Nitrate Reduction to Ammonium and Ammonia-Oxidizing Archaea in US Great Basin Hot Springs

Nitrogen Cycling in Geothermal Hot Springs

Introduction to Nitrogen Biogeochemical Cycle

The nitrogen biogeochemical cycle is a fundamental process in both terrestrial and aquatic environments, involving the transformation of nitrogen between various chemical forms. In geothermal hot springs, unique microbial communities drive these transformations, often under extreme conditions.

• Nitrogen cycle: The series of processes by which nitrogen is converted between its various chemical forms, including ammonium, nitrite, nitrate, and nitrogen gas.

• Geothermal habitats: Environments characterized by high temperatures and unique chemical conditions, supporting thermophilic microorganisms.

• Ammonia-oxidizing archaea (AOA): Microorganisms capable of oxidizing ammonia to nitrite, playing a key role in nitrogen cycling in extreme environments.

• Example: Hot springs in the US Great Basin, such as GBS (Great Basin Spring) and SSW (South Spring Well), are studied for their active nitrogen cycling.

Nitrate Reduction to Ammonium (DNRA)

Definition and Mechanism

Dissimilatory nitrate reduction to ammonium (DNRA) is a microbial process where nitrate ($NO_3^-$) is reduced to ammonium ($NH_4^+$) rather than nitrogen gas. This process is significant in environments where denitrification is limited, such as hot springs.

• DNRA: The reduction of nitrate to ammonium by microorganisms, often under anaerobic conditions.

• Equation:

$NO_3^- + 2H^+ + 2e^- \rightarrow NO_2^- + H_2O$ $NO_2^- + 6H^+ + 6e^- \rightarrow NH_4^+ + 2H_2O$

• Importance: DNRA retains nitrogen in a bioavailable form (ammonium), supporting primary productivity.

• Example: DNRA rates measured in GBS and SSW hot springs indicate active ammonium production.

Ammonia Oxidation and Ammonia-Oxidizing Archaea (AOA)

Ammonia Oxidation Rates and Microbial Communities

Ammonia oxidation is the process by which ammonia ($NH_3$) is converted to nitrite ($NO_2^-$) by specialized microorganisms, including archaea and bacteria. In the studied hot springs, AOA are abundant and contribute significantly to this process.

• Ammonia oxidation: $NH_3 + O_2 \rightarrow NO_2^- + 3H^+$

• AOA populations: Quantitative PCR and gene sequencing reveal high numbers of AOA in sediment samples.

• Experimental findings: Ammonia oxidation rates are higher in sediment slurries compared to spring water alone.

• Example: Figure 1A shows the decrease in $^{15}NH_4^+$ over time, indicating active ammonia oxidation.

Experimental Methods and Data Interpretation

Measurement Techniques

Rates of nitrogen transformations were measured using isotopic tracers ($^{15}N$), gene quantification (qPCR), and chemical analyses. These methods allow for precise determination of microbial activity and nitrogen fluxes.

• Isotopic tracer experiments: Addition of $^{15}NH_4^+$ or $^{15}NO_3^-$ to samples to track transformation rates.

• qPCR: Quantitative polymerase chain reaction used to estimate abundance of functional genes (e.g., amoA for ammonia oxidation).

• Chemical analysis: Measurement of nitrogen species concentrations over time.

Results and Discussion

Key Findings

The study found that both DNRA and ammonia oxidation are active in US Great Basin hot springs, with AOA playing a major role. Ammonium production and consumption rates are influenced by temperature, microbial community composition, and environmental conditions.

• Ammonium production: DNRA is a significant source of ammonium in sediments.

• Ammonia oxidation: AOA are abundant and drive ammonia oxidation, especially in sediment slurries.

• Environmental implications: These processes support primary productivity and influence nitrogen availability in geothermal ecosystems.

• Example: Figure 2A and 2B show the effect of acetylene (an inhibitor) on $NO_3^-$ production, confirming the role of ammonia oxidation.

Comparison of Nitrogen Transformation Rates

Tabular Summary of Key Rates and Microbial Abundance

The following table summarizes the main findings regarding nitrogen transformation rates and microbial populations in the studied hot springs.

Process	Location	Rate (nmol N g-1 h-1)	AOA Abundance (copies g-1)
Ammonia Oxidation	GBS (17°C)	6.7 ± 0.3	1.3 × 107
Ammonia Oxidation	SSW (60°C)	4.1 ± 0.1	1.7 × 107
DNRA	GBS (17°C)	226 ± 25	—
DNRA	SSW (60°C)	—	—

Additional info: Table entries inferred from figure legends and text; actual rates may vary with experimental conditions.

Summary and Ecological Significance

Conclusions

Nitrate reduction to ammonium (DNRA) and ammonia oxidation are both active and significant processes in US Great Basin hot springs. The abundance of ammonia-oxidizing archaea highlights their ecological importance in nitrogen cycling under extreme conditions. These processes contribute to the retention and transformation of nitrogen, supporting microbial and primary productivity in geothermal environments.

• Key terms: DNRA, ammonia oxidation, AOA, geothermal hot springs, nitrogen cycle.

• Applications: Understanding these processes informs ecological management and biogeochemical modeling of extreme environments.