3.2 Microbial communities under well-performed and disturbed
states targeting OLR
Linking the microbial dynamics to process perturbations is fundamental
in order to understand and deal with process instability. Therefore, two
OLRs were chosen in order to investigate the microbial community
structure, i.e. during process stability at 10.2 g VS
L-1 d-1, and subsequent process
disturbances (17 g VS L-1 d-1). In
Fig. 2, bacterial community compositions slightly changed under the two
OLR conditions although the abundances were different.Thermotogae (phyla Thermotogae ) were the predominant
bacterial class, followed by Clostridia (phyla Firmicutes )
and Synergistia (phyla Synergistetes ) at both OLRs. It has
been reported that a significant number of members belonging to phylumThermotogae frequently seem to be higher in thermophilic
digesters (Shi et al., 2018). This phenomenon may be attributed to their
ability to encode thermo-stable enzymes that play a role in conversion
processes (Conners et al., 2006). At an OLR of 10.2 g VS
L-1 d-1, Defluviitoga(79.7%) dominated the microbial genera (Fig. 2a). At an OLR of 17.1 g
VS L-1
d-1, Defluviitoga decreased to 49.8% but there
were significant increases in Clostridium andAnaerobaculum (Fig. 2b), implying that these microorganisms were
functionally redundant. Accordingly, it was obvious that the increase in
OLR resulted in alternation of the microbial abundant during the
experiment. Furthermore, the decline in relative abundance ofDefluviitoga was associated with process failure, signifying its
significant role in substrate metabolism and process stability.Defluviitoga spp. are able to utilize a wide range of
carbohydrate as electron donors (Hania et al., 2012), thus the C-rich
straw in the substrate mixture might promote the enrichment of this
genus. Defluviitoga spp. frequently exists not only in
thermophilic AD (Hania et al., 2012), but also in metabolic association
with hydrogenotrophic methanogens (Maus et al., 2016).
In this study, genera Tepidanaerobacter, Clostridium andSyntrophaceticus , which were three possible syntrophic acetate
oxidation bacteria (SAOB), were detected at an OLR of 10.2 and 17.1 g VS
L-1 d-1. As depicted in Table 3,Clostridium sp. represented 2.8% and 11% of total bacterial at
10.2 and 17.1g VS-1 L-1
d-1 of OLR, respectively. Consequently, the reduction
in the relative abundance of most dominant carbohydrate-fermenting
species (Defluviitoga ) might be linked to simultaneous increase
in relative abundance of Clostridium sp. and meanwhile promoted
carbohydrate availability in digester. The increased presence of SAOB
indicated the existence of SAOB combined with a hydrogenotrophic
methanogenesis (HM) pathway for methane formation. SAOB of generaTepidanaerobacter and Syntrophaceticus were also detected
even though they were at less than 0.5% during the two OLRs.Syntrophomonas , which is known to function as a syntrophic
microbe to degrade complex organic matters to simple fatty acids (Hansen
et al., 1999), was detected with little change in its relative abundance
with the OLRs of 10.2 and 17.1 g VS L-1
d-1 (0.7% and 1%, respectively).
Hydrogenotrophic methanogens (including the genera Methanoculleusand Methanothermobacter) represented 88.2% and 53.5 % of all
archaea population for OLRs of 10.2 and 17.1 g VS
L-1d-1, respectively (Fig. 2c, and
2d). The genus Methanoculleus clearly dominated at OLR of 10.2 g
VS L-1d-1 (86.9%) and OLR of 17.1 g
VS L-1d-1 (52.8%).Methanoculleus has been reported to be a very efficient
hydrogen-utilizing methanogen during the thermophilic AD of chicken
manure (Bayrakdar et al., 2017) and it acts as an important partner withClostridium ultunenes (Yin et al., 2018). The increased
OLR to 17.1 g VS L-1d-1, as
previously stated, reduced the presence of Methanoculleus , and
coincided with the VFA accumulation and the reduction of methane
production (Fig. 1a and 1c). Methylotrophic Methanomassiliicoccuswere represented at relatively low levels at the two OLRs, however,
their relative abundance was obviously enhanced at highest OLR.Methanomassiliicoccus is a H2-dependent
methanogen which can consume methylated compounds to produce methane
(Liu et al., 2016). Nevertheless, it has still not been
identified to act as a hydrogenotrophic partner for SAO and it needs to
be further verified (Westerholm et al., 2016). In fact, the important
role of both Methanoculleus and Methanomassiliicoccus have
been previously reported in AD of lignocellulosic biomass (Li et al.,
2018a). The acetoclastic methanogens, Methanosarcina accounted
for only 2% and 4% of the total archaea at an OLR of 10.2 and 17.1 g
VS L-1d-1, respectively (Fig. 2). In
this study, the high abundance of hydrogenotrophic methanogens, and a
low representation of acetoclastic methanogens at OLR of 10.2 g VS
L-1d-1, alongside a low
concentration of acetate (338 mg L-1), strongly
indicated that the acetate conversion may be a two-stage process.