Abstract
Conventional anaerobic digesters (ADs) treating dairy manure are fed with raw or fermented manure rich in volatile fatty acids (VFAs). In contrast, pre-fermented AD (PF-AD) is fed with the more recalcitrant, fiber-rich fraction of manure that has been pre-fermented and depleted of VFAs. Thus, the substrate of PF-AD may be likened to a lean diet rich in fibers while the pre-fermentation stage fermenter is fed a relatively rich diet containing labile organic substances. Previous results have shown that conventional and pre-fermented ADs fed with raw or pre-fermented manure, respectively, produced comparable methane yields. The primary objective of this study was to characterize, using next-generation DNA sequencing, the bacterial communities in various bioreactors (pre-fermentation stage fermenter; various operational arrangements PF-AD; conventional single-stage AD; and a full scale AD) and compare the Firmicutes to Bacteroidetes (F/B) ratios in these different systems. Firmicutes and Bacteroidetes constituted the two most abundant phyla in all AD samples analyzed, as well as most of the samples analyzed in the fermenters and manure samples. Higher relative abundance of Bacteroidetes, ranging from 26% to 51% of bacteria, tended to be associated with PF-AD samples, while the highest relative abundance of Firmicutes occurred in the fermenter (maximum of 76% of bacteria) and manure (maximum of 66% of bacteria) samples. On average, primary stage fermenters exhibited microbiological traits linked to obesity: higher F/B ratios and a ‘diet’ that is less fibrous and more labile compared to that fed to PF-AD. On the other hand, microbial characteristics associated with leanness (lower F/B ratios combined with fibrous substrate) were associated with PF-AD. We propose that bacterial communities in AD shift depending on the quality of substrate, which ultimately results in maintaining VFA yields in PF-AD, similar to the role of bacterial communities and a high fiber diet in lean mice.
Highlights
Obesity and or leanness are not traits commonly associated with engineered systems like anaerobic digesters (ADs)
When comparing the different sample types by Chao1 richness, the differences observed in operational taxonomic units (OTUs) richness were even more apparent (Table S1), suggesting that much of the diversity in Large Scale AD (LS-AD) lies in the rare phylotypes
Animal-to-animal variation in fecal bacterial community structure at the species level has been observed in cows [17], which probably reflects the process of speciation occurring at the individual cow level
Summary
Obesity and or leanness are not traits commonly associated with engineered systems like anaerobic digesters (ADs). AD microbial consortia may be greatly influenced by substrate qualities that in the gut, elicit metabolic responses that lead to either obesity or leanness. ADs rely on the activities of different functional groups of anaerobic microbes that work together to degrade organic matter to produce methane. ADs and many animal digestive systems are faced with similar metabolic challenges that require similar biochemical reactions involving hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The first three processes lead to the production of volatile fatty acids (VFAs) that are critical intermediates in both animal and engineered systems. Regardless of the system bacteria play similar roles in the generation of VFAs from organic matter
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