Diversity and Ubiquity of Bacteria Capable of Utilizing Humic Substances as Electron Donors for Anaerobic Respiration

Abstract

Previous studies have demonstrated that reduced humic substances (HS) can be reoxidized by anaerobic bacteria such as Geobacter, Geothrix, and Wolinella species with a suitable electron acceptor; however, little is known of the importance of this metabolism in the environment. Recently we investigated this metabolism in a diversity of environments including marine and aquatic sediments, forest soils, and drainage ditch soils. Most-probable-number enumeration studies were performed using 2,6-anthrahydroquinone disulfonate (AHDS), an analog for reduced HS, as the electron donor with nitrate as the electron acceptor. Anaerobic organisms capable of utilizing reduced HS as an electron donor were found in all environments tested and ranged from a low of 2.31 ؋ 10 1 in aquifer sediments to a high of 9.33 ؋ 10 6 in lake sediments. As part of this study we isolated six novel organisms capable of anaerobic AHDS oxidation. All of the isolates coupled the oxidation of AHDS to the reduction of nitrate with acetate (0.1 mM) as the carbon source. In the absence of cells, no AHDS oxidation was apparent, and in the absence of AHDS, no cell density increase was observed. Generally, nitrate was reduced to N 2 . Analysis of the AHDS and its oxidized form, 2,6-anthraquinone disulfonate (AQDS), in the medium during growth revealed that the anthraquinone was not being biodegraded as a carbon source and was simply being oxidized as an energy source. Determination of the AHDS oxidized and nitrate reduced accounted for 109% of the theoretical electron transfer. In addition to AHDS, all of these isolates could also couple the oxidation of reduced humic substances to the reduction of nitrate. No HS oxidation occurred in the absence of cells and in the absence of a suitable electron acceptor, demonstrating that these organisms were capable of utilizing natural HS as an energy source and that AHDS serves as a suitable analog for studying this metabolism. Alternative electron donors included simple volatile fatty acids such as propionate, butyrate, and valerate as well as simple organic acids such as lactate and pyruvate. Analysis of the complete sequences of the 16S rRNA genes revealed that the isolates were not closely related to each other and were phylogenetically diverse, with members in the alpha, beta, gamma, and delta subdivisions of the Proteobacteria. Most of the isolates were closely related to known genera not previously recognized for their ability to couple growth to HS oxidation, while one of the isolates represented a new genus in the delta subclass of the Proteobacteria. The results presented here demonstrate that microbial oxidation of HS is a ubiquitous metabolism in the environment. This study represents the first description of HS-oxidizing isolates and demonstrates that microorganisms capable of HS oxidation are phylogenetically diverse.Humic substances (HS) are ubiquitous in the environment and can be readily isolated from nearly all soils, waters, and sediments. The natural organic matter content of many soils and sedi...

Electron Shuttles Enhance Anaerobic Ammonium Oxidation Coupled to Iron(III) Reduction

“…35,38 In our study, several iron(III)-reducing bacteria (Geobacter and Geothrix) are also capable of using nitrate as an electron acceptor, 43,48 which may lead to the reduction of nitrate to nitrite and ultimately dinitrogen. 12,49 However, almost no known anammox bacteria were detected in both enrichments with all treatments, which was likely due to the extremely low abundance of anammox bacteria.…”

Ferrihydrite-associated organic matter (OM) stimulates reduction by <i>Shewanella oneidensis</i> MR-1 and a complex microbial consortia

“…Humic acids therefore possess a redox-mediating ability which positively affects the cycling of redox-active minerals, including Fe(III) and Mn(IV) / Mn(III), in conjunction with transformation of organic and inorganic compounds. Subsequent studies have shown that reduced forms of humic substances can serve as electron donors for anaerobic organisms growing on a variety of terminal electron acceptors, including nitrate and fumarate (Coates et al, 2002). Under these conditions, microorganisms utilize the reduced humic substances as an energy source, while using a readily degradable carbon source, such as acetate, for growth.…”

“…Previous studies have revealed that some Fe(III)-reducing microorganisms, such as S. alga, G. metallireducens, and Geothrix fermentans, have the ability to undergo dissimilatory humic reduction and to couple reduced humic oxidation with nitrate reduction. Moreover, Paracoccus denitrificans, a known denitrifier, is unable to utilize humics or Fe(III) as a terminal electrons acceptors; however, these microorganisms are capable of using reduced humics as an electron donor for denitrification (Coates et al, 2002;Lovley et al, 1999).…”

Anaerobic Methane Oxidation Driven by Microbial Reduction of Natural Organic Matter in a Tropical Wetland

“…6b). Humus-reducing bacteria that proliferated throughout the 5 months of enrichment included genera from the Desulfuromonadales (29,30), Clostridiales (14,28), and Propionibacteriales (31) orders, which increased by 27%, 7%, and 12%, respectively, with respect to the original composition (Fig. 6c).…”