Exploring the native pulp and paper sludge microbiome to inspire new biotechnologies for waste minimization.

Thousands of metric tonnes of diverse sludge wastes are generated annually in the pulp and paper industry. Due to a high moisture content and an abundance of inorganic material, many types of sludge are hard to recycle and instead accumulate in landfills, causing environmental damage. During storage, forestry sludge waste appears recalcitrant to natural attenuation, indicating limited degradation of wood- and process-derived fibers and polymers by environmental microbes. Intentional enzymatic or microbial hydrolysis of carbohydrates within the sludge may, however, be a feasible approach to reduce waste volume and prevent transfer to landfill. Here, we show that a previously validated biomass-degrading enzyme cocktail lacks efficacy on a metal-rich sludge obtained from a Swedish pulp and paper mill, possibly due to enzyme inhibition. Hypothesising that microbes dwelling within sludge may host enzymes better adapted to this complex contaminated substrate, we assessed whether a native sludge microbiome could be identified, and whether it degrades carbohydrates during incubation in microcosms. Marker gene profiling revealed diverse bacterial and fungal communities undergoing genus-level changes over time, and the most abundant species could be enriched via serial cultivation with pulp-derived carbon sources. Complementary chemical analyses showed that biopolymers were largely removed after a 10-week incubation, leading to sludge solubilization and volume reduction. This confirms the capacity for fiber degradation by native microbiomes and suggests the waste as a potential source of microbes and enzymes capable of sludge polymer degradation, the mechanism of which remains to be explored, but which could reduce the need for future landfilling.IMPORTANCEAccording to European and Swedish guidelines, the top priority in waste handling is prevention, followed by reuse, recycling, energy recovery, and, as a last resort, landfill. While effective in municipal contexts, these guidelines are difficult to apply to pulp and paper industries when managing heterogeneous sludge wastes. Process-derived sludges are hugely abundant but have low economic value as their high moisture content prevents combustion, and the complex mixture of organic fibers prevents metal recovery. According to industrial reports, less than 10% of sludge is used for energy, and under 50% is recycled. Our results demonstrate that biological treatment of sludge could be a method of waste reduction to reduce landfilling, specifically targeting hygroscopic carbohydrate-based polymers. Mapping the microorganisms in this under-explored industrial waste material, using combined "omic" technologies and chemical analysis, lays a foundation for discovering robust organisms and enzymes that withstand harsh conditions, such as low water activity and high metal content.