Abstract
The utilization of extracellular electron transfer by microorganism is highly engaging for remediation of toxic pollutants under “energy-starved” conditions. Humin, an organo-mineral complex of soil, has been instrumental as an external electron mediator for suitable electron donors in the remediative works of reductive dehalogenation, denitrification, and so forth. Here, we report, for the first time, that humin assists microbial acetogenesis as the extracellular electron donor using the electron acceptor . Humin was obtained from Kamajima paddy soil, Japan. The anaerobic acetogenic consortium in mineral medium containing as the inorganic carbon source used suspended humin as the energy source under mesophilic dark conditions. Retardation of acetogenesis under the -deficient conditions demonstrated that humin did not function as the organic carbon source but as electron donor in the -reducing acetogenesis. The consortium with humin also achieved anaerobic dechlorination with limited methanogenic activity. Total electron-donating capacity of humin was estimated at about 87 µeeq/g-humin. The metagenomic sequencing of 16S rRNA genes showed the predominance of Firmicutes (71.8 ± 2.5%) in the consortium, and Lachnospiraceae and Ruminococcaceae were considered as the -reducing acetogens in the consortium. Thus, microbial fixation of using humin introduces new insight to the holistic approach for sustainable treatment of contaminants in environment.
Highlights
The anaerobic nonphototrophic CO2 fixers are abundant in nature [1,2], and though energetics of CO2 reduction to acetate is less favorable than methanogenesis, the ability to use diverse electron donors sets the acetogens apart from the other microbial populations [3,4]
10% (v/v) transfer of the culture was carried out across the generation using the medium in absence of acetate, steady acetate concentration detected after two weeks of incubation demonstrated acetate production by the consortium PCPA0
3 The equivalent contribution as acetate production from phenol has been calculated based on benzyl-CoA pathway [44], and the equivalent contribution as electrons was subtracted from the estimated electron-donating capacity of humin. It was demonstrated for the first time that humin supported microbial CO2 -reducing acetogenesis by donating electrons as electron mediator (EEM) to the consortium PCPA0
Summary
The anaerobic nonphototrophic CO2 fixers are abundant in nature [1,2], and though energetics of CO2 reduction to acetate is less favorable than methanogenesis, the ability to use diverse electron donors sets the acetogens apart from the other microbial populations [3,4]. CO2 fixation [7], reductive acetogenesis attracted much attention, for the autotrophic. In dark subsurface environments where organic substrates and energy are limited, acetate produced by the autotrophic WLP can be utilized by anaerobic microbial consortium as a carbon and energy source for reductive dehalogenation (organohalide respiration) in bioremediation of toxic substances [9,10,11]. Humic substances (HSs) are the key component of the carbon cycle on the earth, attained by virtue of biogeochemical transformation and long residence time of soil organic matter with mineral. Public Health 2020, 17, 4211; doi:10.3390/ijerph17124211 www.mdpi.com/journal/ijerph
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