The microbial communities at the phylum level in MFC and MEC are significantly different. In MFCs, Synergistetes were the dominant phylum followed by Bacteroidetes, then Proteobacteria, Firmicutes, finally Chloroflexi. The order of the phyla in both MFCs was the same; however, the percentage was different. In MECs, Proteobacteria was the dominant phylum, followed by Firmicutes, Bacteroidetes, Synergistetes and Chloroflexi.
Proteobacteria in the MECs was observed in higher percentages (24 – 41 %) than in the MFCs (13 – 15 %), as Proteobacteria species are well known for their ability to produce and consume H2 (Schaetzle et al., 2008). The presence of Chloroflexi in
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% MEC1 MEC2 MFC1 MFC2
Betaproteobacteria (N/A) Thauera Petrimonas
Synergistia (N/A) Negativicutes (N/A) Desulfovibrio
Clostridia (N/A0 Butyricicoccus Cloacibacillus
Acetobacterium Firmicutes_unclassified Deltaproteobacteria (N/A)
Anaerolineae (N/A) Bacteroidetes_unclassified Meniscus
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MFCs and MECs is not unusual, as they are common in biological nutrient removal processes. Chloroflexi has been found as a dominant phylum in the anode biofilm in different MFC studies (De Schamphelaire et al., 2010); in this study, it was present in the MFCs (6 %) at higher percentages than in the MECs (< 1 %). Chloroflexi plays an important role in wastewater treatment (responsible for the degradation of carbohydrates) and nitrogen removal (Meng et al., 2013). Finally, the abundance of Synergistetes in the MFCs (27 – 32 %) was higher than in the MECs (5 – 6 %). Further studies are needed to understand the role of Synergistetes in BESs.
To provide a better understanding of the system, genera were analysed. A hierarchy cluster heat map highlighted the top 50 families, accounting for the most read sequences (Fig.6.4). The heat map showed clear distinctions in community structure between the anode of the MFCs and the anode of the MECs, despite the fact that they were inoculated with the same wastewater. In genus level analysis, the dominant genera in both systems were different. There were some genera present in the MFCs only and that could not be detected in the MECs. In total, 1037 OTUs were detected in both the MFCs and the MECs (Fig.6.5). Both systems were sharing 259 OTUs accounting for 25% of the total OTUs, and the majority of the shared OTUs were Proteobacteria and Bacteroidetes. In addition, 318 OTUs were detected only in the MFCs, whereas 201 OTUs were only detected in the MECs.
Proteobacteria and Bacteroidetes are commonly found as dominant phyla in BESs fed with artificial wastewater (Shimoyama et al., 2009), industrial wastewater (Kiely et al., 2011), and acetate (Ishii et al., 2008). Proteobacteria, which include Deltaproteobacteria species, were responsible for direct electricity production. Similarly, in the Bacteroidetes phylum, Shimoyama et al. (2009) stated that Bacteroidetes, which is correlated with electricity generation from artificial wastewater, was an abundant phylum in a continuous-flow cassette-electrode MFC. Therefore, Proteobacteria and Bacteroidetes phyla are the most important phyla in BESs, as both MFCs and MECs contain those phyla.
At the genera level, statistical analysis was conducted to identify the difference in genus between the microbial communities in the MFCs and the MECs. There was a statistically significant difference (P<0.05) between the two communities, as some genus were found in one system but could not be found in the other. The Bacteroides
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gene, which is known to be able to produce H2 by fermentation and to produce
electricity in MFCs (Kim et al., 2006), was found only in the MFCs in small quantities (< 1%). Acidaminococcus from the Firmicutes phylum was detected only in the MECs, which shows their importance in the operation of MECs. The role of
Acidaminococcus in MECs is amino acid degradation, and it is involved in enhancing
hydrogen productivity (Lay et al., 2010). The genes Desulfocapsa and
Telmatospirillum from the Proteobacteria phylum were found in the MECs in higher
abundance than in the MFCs.
Figure 6.4 Heat map graph of hierarchy cluster for the top fifty families.
M FC1 -w M FC1 -b M FC1 -a M FC2 -w M FC2 -a M FC2 -b M EC 2 -w M EC 1 -b M EC 1 -w M EC 2 -b M EC 1 -a M EC 2 -a
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Desulfobulbus is a well-known gene that plays an important role in transferring
electrons to the anode; it belongs to the sulphate reducers class, and it is involved in electricity generation in BESs (Lovley, 2006). It was observed in this study that the MFCs and the MECs contained a low percentage of Desulfobulbus. In addition, it has been reported that Desulfobulbus is able to produce electricity in an MFC, using the anode electrode as a direct electron acceptor (Holmes et al., 2004).
The Pseudomonas genus was found in both systems at a low percentage (<1 %); however, it was present in the MECs with a higher percentage than in the MFCs.
Pseudomonas have commonly been found in MEC studies. The presence of Pseudomonas can support electricity generation by self-producing mediators to
shuttle electrons outside the cell or between cells (Boon et al., 2008). In addition,
Pseudomonas is responsible for organic matter degradation and metal resistance. The Thauera genus, which belongs to beta-proteobacteria, was observed in both MFCs
(0.3 – 4 %); it is well known as a denitrifier that is able to degrade aromatic compounds (Liu et al., 2006). Different studies have reported the presence of
Thauera in MFCs that are treating wastewater with high nitrogen concentration,
where it has demonstrated the ability to degrade organic compounds in wastewater and enhance nitrogen removal (Sayess et al., 2013). Propionivibrio, which belongs to the class Betaproteobacteria, was detected in the MFCs only, so this bacteria could be used in bioenergy production (Wang et al., 2016a)
Acidaminococcus are fermentative bacteria responsible for amino acid degradation
and are involved in enhancing hydrogen productivity (Lay et al., 2010). The
318
OTU
201
OTU
MEC
259MFC
Figure 6.5 Venn diagram of bacterial communities with shared and unique OTUs among MFC and MEC anode biofilm
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Acidaminococcus genus was detected only in the MECs, and the presence of this
gene might be linked to the system operational conditions.
Cluster analysis of the anodic biofilm of both the MFCs and the MECs showed that the bacterial communities were different. As with the Acidaminococcus genus, the differences found between the communities in the MFCs and the MECs might be due to the differences in the system operational conditions. All the reactors had the same design, and the anode chambers were inoculated with the same wastewater and fed with same synthetic wastewater.
Principal Coordinates Analysis (PCoA) was used to visualise and compare the dissimilarity between two different systems (Fig 6.6). The two first ordination axes explained 41% of the variability found in the microbial community compositions. The duplicate reactors MFCs were clustered together, which means that the microbial communities were similar; the same observation was found with the MECs. PCoA showed a clear separation between the communities in the MFCs and those in the MECs. The analysis shows great diversity in the communities of the anode in the MFCs and in the MECs. These results showed the impact of the reactor operational conditions on the community structure.
In Figure 6.6, the green circles represent the anode electrodes from the MECs, and the green triangle represents the water sample from the anode chamber in the MECs. The red circles represent the anode electrodes from the MFCs, and the red triangle represents the water sample from the anode chamber in the MFCs. It can be seen clearly that the biofilm communities of the MECs are located in the top right corner, and the biofilm communities of the MFCs are located in the top left corner of the plot.
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