Enriching distinctive microbial communities from marine sediments via an electrochemical-sulfide-oxidizing process on carbon electrodes

Abstract

Sulfide is a common product of marine anaerobic respiration, and a potent reactant biologically and geochemically. Here we demonstrate the impact on microbial communities with the removal of sulfide via electrochemical methods. The use of differential pulse voltammetry revealed that the oxidation of soluble sulfide was seen at +30 mV (vs. SHE) at all pH ranges tested (from pH = 4 to 8), while non-ionized sulfide, which dominated at pH = 4 was poorly oxidized via this process. Two mixed cultures (CAT and LA) were enriched from two different marine sediments (from Catalina Island, CAT; from the Port of Los Angeles, LA) in serum bottles using a seawater medium supplemented with lactate, sulfate, and yeast extract, to obtain abundant biomass. Both CAT and LA cultures were inoculated in electrochemical cells (using yeast-extract-free seawater medium as an electrolyte) equipped with carbon-felt electrodes. In both cases, when potentials of +630 or +130 mV (vs. SHE) were applied, currents were consistently higher at +630 then at +130 mV, indicating more sulfide being oxidized at the higher potential. In addition, higher organic-acid and sulfate conversion rates were found at +630 mV with CAT, while no significant differences were found with LA at different potentials. The results of microbial-community analyses revealed a decrease in diversity for both CAT and LA after electrochemical incubation. In addition, some bacteria (e.g., Clostridium and Arcobacter) not well-known to be capable of extracellular electron transfer, were found to be dominant in the electrochemical cells. Thus, even though the different mixed cultures have different tolerances for sulfide, electrochemical-sulfide removal can lead to major population changes.

Keywords: carbon electrodes; differential pulse voltammetry; electrochemical sulfide oxidation; marine sediments; microbial community analyses.

Figure 1

(A) Differential pulse voltammogram of sulfide oxidation at different pHs. The numbers in the parentheses denote (actual pH value, peak potential). Solid line, pH 8; dash line, pH 7; dash-dot line pH 6; short dash line, pH 5; dash-dot-dot line, pH 4; dot line, sulfide-free test at pH 8. (B) Responding current during electrochemical sulfide oxidation with 630 mV poised on the graphite electrodes. The broken and solid lines represent the results collected at pH = 8 and 4, respectively. The arrow indicates high concentration of sulfide spiked (resulting 1.6 mM) at t = 0 hr.

Figure 2

Time course profiles of current, lactate, and acetate for CAT and LA tested in electrochemical cells. (A,B) Current obtained at +630 mV, red line; current obtained at +130 mV, blue line; (C,D) lactate at +630 mV, red triangle; sulfate at +630 mV, red circle; acetate at +630 mV, red X; lactate at +130 mV, blue triangle; sulfate at +130 mV, blue circle; acetate at +130 mV, blue X.

Figure 3

Microbial community analyses (A) CAT in serum bottles; (B) CAT in electrochemical cells, +630 mV; (C) LA in serum bottles; (D) LA in electrochemical cells, +130 mV; (E)LA in electrochemical cells, +630 mV.

Figure 4

SEM images of biofilms on graphite-felt electrodes. (A) Developed biofilms of CAT under the condition of +630 mV; (B) developed biofilms of LA under the condition of +130 mV; (C,D) developed biofilms of LA under the condition of +630 mV that some nano-wire-like structures could be seen. The white arrows indicate the significant nanowire appendages lining on graphite fibers.