Hydrogen-dependent dissimilatory nitrate reduction to ammonium in Arctic marine sediments

Hydrogen-dependent dissimilatory nitrate reduction to ammonium in Arctic marine sediments

The Arctic region is experiencing rapid environmental changes, including warming temperatures, melting sea ice, and alterations to marine ecosystem dynamics. One particularly important component of Arctic biogeochemistry is the cycling of nitrogen (N) in marine sediments. Nitrogen transformations play a crucial role in regulating the availability of nutrients, supporting primary production, and influencing greenhouse gas emissions in these high-latitude systems.

Microbial Processes

A key process in the benthic N cycle is dissimilatory nitrate reduction to ammonium (DNRA), where nitrate (NO3-) is converted to ammonium (NH4+) via microbial respiration. This process is distinct from denitrification, which reduces nitrate to dinitrogen gas (N2), representing a net removal of fixed N from the system. DNRA retains N in a biologically available form, potentially fueling eutrophication, whereas denitrification removes N and helps mitigate these impacts.

The relative importance of these two nitrate reduction pathways is influenced by a variety of environmental factors, including oxygen availability, the presence of electron donors (e.g., organic carbon, hydrogen), and the composition of the microbial community. In Arctic marine sediments, where temperatures are cold and organic matter inputs can be variable, understanding the controls on DNRA and denitrification is particularly important.

Sediment Characteristics

Arctic marine sediments are characterized by low temperatures, relatively low organic matter content, and the presence of hydrogen (H2) as an important electron donor. The cold temperatures slow down microbial metabolic rates, while the limited organic matter availability can constrain heterotrophic processes like denitrification that rely on organic carbon as the primary electron donor.

In this context, hydrogen-based DNRA may become a more prominent pathway for nitrate reduction. Certain microbes can use H2 as an electron donor to drive the reduction of nitrate to ammonium, a process that is often favored over denitrification in cold, organic-poor environments.

Environmental Factors

The relative importance of DNRA versus denitrification in Arctic marine sediments is likely influenced by a variety of environmental factors, including temperature, organic matter availability, oxygen levels, and the composition of the microbial community. Understanding how these parameters interact to regulate N cycling is crucial for predicting the response of Arctic ecosystems to ongoing environmental changes.

Arctic Marine Sediments

Geochemical Profiles

Studies in Arctic marine sediments have revealed distinct geochemical profiles that suggest the importance of hydrogen-based DNRA. Vertical profiles of dissolved oxygen, nitrate, and ammonium often show a decoupling between nitrate reduction and ammonium production, with ammonium accumulating in the deeper, more anoxic sediment layers. This pattern is indicative of DNRA outpacing denitrification as the dominant nitrate reduction pathway.

Microbial Community Composition

Analyses of the microbial community composition in Arctic marine sediments have identified the presence of Sulfurospirillum and Desulfovibrio species, known for their ability to couple the oxidation of H2 to the reduction of nitrate via DNRA. These microbes may be well-adapted to the cold, organic-poor conditions typical of Arctic seafloors, allowing them to outcompete denitrifiers in certain environments.

Nutrient Cycling

The prevalence of hydrogen-based DNRA in Arctic marine sediments has important implications for the cycling of nitrogen and other nutrients. By retaining fixed N in the form of ammonium, DNRA can influence the availability of N for primary producers, as well as the production of greenhouse gases like nitrous oxide (N2O) that can be generated as byproducts of nitrogen transformations.

Nitrogen Transformation Pathways

Nitrate Reduction

In Arctic marine sediments, the reduction of nitrate can proceed via several pathways, including denitrification, anammox (anaerobic ammonium oxidation), and DNRA. The relative importance of these processes is shaped by the interplay of environmental factors, such as oxygen availability, organic matter content, and the presence of alternative electron donors like hydrogen.

Ammonium Production

The DNRA pathway results in the direct conversion of nitrate to ammonium, which can then be assimilated by primary producers or subjected to further transformations, such as nitrification (the oxidation of ammonium to nitrite and nitrate). This ammonium-based N cycling can have important implications for ecosystem productivity and greenhouse gas emissions.

Hydrogen Utilization

The ability of certain microbes to couple the oxidation of H2 to the reduction of nitrate via DNRA represents a unique adaptation to the cold, organic-poor conditions of Arctic marine sediments. By exploiting this alternative electron donor, these microbes may outcompete denitrifiers and other nitrate-reducing organisms in specific environmental settings.

Ecological Implications

Benthic-Pelagic Coupling

The prevalence of hydrogen-based DNRA in Arctic marine sediments can influence the coupling between benthic and pelagic ecosystems. By retaining fixed N in the form of ammonium, DNRA can affect the availability of N for primary producers in the overlying water column, potentially altering patterns of productivity and community structure.

Greenhouse Gas Emissions

The production of ammonium via DNRA, rather than the loss of fixed N through denitrification, may also influence the generation of greenhouse gases like nitrous oxide (N2O) in Arctic marine sediments. The fate of ammonium, whether it is assimilated, nitrified, or subjected to other transformations, can determine the net climate impact of these microbial processes.

Ecosystem Services

Understanding the dynamics of nitrogen cycling, including the relative importance of DNRA versus denitrification, is crucial for predicting the response of Arctic marine ecosystems to ongoing environmental changes. These processes can have far-reaching implications for the provision of key ecosystem services, such as nutrient regulation, carbon sequestration, and greenhouse gas mitigation.

The unique characteristics of Arctic marine sediments, including the prevalence of hydrogen as an electron donor, highlight the need for further research to elucidate the complex interplay of environmental factors that regulate microbial nitrogen transformations in these high-latitude systems. By gaining a better understanding of these processes, we can better anticipate the ecological and biogeochemical implications of environmental change in the Arctic.

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