M. Molnár, K. Gruiz, É. Fenyvesi
Petroleum hydrocarbons comprise diverse groups of compounds including alkanes, alkenes, aromatic constituents, and heterocyclic compounds. Among petroleum hydrocarbons n-alkanes are degraded faster than branched alkanes and aromatics compounds. Long-chain n-alkanes (C10–C24) are degraded most rapidly. Short-chain alkanes (less than C9) are toxic to many microorganisms, but they evaporate rapidly from petroleum-contaminated sites. Oxidation of alkanes is classified as being terminal or diterminal, the monoterminal oxidation being the main pathway. It proceeds via the formation of the corresponding alcohol, aldehyde, and fatty acid (see Figure 5.6). Beta-oxidation of the fatty acids results in the formation of acetyl-CoA (Fritsche & Hofrichter, 2008). Cyclic alkanes representing minor components of mineral oil are relatively resistant to microbial attack. The absence of an exposed terminal methyl group complicates the primary attack. A few species are able to use cyclohexane as sole carbon source; more common is its cometabolism (see later) with mixed cultures.
Advanced Biomethane Processes
Published in Eduardo Rincón-Mejía, Alejandro de las Heras, Sustainable Energy Technologies, 2017
Sevcan Aydin, Bahar Yavuzturk Gul, Aiyoub Shahi
Several groups of facultative and anaerobic microorganisms take part in the stages of the anaerobic digestion process to degrade organic material. The synergetic community of microorganisms found in an anaerobic digester conducts the process of fermenting organic matter into methane. Anaerobic digestion is mediated during the stages of hydrolisis, acidogenesis, acetogenesis, and methanogenesis by anaerobic microorganisms. Mentioned stages are shown in Figure 21.1 (Aydin et al., 2015). Complex particulate materials such as lipids, carbohydrates, and proteins must be hydrolyzed to soluble organic matter that can be absorbed by microbial cells. This hydrolysis step is done by specific extracellular enzymes that are produced by hydrolytic fermentative bacteria under anaerobic conditions in anaerobic digesters. pH, temperature, substrate composition, cell residence time, and the by-products of hydrolytic bacteria are important factors affected by the reaction rates of extracellular enzymes in hydrolysis. The microbial community of the hydrolysis stage is considerably heterogenic. It was found that the compounds containing cellulose are degraded by Clostridium spp., but, Bacillus spp. are responsible for the degradation of protein and fats. The most widespread hydrolytic microorganisms are classified as cellulolytic (Clostridium thermocellum), proteoytic (Clostridium bifermentas, Peptococcus spp.), lipolytic (genera of Clostridia and Micrococci) and aminolytic (Clostridium butyricum, Bacillus subtilis) bacteria. It was found that several anaerobic fungi also degrade cellulose and hemicelluloses (Yuan and Zhu, 2016). The hydrolytic microorganisms are also capable of degradation of some intermediate products to simple volatile fatty acids (VFAs), lactic acid, carbon dioxide, hydrogen, ethanol, ammonia, and hydrogen sulfide. After hydrolysis, the soluble monomers that are generated are broken down to short-chain organic acids, alcohols, hydrogen, and carbon dioxide by facultative and obligatory anaerobic bacteria in the second step, defined as acidogenesis. The concentration of hydrogen ions is important for determining the type of end products that will be generated. The partial pressure of hydrogen must be high in order to form acetate. Acidogenic or fermentative bacteria can metabolize amino acids and sugars to intermediary products like acetate and hydrogen. While single amino acids are produced by Clostridia, Mycoplasmas and Streptococci, butanol, butyric acid, acetone, and iso-propanol are usually produced by Clostridum sp. Butyrate is produced by Butyribacterium; acetone and butanol are produced by Clostridium acetobutylicum. Clostridium butylicum also produces butanol, hydrogen, carbon dioxide, and iso-propanol. During these reactions, different pathways are used. In the degradation of carbohydrates, propionic acid is formed by the succinate pathway and the acrylic pathway. Butyric acid and fatty acids are degraded by the beta oxidation reaction. Proteins are degraded by the Stickland reaction, and if cysteine is degraded, hydrogen sulfide can be formed (Zahedi et al., 2016).
Comparison of biodegradation of lubricant wastes by Scenedesmus vacuolatus vs a microalgal consortium
Published in Bioremediation Journal, 2019
Stella Beverly Eregie, Sumaiya F. Jamal-Ally
To date, no catabolic pathways have been described regarding microalgal-mediated biodegradation. Also, the knowledge about the enzymatic reactions involved in the degradation process are still quite scarce. However, the catabolic pathway used by microalgae to degrade hydrocarbon compounds was found to be similar to that of the bacteria, fungal and mammalian systems (Prasad 2015). Algal-mediated biodegradation involves the conversion of hydrocarbon compounds to hydroxylated/oxidized intermediates. From research done, two classes of enzymes are seen to be responsible for biodegradation and these are monooxygenase and dioxygenase enzymes. These enzymes facilitate the degradation of hydrocarbon compounds by microalgae. Most research on the degradation of hydrocarbon compounds by microalgae has focused on the initial step of oxidation/subterminal oxidation (Chikere, Okpokwasili, and Chikere 2011). For aliphatic compounds (n-alkanes), the monooxygenase or dioxygenase takes on the methyl group to form an alcohol and the alcohol formed is further broken down to aldehyde and fatty acid. The fatty acid formed is further oxidized through a beta-oxidation pathway, to form acetyl-CoA or propionyl-CoA which is then broken down through a tricarboxylic acid cycle to carbon dioxide and water (Chikere, Okpokwasili, and Chikere 2011). For aromatic compounds, no catabolic pathways for the degradation of aromatic compounds had been described. However, recently, Subashchandrabose et al. (2013) reported the biodegradation of high-molecular-weight polyaromatic hydrocarbons such as benzo(a)-pyrene by Selenastrum capricornutum, on benzo(a)- pyrene. They found that the microalgal used a dioxygenase system to oxidize the compound to cis-dihydrodiols which were then converted to sulfate ester and α and β-glucoside conjugates. Nevertheless, further studies are required to confirm this mechanism.