Enzyme adaptation in Sphagnum peatlands questions the significance of dissolved organic matter in enzyme inhibition.

Peatlands store a large proportion of global soil carbon in the form of peat because decomposition of plant organic matter is slow. In Sphagnum-dominated peatlands, dissolved organic matter (DOM) is traditionally considered an important inhibitor of hydrolytic enzymes due to the polyphenolic polymers it contains. Interestingly, the acidic character of the polymers in such DOM has never been tested for its enzyme-inhibitory properties. We raised two principal hypotheses: (1) not only the polyphenolic but also the acidic character of DOM inhibits the activity of extracellular enzymes in Sphagnum-dominated peatlands; (2) environmental, peat-extracted enzymes will show adaptation to their environment. We tested the inhibition of commercial acid phosphatase and cellobiohydrolase, and environmental phosphatase and β-glucosidase by following dissolved substances: (1) polyphenol-free polycarboxylates from Sphagnum cell walls, i.e. sphagnan, (2) environmental DOM (peat-DOM) containing polymers of polyphenolic-polycarboxylate nature, (3) tannic acid (carboxyl-free polyphenolic oligomer) and (4) monomeric phenolic acids. Sphagnan strongly inhibited commercial acid phosphatase, to a similar extent as peat-DOM and more strongly than tannic acid and a polycarboxylate from another moss (Leucobryum glaucum). Monomeric phenolic acids were weak inhibitors. Commercial cellobiohydrolase was only partially inhibited by sphagnan or peat-DOM. Environmental phosphatase and β-glucosidase were consistently slightly inhibited by tannic acid, but not by sphagnan or peat-DOM. Inhibition of commercial phosphatase by sphagnan and peat-DOM was counteracted by a polycation chitosan, indicating the electrostatic nature of carboxylate-mediated inhibition. Our results question the polyphenol-mediated enzyme inhibition in Sphagnum-dominated peatlands as (1) the DOM had a strong inhibitory potential due to its polycarboxylates; nevertheless, (2) the peat microbial communities exhibited enzyme resistance to both polyphenol and polycarboxylate polymers in peat-DOM.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.