Shewanella algae Relatives Capable of Generating Electricity from Acetate Contribute to Coastal-Sediment Microbial Fuel Cells Treating Complex Organic Matter

Abstract

To identify exoelectrogens involved in the generation of electricity from complex organic matter in coastal sediment (CS) microbial fuel cells (MFCs), MFCs were inoculated with CS obtained from tidal flats and estuaries in the Tokyo bay and supplemented with starch, peptone, and fish extract as substrates. Power output was dependent on the CS used as inocula and ranged between 100 and 600 mW m^-2 (based on the projected area of the anode). Analyses of anode microbiomes using 16S rRNA gene amplicons revealed that the read abundance of some bacteria, including those related to Shewanella algae, positively correlated with power outputs from MFCs. Some fermentative bacteria were also detected as major populations in anode microbiomes. A bacterial strain related to S. algae was isolated from MFC using an electrode plate-culture device, and pure-culture experiments demonstrated that this strain exhibited the ability to generate electricity from organic acids, including acetate. These results suggest that acetate-oxidizing S. algae relatives generate electricity from fermentation products in CS-MFCs that decompose complex organic matter.

Keywords: coastal sediment; electrode-plate culture; exoelectrogen; metabarcoding; microbial fuel cell.

Fig. 1.

Electricity generation in CS-MFCs. (a) Changes in J (a) and P_max (b). COD concentrations in refreshed media and external resisters for CS-MFCs are indicated above the J curves. On day 106, portions of anodes were cut from these MFCs, and they were re-started.

Fig. 2.

Metabarcoding of 16S rRNA gene amplicons for bacterial populations in original sediments (OS), anode biofilms (AB), and planktonic microbes (PM) in CS-MFCs. Sequences were classified at the genus level.

Fig. 3.

A phylogenetic tree based on 16S rRNA gene sequences showing relationships among bacterial strains in the genus Shewanella. Alteromonas mediterranea is used as the outgroup. Bootstrap values (100 trials) are indicated at branching points. Sequence divergence is indicated with bars, and accession numbers are shown in parentheses.

Fig. 4.

Current generation by Shewanella sp. OR-1, Shewanella algae BrY, and Shewanella oneidensis MR-1. Representative J curves (a, c, e, g, and i) and mean J_max values (n=3, bars indicate SDs) (b, d, f, h, and j) are shown. The substrates used (10‍ ‍mM) were no substrate (a, b), glucose (c, d), lactate (e, f), propionate (g, h), and acetate (i, j).