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

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.