Atmospheric N deposition increases bacterial laccase-like multicopper oxidases: implications for organic matter decay

• Published in: Applied and Environmental Microbiology 2014-07, Vol.80 (14), p.4460-4468
• Main Authors: Freedman, Zachary, Zak, Donald R
• Format: Article
• Language: English
• Subjects: Actinobacteria; Actinobacteria - enzymology; Actinobacteria - genetics; Atmosphere; Bacteria; Basidiomycetes; Carbon - metabolism; Decomposition; DNA, Bacterial - genetics; Enzymes; Forests; Fungi; High-Throughput Nucleotide Sequencing; Laccase - genetics; Laccase - metabolism; Michigan; Microbial Ecology; Microbiology; Nitrogen; Nitrogen - metabolism; Polymerase Chain Reaction; Proteobacteria; Sequence Analysis, DNA; Soil Microbiology; Terrestrial ecosystems
• ISSN: 0099-2240; 1098-5336; 1098-6596
• DOI: 10.1128/aem.01224-14

Anthropogenic release of biologically available nitrogen (N) has increased dramatically over the last 150 years, which can alter the processes controlling carbon (C) storage in terrestrial ecosystems. In a northern hardwood forest ecosystem located in Michigan in the United States, nearly 20 years of experimentally increased atmospheric N deposition has reduced forest floor decay and increased soil C storage. This change occurred concomitantly with compositional changes in Basidiomycete fungi and in Actinobacteria, as well as the downregulation of fungal lignocelluloytic genes. Recently, laccase-like multicopper oxidases (LMCOs) have been discovered among bacteria which can oxidize β-O-4 linkages in phenolic compounds (e.g., lignin and humic compounds), resulting in the production of dissolved organic carbon (DOC). Here, we examined how nearly 2 decades of experimental N deposition has affected the abundance and composition of saprotrophic bacteria possessing LMCO genes. In our experiment, LMCO genes were more abundant in the forest floor under experimental N deposition whereas the abundances of bacteria and fungi were unchanged. Experimental N deposition also led to less-diverse, significantly altered bacterial and LMCO gene assemblages, with taxa implicated in organic matter decay (i.e., Actinobacteria, Proteobacteria) accounting for the majority of compositional changes. These results suggest that experimental N deposition favors bacteria in the forest floor that harbor the LMCO gene and represents a plausible mechanism by which anthropogenic N deposition has reduced decomposition, increased soil C storage, and accelerated phenolic DOC production in our field experiment. Our observations suggest that future rates of atmospheric N deposition could fundamentally alter the physiological potential of soil microbial communities.