Relative Weight of Organic Waste Origin on Compost and Digestate 16S rRNA Gene Bacterial Profilings and Related Functional Inferences.

Axel Aigle,Emilie Bourgeois,Sabine Houot,Wessam Galia,Benoit Cournoyer,Emmanuel Doelsch,Dominique Patureau,Laurence Marjolet

doi:10.3389/fmicb.2021.667043

Abstract

Even though organic waste (OW) recycling via anaerobic digestion (AD) and composting are increasingly used, little is known about the impact of OW origin (fecal matters and food and vegetable wastes) on the end products’ bacterial contents. The hypothesis of a predictable bacterial community structure in the end products according to the OW origin was tested. Nine OW treatment plants were selected to assess the genetic structure of bacterial communities found in raw OW according to their content in agricultural and urban wastes and to estimate their modifications through AD and composting. Two main bacterial community structures among raw OWs were observed and matched a differentiation according to the occurrences of urban chemical pollutants. Composting led to similar 16S rRNA gene OTU profiles whatever the OW origin. With a significant shift of about 140 genera (representing 50% of the bacteria), composting was confirmed to largely shape bacterial communities toward similar structures. The enriched taxa were found to be involved in detoxification and bioremediation activities. This process was found to be highly selective and favorable for bacterial specialists. Digestates showed that OTU profiles differentiated into two groups according to their relative content in agricultural (manure) and urban wastes (mainly activated sludge). About one third of the bacterial taxa was significantly affected by AD. In digestates of urban OW, this sorting led to an enrichment of 32 out of the 50 impacted genera, while for those produced from agricultural or mixed urban/agricultural OW (called central OW), a decay of 54 genera over 60 was observed. Bacteria from activated sludge appeared more fit for AD than those of other origins. Functional inferences showed AD enriched genera from all origins to share similar functional traits, e.g., chemoheterotrophy and fermentation, while being often taxonomically distinct. The main functional traits among the dominant genera in activated sludge supported a role in AD. Raw OW content in activated sludge was found to be a critical factor for predicting digestate bacterial contents. Composting generated highly predictable and specialized community patterns whatever the OW origin. AD and composting bacterial changes were driven by functional traits selected by physicochemical factors such as temperature and chemical pollutants.

Highlights

Organic wastes (OWs) have increased 10-fold since the last century and are likely to double by 2025 as a result of the growth of the world population (Hoornweg et al, 2013)
To go deeper into the understanding of these community changes, the impact of anaerobic digestions (AD) on the structuration of end product microbiomes was assessed by grouping 16S rRNA gene reads according to their allocations at the genus level, and making comparisons by DESEQ2. These analyses revealed that about one third of the total relative abundance of the digestate bacterial taxa were significantly affected by the AD treatment whatever the raw OW origin
AD and composting induced bacterial changes that were found driven by pH, temperature, and the C- and energy sources which require specialized metabolic properties

Summary

Introduction

Organic wastes (OWs) have increased 10-fold since the last century and are likely to double by 2025 as a result of the growth of the world population (Hoornweg et al, 2013). From a waste management standpoint, aerobic (composting) and anaerobic digestions (AD) are the most obvious operational processes prior to soil applications These recycling treatments can be combined by performing a composting of digestates. The production of acetate from the fatty acids will be performed by homoacetogenic bacteria like Butyribacterium and Acetobacterium, that of H2 will be performed by the acetogenic microbiota such as Syntrophomonas, and that of CO2 and H2S will be performed by sulfate-reducing bacteria such as Desulfovibrio and Desulfobacter. Some of these compounds will be used by methanogens to generate CH4. Hydrogenotrophic methanogens such as Methanobacterium and Methanogenum sp. convert H2 and CO2 into CH4, and acetotrophic ones such as Methanosarcina will convert acetate into CH4 (Demirel and Scherer, 2008; Lykidis et al, 2011)

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Conclusion