Groundwater Nitrogen Fixation Is Associated with Methane and Sulfur Cycling

Area of Science:

• Environmental microbiology
• Geochemistry
• Biogeochemical cycles

Background:

• Groundwater is the largest freshwater reservoir, typically oligotrophic, but some habitats support high productivity.
• Nitrogen fixation is crucial for microbial life in nitrogen-limited conditions, potentially important in productive groundwater.

Purpose of the Study:

• To investigate the diversity, activity, and metabolic functions of nitrogen-fixing microorganisms (diazotrophs) in groundwater.
• To understand the role of nitrogen fixation in groundwater ecosystems, particularly in relation to methane and sulfur cycling.

Main Methods:

• Analysis of 25 groundwater metagenomes and five proteomes.
• Identification and characterization of nitrogenase iron proteins (NifH) and diazotroph genomes.
• Metabolic profiling of identified microbial communities.

Main Results:

• Identified 203 NifH proteins and 140 diazotroph genomes across diverse bacterial and archaeal taxa, potentially comprising up to 50% of the community.
• Methanotrophic Methylomonadaceae were the most abundant and active diazotrophs in methane-rich groundwater, linking nitrogen fixation to the methane cycle.
• Sulfur-cycling diazotrophs (oxidizers and reducers) were widespread, demonstrating metabolic versatility under varying geochemical conditions.

Conclusions:

• Nitrogen fixation is a significant process in groundwater ecosystems, especially in nitrogen-depleted aquifers utilizing methane.
• The study highlights a strong interdependence between nitrogen fixation and methane/sulfur cycles, driven by diverse microbial groups.
• Proteomic data confirmed active nitrogen fixation by both methane- and sulfur-associated diazotrophs.