How resilient are microbial communities to temperature changes during composting?

The resilience of compost systems to perturbation is usually attributed to the highly active and diverse microbial population. Composting is characterized by distinct temperature changes that are associated with a succession of microbial communities adapted to the prevailing temperature. The temperature fluctuations are spatially and temporally variable. The transition between mesophilic and thermophilic communities can result in loss of degradation efficiency. We tested the resilience of microbial communities to temperature fluctuations in laboratory reactors containing tree bark and pulverized wood amended with pot ale liquor, a waste by-product of the whisky manufacturing process. The laboratory reactors were operated in fixed temperature mode at 50°C, 35°C, and 20°C, in three replicates of each treatment, from which COD, suspended solids, BOD and pH of the resultant leachate were monitored on a daily basis. After running at fixed temperatures for 45 days, the reactors, initially operated at 50, 35 and 20°C, were switched to 20, 50 and 35°C, respectively. A further temperature switch was made after 90 days so that, at the conclusion of the experiment after 135 days, each triplicate set of incubators had experienced a 45-day period at each of the three temperature settings. Microbial community structure, measured using phospholipid fatty acid analysis, showed that communities at different temperatures were quite distinct. The communities conditioned at low temperature adapted to the higher temperature imposed and were indistinguishable from other thermophilic populations.
In contrast, the thermophilic communities that had been conditioned at high temperature and switched to low temperature evolved a mesophilic community that was distinct from the mesophilic communities previously conditioned at low temperature. The loss in process efficiency after the thermophilic phase may therefore be limited by the recovery of surviving populations and/or by slow recolonisation from low-temperature zones.