Conductive Fe3O4 Nanoparticles Accelerate Syntrophic Methane Production from Butyrate Oxidation in Two Different Lake Sediments

Abstract

Syntrophic methanogenesis is an essential link in the global carbon cycle and a key bioprocess for the disposal of organic waste and production of biogas. Recent studies suggest direct interspecies electron transfer (DIET) is involved in electron exchange in methanogenesis occurring in paddy soils, anaerobic digesters, and specific co-cultures with Geobacter. In this study, we evaluate the possible involvement of DIET in the syntrophic oxidation of butyrate in the enrichments from two lake sediments (an urban lake and a natural lake). The results showed that the production of CH4 was significantly accelerated in the presence of conductive nanoscale Fe3O4 or carbon nanotubes in the sediment enrichments. Observations made with fluorescence in situ hybridization and scanning electron microscope indicated that microbial aggregates were formed in the enrichments. It appeared that the average cell-to-cell distance in aggregates in nanomaterial-amended enrichments was larger than that in aggregates in the non-amended control. These results suggested that DIET-mediated syntrophic methanogenesis could occur in the lake sediments in the presence of conductive materials. Microbial community analysis of the enrichments revealed that the genera of Syntrophomonas, Sulfurospirillum, Methanosarcina, and Methanoregula were responsible for syntrophic oxidation of butyrate in lake sediment samples. The mechanism for the conductive-material-facilitated DIET in butyrate syntrophy deserves further investigation.

Keywords: Fe3O4; butyrate; direct interspecies electron transfer; lake sediments; methane; methanogenesis; syntrophy.

FIGURE 1

Effects of conductive Fe₃O₄ nanoparticles on the production of CH₄ in the enrichments from Weiming Lake sediment, WML (A–D), and Erhai Lake sediment, EHL (E–H). Error bars represent the standard deviation of replicate experiments.

FIGURE 2

Effects of conductive Multi-walled CNTs on the production of CH₄ in the third and fourth enrichments from WML (A,B) and EHL (C,D).

FIGURE 3

Spatial distribution of archaeal (Arc915-FITC, green) and bacterial (EUB338mix-Cy3, red) cells identified by FISH in WML and EHL enrichments with CK, Fe₃O₄ and MWCNTs treatments. (CK treatment: A,B; Fe₃O₄ treatment: C,D; WMCNTs treatment: E,F). The Fe₃O₄ nanoparticles and the MWCNTs appear black in the FISH images from the corresponding treatments.

FIGURE 4

Scanning electron micrographs (SEM) of cell aggregates or cell/material mixtures in the enrichment cultures from WML and EHL (WML: A,C,E; EHL: B,D,F). White arrows indicate cells and the black arrows indicate Fe₃O₄ nanoparticles.

FIGURE 5

Neighbor-joining phylogenetic tree of representative bacterial (A) and archaeal (B) 16S rRNA gene clones obtained from the fourth enrichments of WML with the addition of Fe₃O₄ nanoparticles. Clones obtained in this study are indicated in boldface and their relative abundances are given in parentheses. GenBank accession numbers of reference sequences are indicated.

FIGURE 6

The community composition and relative abundance of bacteria (A) and archaea (B) at genus level in the fourth enrichments of EHL as determined by Illumina Miseq sequencing. The genus whose relative abundance was less than 3% was included in the group “Other.” More than 30,000 sequences were obtained for the bacteria and archaea phylogenetic classifications at genus level separately.