Anoxic Biodegradation of Isosaccharinic Acids at Alkaline pH by Natural Microbial Communities

Abstract

One design concept for the long-term management of the UK’s intermediate level radioactive wastes (ILW) is disposal to a cementitious geological disposal facility (GDF). Under the alkaline (10.0<pH>13.0) anoxic conditions expected within a GDF, cellulosic wastes will undergo chemical hydrolysis. The resulting cellulose degradation products (CDP) are dominated by α- and β-isosaccharinic acids (ISA), which present an organic carbon source that may enable subsequent microbial colonisation of a GDF. Microcosms established from neutral, near-surface sediments demonstrated complete ISA degradation under methanogenic conditions up to pH 10.0. Degradation decreased as pH increased, with β-ISA fermentation more heavily influenced than α-ISA. This reduction in degradation rate was accompanied by a shift in microbial population away from organisms related to Clostridium sporosphaeroides to a more diverse Clostridial community. The increase in pH to 10.0 saw an increase in detection of Alcaligenes aquatilis and a dominance of hydrogenotrophic methanogens within the Archaeal population. Methane was generated up to pH 10.0 with acetate accumulation at higher pH values reflecting a reduced detection of acetoclastic methanogens. An increase in pH to 11.0 resulted in the accumulation of ISA, the absence of methanogenesis and the loss of biomass from the system. This study is the first to demonstrate methanogenesis from ISA by near surface microbial communities not previously exposed to these compounds up to and including pH 10.0.

Microbial Community Evolution Is Significantly Impacted by the Use of Calcium Isosaccharinic Acid as an Analogue for the Products of Alkaline Cellulose Degradation

“…In addition, our study, when coupled to recent findings by Kuippers et al [14] and Rout et al [8, 9] show that both α-ISA alone and a mixed carbon source as part of CDP are likely to support a methanogenic system within the operational period of such a facility. Irrespective of the source of ISA (single/mixed diastereomers), the degradation rates observed here were similar.…”

“…The other carbon sources present within CDP are therefore likely to not be metabolized to methane, and may indeed be recalcitrant to these organisms. The recalcitrance of some of the components of CDP has previously been observed [8, 10]. Within this study, acetic acid accumulated to 63.7% (α-ISA) and 61.9% (CDP) of theoretical levels, suggesting that a degree of acetate oxidation was occurring.…”

“…The method used for the generation of CDP has been shown previously produce a liquor containing a number of organic acids of which ISAs represent >70% of the total organic carbon [8]. There was a significant proportion of organic carbon left uncharacterized within this study as a result of the detection limits of the equipment used, previous research has shown that the number of the organic acids generated from the alkaline hydrolysis of cellulose can be extensive [33].…”

“…This has resulted in a number of studies attempting to identify the impact of microbial intrusion and metabolism of ISA’s. The studies of Rout et al [8–10] and Charles et al [11] adopted the use of a synthetic CDP leachate [12] containing a mixture of both diastereomers of ISA, whilst the work of Bassil et al [13] and Kuippers et al [14] have chosen α-ISA as carbon source. A number of these studies have used the sediment from the same geographic location [15].…”

Floc Formation Reduces the pH Stress Experienced by Microorganisms Living in Alkaline Environments

“…These conditions will also result in alkaline hydrolysis of the cellulose component of the ILW, producing a range of low-molecular-weight organic compounds collectively known as cellulose degradation products (CDP) (5). The biodegradation of CDP has received considerable attention in recent years with a number of authors reporting alkaliphilic degradation under a wide range of growth conditions (6,7), including alkaliphilic methanogenic consortia operating at a pH of 11.0 (8). In some cases, these alkaliphilic communities have been shown to form flocs where the bacteria are encased in a matrix of extracellular polymeric substances (EPS) (9).…”