NifH-Harboring Bacterial Community Composition across an Alaskan Permafrost Thaw Gradient

Area of Science:

• Soil microbiology and biogeochemistry
• Permafrost ecology
• Nitrogen cycling

Background:

• Nitrogen (N) is frequently a limiting nutrient in permafrost soils.
• Understanding nitrogen fixation is crucial for soil fertility and ecosystem function in thawing permafrost.
• The genetic potential for nitrogen fixation can be assessed by targeting the nifH gene.

Purpose of the Study:

• To investigate the nitrogen (N₂)-fixing genetic potential in Alaskan boreal permafrost soils along a thaw gradient.
• To identify the microbial taxa harboring nitrogen-fixing genes (nifH) and their distribution relative to permafrost thaw and water table depth.
• To compare N₂-fixing communities across different Alaskan and non-Alaskan soil environments.

Main Methods:

• Sequencing of nifH gene fragments from soil samples collected across a permafrost thaw gradient in Alaskan boreal soil.
• Analysis of samples from minimally, moderately, and extensively thawed sites to a depth of 79 cm, including zones above and below the water table.
• Bioinformatic analysis including translation with frameshift correction, sequence clustering into Operational Taxonomic Units (OTUs), and comparison with existing datasets.

Main Results:

• A total of 112,476 nifH sequences were clustered into 1,631 OTUs.
• NifH sequence composition varied significantly with sample depth relative to the water table. Alpha- and Beta-Proteobacteria were more abundant above the water table, while Delta-Proteobacteria were more abundant below.
• Sequences distantly related to Verrucomicrobia-Opitutaceae were the most dominant below the water table, constituting one-third of the total sequences. Alaskan sites showed distinct N₂-fixing communities compared to non-Alaskan sites, with unique halophilic, sulfate-, and iron-reducing taxa present in Alaska.

Conclusions:

• Permafrost thaw significantly alters the class-level composition of nitrogen (N₂)-fixing communities in thawed soil layers.
• The distribution of these N₂-fixing communities is strongly correlated with water table depth.
• Alaskan permafrost soils harbor distinct nitrogen-fixing microbial communities compared to other global sites, suggesting unique adaptations to Arctic and sub-Arctic environments.