Natural organic matter as global antennae for primary production.

Humic substances (HS) are high-molecular-weight complex refractory organics that are ubiquitous in terrestrial and aquatic environments. While resistant to microbial degradation, these compounds nevertheless support microbial metabolism via oxidation or reduction of their (hydro)quinone moieties. As such, they are known to be important electron sinks for respiratory and fermentative bacteria and electron sources for denitrifying and perchlorate-reducing bacteria. HS also strongly promote abiotic reduction of Fe(III) when irradiated with light. Here, we show that HS-enhanced Fe(III) photoreduction can also drive chemolithotrophic microbial respiration by producing Fe(II), which functions as a respiratory electron donor. Due to their molecular complexity, HS absorb most of the electromagnetic spectrum and can act as broad-spectrum antennae converting radiant energy into bioavailable chemical energy. The finding that chemolithotrophic organisms can utilize this energy has important implications for terrestrial, and possibly extraterrestrial, microbial processes and offers an alternative mechanism of radiation-driven primary productivity to that of phototrophy.

Humic acid-oxidizing, nitrate-reducing bacteria in agricultural soils.

UNLABELLED: This study demonstrates the prevalence, phylogenetic diversity, and physiology of nitrate-reducing microorganisms capable of utilizing reduced humic acids (HA) as electron donors in agricultural soils. Most probable number (MPN) enumeration of agricultural soils revealed large populations (10(4) to 10(6) cells g(-1) soil) of microorganisms capable of reducing nitrate while oxidizing the reduced HA analog 2,6-anthrahydroquinone disulfonate (AH(2)DS) to its corresponding quinone. Nitrate-dependent HA-oxidizing organisms isolated from agricultural soils were phylogenetically diverse and included members of the Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. Advective up-flow columns inoculated with corn plot soil and amended with reduced HA and nitrate supported both HA oxidation and enhanced nitrate reduction relative to no-donor or oxidized HA controls. The additional electron donating capacity of reduced HA could reasonably be attributed to the oxidation of reduced functional groups. Subsequent 16S rRNA gene-based high-density oligonucleotide microarray (PhyloChip) indicated that reduced HA columns supported the development of a bacterial community enriched with members of the Acidobacteria, Firmicutes, and Betaproteobacteria relative to the no-donor control and initial inoculum. This study identifies a previously unrecognized role for HA in stimulating denitrification processes in saturated soil systems. Furthermore, this study indicates that reduced humic acids impact soil geochemistry and the indigenous bacterial community composition. IMPORTANCE: This study identifies a new metabolic capacity in soil microbial communities that may be responsible for the mediation of significant nitrogen losses from soil systems. Nitrate-dependent humic acid (HA)-oxidizing organisms isolated from agricultural soils were phylogenetically diverse and included members of Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. Advective up-flow columns inoculated with corn plot soil and amended with reduced HA and nitrate supported both HA oxidation and enhanced nitrate reduction relative to no-donor or oxidized HA controls. The additional electron donating capacity of reduced HA could reasonably be attributed to the oxidation of reduced functional groups.

Environmental and taxonomic bacterial diversity of anaerobic uranium(IV) bio-oxidation.

Microorganisms in diverse terrestrial surface and subsurface environments can anaerobically catalyze the oxidative dissolution of uraninite. While a limited quantity (∼5 to 12 µmol liter(-1)) of uranium is oxidatively dissolved in pure culture studies, the metabolism is coupled to electron transport, providing the potential of uraninite to support indigenous microbial populations and to solubilize uranium.

Electrochemical stimulation of microbial perchlorate reduction.

As part of our studies into the diversity of dissimilatory perchlorate reducing bacteria (DPRB) we investigated the reduction of perchlorate in the cathodic chamber of a bioelectrical reactor (BER). Our results demonstrated that washed cells of Dechloromonas and Azospira species readily reduced 90 mg L(-1) perchlorate in the BER with 2,6-anthraquinone disulfonate (AQDS) as a mediator. No perchlorate was reduced in the absence of cells or AQDS, or in an open-circuit control. Similar results were observed when a natural microbial community was inoculated into a fed-batch BER. After 70 days of operation, a novel DPRB, strain VDY, was isolated which readily reduced perchlorate in a mediatorless BER. Continuous up-flow BERs (UFBERs) were seeded with active cultures of strain VDY, and perchlorate at a volumetric loading of 60 mg L(-1) day(-1) was successfully removed. Gas phase analysis indicated that low levels of H2 produced at the cathode surface through electrolysis may mediate this metabolism. The results of these studies demonstrate that biological perchlorate remediation can be facilitated through the use of a cathode as the primary electron donor, and that continuous treatment in such a system approaches current industry standards. This has important implications for the continuous treatment of this critical contaminant in industrial waste streams and drinking water.