The maximum power densities of GAC, GS,CFC and GG packed MFCs showed a decreasing trend. The specific areas changed in the similar way as the power densities of MFCs with different cathodic materials. It indicated that the power density had positive correlation with specific area, which can be assured that high specific area is profitable for microbial attachment and biological catalytic processes.
Shannon diversity indices results suggested that the diversity was the highest on GS and lowest on GAC. The results showed that biocathode materials had effects on the microbial diversity and evenness. Results on electrical power showed that the power density was the highest in GAC packed MFC, followed by GS, CFC and GG (Additional file 1
: Table S1). It indicated that the microbial diversity and evenness in biocathode material was not positively related to the power production. Other studies about anodic microbes also showed that numerical abundance of microorganisms in anodic biofilms was not correlated with the current (Kiely et al. 2010
Phylogenetic analysis indicated that the biofilm developing on each cathodic surface had some differences. It could be assumed that the microbial members had different affinities for the materials. Bacteroidetes and Betaproteobacteria were predominant in the biofilms sampled from four cathode materials. In a related study, it was also reported that the microbial community composition of an oxygen reducing biocathode was dominated by Bacteroidetes (Rabaey et al. 2008
). When nitrate was served as the terminal electron acceptor on the biocathode, Chen et al. (2010)
analyzed the microbial community dynamics and the results showed that Betaproteobacteria and Bacteroidetes were the most abundant division of the community. Another study showed that Gammaproteobacteria was the most abundant, followed by uncultured Bacteria and Bacteroidetes (Chen et al. 2008
). These results indicated that the microorganisms belonging to proteobacteria and Bacteroidetes play important roles in catalyzing oxygen or nitrate reduction in cathodic compartment.
Electrode materials had an important effect on the type of microbial species in MFCs reactors. Different electrode materials have different microscopic surface structure and conductivity, which in turn affects the adhesion of specific microbes (Rabaey et al. 2004
,Zhou et al. 2011
). In GAC, GS and CFC packed MFCs, Comamonas
of Betaproteobacteria was the dominated genus; Acidovorax
was the most obvious microbes of GG packed MFC. The powder density was the highest in GAC packed MFC, followed by GS, CFC and GG. Therefore microbes Comamonas
and its abundance in cathodic materials might have some positive relation to the power generation in GAC, GS and GG packed MFCs, while in GG packed MFC Acidovorax
may be correlated with electron transfer. A recent study showed that Comamonas testosteroni
displayed a higher power production performance under a high pH condition in the anode chamber (Juang et al. 2011
). The similarities between anode and cathode reducing/oxdizing populations may indicate the capability of many organisms to perform electron transfer both to and from electrodes, such as Shewanella putrefaciens
and Geobacter sulfurreducens
. Some electrochemically active bacteria in biocathodes have been reported, including Gram-negative and positive bacteria, such as Acinetobacter calcoaceticus
, Sphingobacterium multivorum
, Micrococcus luteus
and Bacillus subtilis
, which can catalyze oxygen reduction in biocathode MFCs (Rabaey et al. 2008
,Cournet et al. 2010
). Therefore, the cathodic biofilm in a MFC is composed of diverse populations of bacteria, and they may work together to electron transfer and power production.
In conclusion, electrical results showed that the power density was the highest in GAC packed MFC, followed by GS, CFC and GG. Different biocathode materials had effects on the microbial diversity and evenness, but the differences in microbial diversity and evenness of different biocathode materials were not positively related to the power production. Biocathode materials had an important effect on the type of microbial species in MFCs reactors. The microbes belonging to Bacteroidetes and Proteobacteria were the dominant phyla in the four materials packed biocathode MFCs. Comamonas of Betaproteobacteria might have important effects on electron transfer process of GAC, GS and CFC packed biocathode MFCs, while in GG packed MFC Acidovorax may be correlated with power generation.