The concept of DIET has garnered significant attention in recent years. Unlike IET, the transfer efficiency of DIET is not limited by the mass transfer rate and typically occurs between electroactive bacteria and methanogens. There was clear evidence that Geobacter, a type of electroactive bacteria, showed the highest abundance in the lignocellulose-based biochar treatment, increasing 34-fold. This phenomenon can potentially be attributed to an enhancement in the biochar's conductivity following the adsorption of trace elements, resulting in an approximate ten-fold increase in conductivity and improved DIET performance. Furthermore, biochar has the ability to enhance the growth of hydrogenophilic methanogenic archaea, a functional microorganism associated with DIET, and the presence of trace elements showed an enhanced performance on this effect. Trace element stimulated the abundance of Methanobacillus and Spirospira [
6], and the effect of cyanobacteria biochar in Taihu Lake on methane production in the AD process of sludge. The study found that lower vaccination rates (4% and 1%, v/v) increased methane output. Algae-based biochar at lower vaccination rates enriched methane sarcoma better than higher vaccination rates. According to prior study, focused DIET may enable methane synthesis [
35]. The algal biochar, owing to its porous structure and distinct functional groups, exhibits remarkable electron transport capabilities with superior storage capacity. This property expedites the reduction of Fe (III) to Fe (II) [
104], and the cytochrome and ferredoxin involved in the synthesis of Fe (II) are key factors in electron transport [
104]. It is worth mentioning that most microorganisms participating in DIET, such as
Geobacter,
Sphaerochaeta and Sporanaerobacter species, rely on exchangeable (bioavailable) iron and sulfur to complete the process of extracellular respiration [
61]. Surface functional groups may trigger the DIET mechanism between anaerobic microorganisms. The addition of rice straw biochar has no significant effect on the function of bacterial flora, but biochar material with rich specific surface area and hydrophobicity can provide better electronic conductivity as an electronic conduit. Thus, enhancing electron transport capacity between microorganisms [
108]. Besides, it is worth noting that the augmentation of distinct surface functional groups, namely phenolic and lactic acid groups, is the underlying cause of the alteration in redox characteristics of algal biochar. This phenomenon is also responsible for the rise in electron-donating potential. The coenzyme F
420 activity of methanogenic was enhanced, and the methane yield was increased by 58.7%. Thus, algal biochar acts as an effective electron conduit for DIET during anaerobic methane generation [
34]. In thermophilic digesters modified from waste wood pellet biochar, beneficial bacteria (e.g.,
Thermotogae and Defluviitoga) play a leading role in the fermentation of food waste. Due to the good electrical conductivity of biochar, the DIET process between methanogens and their syntrophic partners is promoted. The addition of sawdust biochar effectively shortened the lag period by 27.5-64.4%, and increased the maximum methanogenesis rate by 22.4%-40.3%, due to the enrichment of Anaerolineaceae and Methanosaeta, typical microorganisms for DIET [
12]. Agricultural waste corn straw biochar also promoted the abundance of two electron-synergistic microorganisms, Clostridia and Methanosarcina [
93]. Draff-based biochar has abundant active surface functional groups (such as -CO, pyridine-N, and graphite-N), and it has been shown that increased methane production is associated with DIET through the charge-discharge cycle containing C, N, and O functional groups on the surface [
17]. Furthermore, previous reported that the presence of elements also exerts a significant influence on the development of biochar functional groups, which affect the DIET indirectly [
133]. Accordingly, trace or common elements with bioavailability can enhance DIET through two paths: (1) Enhancing the DIET by abundant functional groups on biochar surface; (2) Enriching the electroactive microorganisms related to DIET and strengthening the cooperative relationship of DIET between electroactive microorganisms and methanogens.