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.

土壤中的腐殖质 (HS) 广泛分布在寒冷环境中，占土壤有机碳的很大一部分。细菌菌株 ( n  = 281) 是在 15 °C 下使用含有腐殖酸 (HA)（HS 的主要成分）的培养基从各种极地土壤样品中分离出来的：217株来自南极，64株来自北极。我们基于 16S rRNA 序列相似性鉴定了 73 种潜在的 HA 降解细菌，这些序列隶属于变形菌门(73.9%)、放线菌门(20.5%) 和拟杆菌门(5.5%)。HA降解菌株进一步分为假单胞菌属（51株）、红球菌属（10 株）或其他（12 株）。大多数菌株在 10 到 25 °C 之间降解 HA，但不超过 30 °C，表明存在冷适应降解。30 个独特的漆酶样多铜氧化酶 (LMCO) 基因片段从 73 种 HA 降解细菌菌株的 71% 中进行 PCR 扩增，所有这些都包括保守的铜结合区 (CBR) I 和 II，这两者都是漆酶活性所必需的。细菌 LMCO 序列不同于已知的真菌漆酶；例如，在细菌 LMCO 中未检测到真菌漆酶中 CBR I 和 CBR II 之间的半胱氨酸残基。这表明 LMCO 的细菌生物标志物作用可以预测随着全球气候变化而导致苔原地区 HS 降解率的变化。计算机辅助分子模型显示，这些 LMCO 包含一个高度保守的铜依赖性活性位点，由 CBR I 和 CBR II 之间的三个组氨酸残基形成。基于系统发育和建模的方法证实了 LMCO 基因在 HA 降解极地土壤细菌中的广泛存在，并将其推定的基因功能与最初的 HS 降解过程联系起来。