Abstract
Humic substances (HS) in soil are widely distributed in cold environments and account for a significant fraction of soil’s organic carbon. Bacterial strains (n = 281) were isolated at 15 °C using medium containing humic acids (HA), a principal component of HS, from a variety of polar soil samples: 217 from the Antarctic and 64 from the Arctic. We identified 73 potential HA-degrading bacteria based on 16S rRNA sequence similarity, and these sequences were affiliated with phyla Proteobacteria (73.9%), Actinobacteria (20.5%), and Bacteroidetes (5.5%). HA-degrading strains were further classified into the genera Pseudomonas (51 strains), Rhodococcus (10 strains), or others (12 strains). Most strains degraded HA between 10 and 25 °C, but not above 30 °C, indicating cold-adapted degradation. Thirty unique laccase-like multicopper oxidase (LMCO) gene fragments were PCR-amplified from 71% of the 73 HA-degrading bacterial strains, all of which included conserved copper-binding regions (CBR) I and II, both essential for laccase activity. Bacterial LMCO sequences differed from known fungal laccases; for example, a cysteine residue between CBR I and CBR II in fungal laccases was not detected in bacterial LMCOs. This suggests a bacterial biomarker role for LMCO to predict changes in HS-degradation rates in tundra regions as global climate changes. Computer-aided molecular modeling showed these LMCOs contain a highly-conserved copper-dependent active site formed by three histidine residues between CBR I and CBR II. Phylogenetic- and modeling-based methods confirmed the wide occurrence of LMCO genes in HA-degrading polar soil bacteria and linked their putative gene functions with initial HS-degradation processes.
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This work was supported by Korea Polar Research Institute (grant PE20170).
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Ha Ju Park: investigation and writing of original draft; Yung Mi Lee: data curation and writing of original draft; Hackwon Do: investigation and writing of original draft; Jun Hyuck Lee: data curation and editing of original draft; Eungbin Kim: data curation and editing of original draft; Hyoungseok Lee: resources, data curation, and editing of original draft; Dockyu Kim: resources, conceptualization, data curation, and writing and editing of original draft.
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Park, H.J., Lee, Y.M., Do, H. et al. Involvement of laccase-like enzymes in humic substance degradation by diverse polar soil bacteria. Folia Microbiol 66, 331–340 (2021). https://doi.org/10.1007/s12223-020-00847-9
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DOI: https://doi.org/10.1007/s12223-020-00847-9