Bark is one of the main defenses of spruce trees (Christmas trees). Bark is also a side-stream from the pulp and paper industry produced in millions of tons each year. Today the bark is mostly as an energy source, which however is an inefficient process. To better utilise bark industrially, basic knowledge of its biological degradation is needed. Now, researchers at Chalmers University of Technology have shown that the first step is the breakdown of the toxic molecules that are present in high levels in the bark.
Bark protects trees from insects, microorganisms, UV radiation, fire, and other external threats. The barrier that the bark forms consist largely of so-called extractives, which are toxic to most microorganisms—either directly by causing cell death or by inactivating essential proteins. However, trees, and consequently bark, eventually break down. What happens during this process?
Decomposition of spruce bark
In a recently published study, researchers from the Divisions of Industrial Biotechnology, Systems and Synthetic Biology, and Forest Products and Chemical Engineering investigated the decomposition of spruce bark— both to find out what happens to the material and which microorganisms are involved?
"We started this study since knowledge about how bark decomposition occurs in nature, especially over time, is scarce. Basic research and knowledge are necessary for future work to utilise the bark instead of burning it," says Amanda Sörensen Ristinmaa, the first author of the study, who recently defended her thesis Microbial strategies for deconstruction of bark components.
Tracked changes in chemical and biological content
The researchers dried and ground spruce bark obtained from Holmen AB, a Swedish forestry company, which was sterilized suing gamma irradiation to ensure that no living microorganisms remained. A culture from the original bark material was added to the sterilized spruce bark, and then, over six months, the researchers took samples to track changes in the chemical and biological content, i.e., the chemical molecules and microorganisms present in the bark culture and how these changed over time.
"Very early in the decomposition process, we saw that resin acids, which are a significant part of the extractives in bark and are known to be very toxic, decreased in concentration. Simultaneously, we could see that a certain type of bacteria began to dominate the culture. When the resin acids were reduced in concentration, the amount of bacterial species increased," says Johan Larsbrink, Associate Professor in molecular enzymology.
A new bacterial species was isolated
The researchers isolated a completely new bacterial species, which they named Pseudomonas abieticivorans, from the culture with resin-degrading bacteria.
"The isolated bacterium has very few types of carbohydrate-degrading enzymes that are usually present in large quantities in microorganisms that degrade wood and other plant material. Instead, we identified all known enzymes needed to break down resin acid molecules. It strengthened our suspicion that it can grow on resin acid as the sole carbon source, which it did, and it seems to play a significant role in bark decomposition," says Amanda Sörensen Ristinmaa.
The name abieticivorans means "eater of abietic acid"—one of the most common resin acids in spruce.
The breakdown of extractives is crucial
Although research on bark decomposition is in its early stages, the researchers propose that the study shows that the breakdown of extractives is crucial. For future work in this area, they would like to replicate studies on spruce bark decomposition but also examine other types of bark to see if the decomposition process is the same or significantly different between different types of bark.
Burning large amounts of bark, as currently done in the industry, is inefficient since bark has a high content of both moisture and minerals, which reduce the energy output and also create large amounts of ash .
"The extractives are very interesting molecules that potentially could be used for various antibacterial materials—which is already their primary function. From a sustainability perspective, it would be better to, in the future, be able to extract them from the residual bark material and burn the rest," says Johan Larsbrink.
More about the study
- Read the scientific publication in Nature Communications: Resin acids play key roles in shaping microbial communities during the degradation of spruce bark
- Authors: Amanda Sörensen Ristinmaa, Albert Tafur Range, Eduard J. Kerkhoven, and Johan Larsbrink. Department of Life Sciences at Chalmers, Alexander Idström and Merima Hasani, Department of Chemistry and Chemical Engineering at Chalmers, Sebastian Valenzuela, Division of Medical Chemistry and Cell Biology at the University of Gothenburg, and Philip B. Pope, Faculty of Biosciences, Norwegian University of Life Sciences.
- The study is funded by the Swedish Energy Agency together with the Swedish Research Council (projects in Energy-Oriented Basic Research), and the Adlerbert Research Foundation.
Contact
- Associate Professor, Industrial Biotechnology, Life Sciences