Nitrogen fixation (diazotrophy), the conversion of di-nitrogen (N2) gas to reactive nitrogen (N) by specialized microbes (diazotrophs), plays an essential role in Earth’s N cycle. Molecular ecology continues to show the common presence of diazotrophs across all of Earth’s biomes. Yet, N2 fixation rates and the functional role(s) of N2 fixation in population, community, and ecosystem ecology remain poorly studied, particularly in the aquatic ecosystems that connect terrestrial landscapes and the open ocean. The Aquatic Nitrogen Fixation Research Coordination Network was established in 2021 to open lines of communication among N2 fixation researchers at scales ranging from headwater streams and wetlands to the coastal oceans. This session is dedicated to all N2 fixation science. We invite abstracts across all scales of study from molecular ecology to global biogeochemistry and using any approaches from surveys to field/lab experiments to mathematical modeling. We are interested in a range of N2 fixation related research. This includes, but not limited to, studies reporting N2 fixation rates, the abundance or activity of diazotrophs, and/or the biodiversity of diazotrophs across the freshwater-marine continuum. We also welcome contributions on the molecular mechanisms of N2 fixation, the stoichiometry of diazotrophs from the physiological to the ecosystem scale, and the interacting biotic and abiotic constraints of diazotrophs in aquatic ecosystems. The session will provide a rare opportunity to share science not only with the ASLO community, but also with the Society for Freshwater Science (SFS) who are hosting a contemporaneous meeting in Philadelphia, PA. Our session will occur simultaneously with an identical SFS session. Some talks will be live streamed in both directions, and we will host questions for speakers in both directions.
Lead Organizer: J. Thad Scott, Baylor University (Thad_Scott@Baylor.edu)
Co-organizers:
K. Riley Book, University of Wisconsin-Madison (krbook@wisc.edu)
Morgan S. Sobol, University of Wisconsin-Madison (msobol@wisc.edu)
Robinson W. Fulweiler, Boston University (rwf@bu.edu)
Presentations
12:30 PM
THE AQUATIC NITROGEN FIXATION RESEARCH COORDINATION NETWORK: GOALS, PLANS AND PROGRESS (7791)
Primary Presenter: Thad Scott, Baylor University (Thad_Scott@Baylor.edu)
The Aquatic N2 Fixation Research Coordination Network (ANF RCN) was established in 2021 to cultivate a new paradigm of the fundamental, yet understudied, role of N2 fixation in ecosystems across the freshwater to marine continuum. The ANF RCN is addressing three research coordination opportunities (RCOs) via the formation of working groups that work over 24 months to set and conduct research tasks, participate in a workshop, and produce paper and data products. RCO1 synthesized the current knowledge of the rates and biodiversity of diazotrophs (organisms that fix N2) for ecosystems along the freshwater-marine continuum. The RCO1 workshop was held in June 2022, and produced a data synthesis of rates from the published literature, and a synthesis of diazotroph biodiversity from archived nifH amplicon and metagenome data. The goal of RCO2 was to characterize the variable stoichiometries of diazotrophs with different metabolic strategies. The RCO2 workshop was held in October 2023 and produced mathematical models describing the stoichiometry of photosynthetic and organo-heterotrophic diazotrophs, and a synthetic overview of the state of knowledge of chemotrophic diazotrophs. RCO3 will develop a set of common mathematical and statistical tools enabling the upscaling of N2 fixation for comparison within and among diverse aquatic ecosystems and in regional and global N budgets, with a workshop planned for March 2025. The ANF RCN invites applicants for RCO3 participation, and also provides opportunities for the entire community of researchers studying nitrogen fixation across the aquascape to share research via networking, focused publications and special conference sessions.
12:45 PM
CURRENT BARRIERS TO SIMULTANEOUS QUANTIFICATION OF N2 FIXATION AND DENITRIFICATION FROM OPEN-CHANNEL DIEL N2 FLUX (AND SOME IDEAS FOR BREAKING THEM) (8492)
Primary Presenter: Michelle Catherine Kelly, Michigan Technological University (mckelly1@mtu.edu)
The contribution of dinitrogen (N2) fixation to stream nutrient budgets could be better quantified with whole-reach scale rate estimates. While the opposite of N2 fixation (denitrification) can be estimated from open-channel diel N2 flux models, this approach needs refinement to account for all the ways that N2 enters, exits, or is transformed in ecosystems. Therefore, we ask: how can we more accurately estimate N2 fixation and denitrification rates from diel N2 concentrations, either by better parameterization of model unknowns or by changes to the model structure? Using syntheses of denitrification and N2 fixation rates in streams, we investigate the degree of divergence between process rates estimated with published N2 flux model structures and those derived from other methods, such as 15N tracers or acetylene chambers. We review the N2 flux and transformation pathways that are not captured in current model structures, and review ways to (a) integrate them into the N2 flux model equation, and (b) estimate rates of these processes in rivers to better constrain Bayesian model priors. Using example data, we build on published N2 flux model equations, demonstrating the impact of changes to the model structure and Bayesian priors on the fit between measured and predicted N2 concentrations. Improved N2 flux models could provide a structure to measure N2 fixation and denitrification on the same temporal and spatial scale as other reach and ecosystem-scale process measurements (eg. metabolism, nutrient uptake), building a framework to understand controls on these processes across ecosystems.
01:00 PM
Ecological and environmental influences on nitrogen fixation evolution (8439)
Primary Presenter: Morgan Sobol, University of Wisconsin - Madison (msobol@wisc.edu)
The evolution of ancient metabolisms played a pivotal role in shaping Earth into the habitable planet it is today. A prime example of this is biological nitrogen fixation (N2-fixation), a metabolism that all life depends on. Unlocking the origin and evolution of N2-fixation is imperative for understanding the processes that shape metabolic evolution on Earth. At the heart of N2-fixation lies nitrogenase, an enzyme found only in prokaryotes and is the sole biological solution capable of reducing N2 to NH3-. Its evolution is characterized by three distinct isozymes. The most ancient form utilizes a molybdenum-iron cofactor, while the others incorporate additional iron or vanadium in place of molybdenum. Yet, questions remain regarding how nitrogenase's evolution was influenced by its physical environment and ecological interactions. Here, we first examined the environmental drivers that shape the distribution and diversity of nitrogenase across today’s aquatic and terrestrial ecosystems. Then, we used ecological modeling to evaluate our hypothesis concerning the impact of resource availability on nitrogenase's evolution. Our results find that the expansion of habitats on early Earth likely spurred the evolution of nitrogenase’s isozymes, underscoring their coevolutionary journey with the environment. This work elucidates how the molecular evolution of critical metabolic innovations, like nitrogenase, is governed by their environmental and ecological surroundings, and offers insights applicable to other biogeochemically significant enzymes and metabolisms.
01:15 PM
A GLOBAL SYNTHESIS OF AQUATIC DIAZOTROPH DISTRIBUTIONS AND METABOLIC DIVERSITY FROM INLAND FRESHWATERS TO THE COASTAL OCEAN (8332)
Primary Presenter: Julian Damashek, Hamilton College (jdamashe@hamilton.edu)
Recent research has broadened our understanding of the ecology of diverse diazotrophs, particularly in the ocean. For example, numerous studies have indicated widespread abundance and activity of heterotrophic marine diazotrophs in both the pelagic and benthic realms, and has emphasized critical roles for syntrophy and symbioses involving diazotrophs and other organisms. However, surprisingly little is known about diazotrophs in freshwater ecosystems as compared to their marine counterparts. Here, we present a meta-analysis of global diazotroph biodiversity along the inland-to-coastal continuum. First, we compiled a database of putative diazotrophs within the Genome Taxonomy DataBase (GTDB), including analysis of metabolic potential in the resulting thousands of genomes. We then screened thousands of metagenomic assemblies from streams, rivers, lakes, estuaries, and coastal oceans for nif genes, and aggregated amplicon data from dozens of projects amplifying nifH from similar environments. We confirm the importance of non-cyanobacteria diazotrophs across inland and coastal ecosystems, with a majority of diazotroph sequences in many water and sediment samples representing heterotrophic or chemotrophic microbes. We also identified ecosystem types with unique diazotrophic communities across the globe, including highly divergent communities in, for example, mangrove sediments and freshwater wetlands. This synthesis allows for an unprecedented and unique view of diazotroph ecology at local, regional, and global scales.
01:30 PM
RIVERINE NITROGEN FIXATION: AN UPDATED SYNTHESIS (8493)
Primary Presenter: Megan Berberich, Michigan Technological University (berberme@mail.uc.edu)
Dinitrogen (N2) fixation is a fundamental process that has historically been understudied in riverine ecosystems. In 2008, Marcarelli and colleagues published a synthesis of riverine N2 fixation rates from 22 streams across 9 studies. Over the last fifteen years, the number of publications that report riverine N2 fixation rates has increased with 30+ new studies that help fill this gap in research on stream nitrogen cycling. We present an updated synthesis of N2 fixation rates in streams and rivers with data derived from published literature and public data repositories, including a dataset that was generated by the Aquatic Nitrogen Fixation Research Coordination Network. This synthesis includes over 800 N2 fixation rates across >100 riverine sites. Median fixation rates are approximately 4 g-N m-2 h-1, which is comparable to the median reported in the 2008 paper of 10 g-N m-2 h-1. The updated dataset spans 4 continents and a wide range of environmental conditions, including nitrate concentrations ranging from 0.9 - 3600 g-N L-1. Preliminary results show a negative relationship between nitrate availability and N2 fixation rates, while in 2008 the limited available data did not detect this relationship. For a subset of streams, we compare N2 fixation to denitrification and dissolved inorganic nitrogen uptake rates to evaluate the contribution of N2 fixation to other nitrogen cycle fluxes. This updated synthesis shows how 15 years of additional studies results in a more complete understanding of the ecological significance and environmental conditions affecting N2 fixation in streams.
01:45 PM
N-fixing trees as a source of nitrate for tropical streams (8494)
Primary Presenter: Marcelo Ardon Sayao, NC State University (mlardons@ncsu.edu)
High abundance of nitrogen fixing trees in tropical areas has been hypothesized to drive higher stream nitrate concentrations compared to temperate streams. However, few studies have examined annual variation in concentration and isotopes of nitrate in tropical watersheds. We examined the del15N and del18O in NO3 for one year in throughfall and stream water in three sites in the Salto watershed in La Selva Biological Station, Costa Rica. del15N of throughfall was more depleted (-4.15) than stream water (4.65 - 4.90), while there was little difference in del18O across all sites (2.05-2.99). Stream water del15N and del18O did not change from a first order stream site to a third order stream, even though the third order stream receives solute rich interbasin groundwater inputs. There were no differences in throughfall or stream del15N and del18O in dry vs wet season. Evaluation of the proportion of N lost via denitrification for this watershed using isotopic models suggest that N-fixation in tropical watersheds can sustain high N exports.
02:00 PM
IRON UPTAKE LEADS TO DIVERGENT RESPONSES IN NITROGEN FIXING MICROORGANISMS IN THE OLIGOTROPHIC OCEAN (8149)
Primary Presenter: Abiel Kidane, Max Planck Institute for Marine Microbiology, Bremen (akidane@mpi-bremen.de)
Biological nitrogen (N2) fixation represents the primary source of new nitrogen (N) in the ocean, playing a crucial role in controlling ocean productivity. The nitrogenase enzyme responsible for N2 fixation has a high iron (Fe) requirement, in addition to Fe in the photosystem, and evidence from experiments on cultivated microorganisms suggests that Fe availability may regulate N2 fixation activity in the ocean. Furthermore, regions with elevated dust (i.e., Fe) fluxes in the ocean exhibit some of the highest N2 fixation rates. Despite this, there is limited evidence supporting a direct link between Fe input and N2 fixation activity by microorganisms in the environment. Utilizing stable isotope incubations (57Fe, 13C-DIC, and 15N2) followed by single-cell uptake measurements (using nanoSIMS) in Northern Atlantic surface waters, we demonstrated that key N2-fixing microorganisms incorporated the added Fe. The assimilated Fe did not exhibit a strong correlation with either N2 and CO2 fixation rates of N2-fixing microorganisms. The addition of Fe stimulated both N2 and CO2 fixation rates in symbiotic Richelia, while globally important Trichodesmium spp and Crocosphaera directed the assimilated Fe towards fueling CO2 fixation, simultaneously reducing their N2 fixation. The divergent responses of various N2-fixing microorganisms to the added Fe were responsible for the observed lack of major responses at the bulk level. This has implications for global biogeochemical models. Predicting N2 fixation in the contemporary and future ocean may require knowledge not only of the structure of the N2-fixing microbial community but also an understanding of the degree of nutrient stress.
02:15 PM
NITROGEN FIXATION IN SHALLOW LAGOONS: RATES, PALYERS AND IMPORTANCE IN NITROGEN CYCLING (8233)
Primary Presenter: Mindaugas Zilius, Klaipeda University (mindaugas.zilius@jmtc.ku.lt)
Lagoons, being situated at the interface between land and marine environments, are potentially important sites for nitrogen (N) turnover due to long retention time and high biogeochemical rates. Most river-dominated lagoons shift toward N limitation in summer, creating favorable conditions for opportunistic microorganisms capable of fixing dissolved dinitrogen (N2). This N-cycling pathway may allow ecosystems to overcome N limitation and maintain high biomass in the water column while adding surplus N. However, the magnitude of N2 fixation is not well constrained in lagoons. Here, we used N isotope tracing and functional genomics to unravel N2 fixation in the three largest European lagoons (Curonian, Vistula and Oder) and elucidate its contribution to ecosystems. Overall, transcriptome analysis shows that diazotrophic communities are active in both sediments and water, with the highest activity during summer compared to cold spring season. The results further indicate that N2 fixation is higher in the euphotic layer of the water column than in dark aphotic layer or sediments. The importance of N fixation to the lagoon N budget varied widely depending on the system and diazotrophic community composition. Nonetheless, some general patterns emerge, such as diazotrophic activity was observed across all seasons, and transcript numbers were considered high even in sediments which are generally not N-limited.
02:30 PM
PHOSPHORUS AND IRON AMENDMENTS AFFECT MULTIPLE NITROGEN CYCLING PROCESSES (8495)
Primary Presenter: Renn Schipper, Kent State University (rschipp1@kent.edu)
Nitrogen (N) is a limiting factor in many ecosystems, as a result, microbes have developed methods to acquire and recycle N. However, these processes require investment and thus can be limited by resources like phosphorus (P) and iron (Fe). We performed a stream-side mesocosm experiment, drawing water from a reference stream (East Branch Chagrin River, Kirtland, Ohio) to examine the influence of P and Fe on N cycling processes. 32 flow-through mesocosms were dosed with nutrient combinations of NO3- (250 g/L), PO43- (30 g/L), and Fe3+ (100 g/L) in a fully factorial design. These mesocosms received amendments for 23 days, after which we collected biofilm and sediment to measure N fixation and denitrification rates (acetylene reduction and block, respectively), NH4+ uptake and regeneration (15-N labeled ammonium), and aminopeptidase activity. Preliminary results show that there were no significant differences between treatments for N fixation (ANOVA, p-value=0.232). Meanwhile, the rates of both NH4 regeneration and uptake were highest for mesocosms that received a combination of P and Fe and lowest among N treatments. These higher rates of regeneration in treatments with Fe suggest that Fe is increasing heterotrophic production and the activity of proteins responsible for recycling of NH4. These results suggest that Fe and P may play a role in alleviating N limitation primary through stimulation of internal recycling rather than increased N fixation.
02:45 PM
Mathematically modeling stoichiometric drivers of heterotrophic N2 fixation (8496)
Primary Presenter: Rebecca Everett, Haverford College (reverett@haverford.edu)
The major source of nitrogen (N) to the biosphere is biological N2 fixationthe microbially-mediated reduction of abundant but inert N2 gas to an assimilable form. While photosynthetic N2-fixers (diazotrophs) have been well-characterized, organoheterotrophic diazotrophs remain understudied despite their likely significance in satiating community N demand on local to global scales. Importantly, while fixed N availability is a strong driver of N2 fixation in photoautotrophs, recent work suggests that heterotrophic diazotrophs are less sensitive to ambient fixed N concentrations, particularly in organic-rich environments. Here, we introduce a mathematical model to predict the behavior of free-living heterotrophic diazotrophs across the aquatic continuum, coupling Monod- and Droop-type functions to predict elemental fluxes with a Gibbs Energy Dissipation model to calculate growth yields from first principles. We will present simulations from the model exploring population steady state conditions under variable chemical (N, C, O) environments and/or physiological traits.
03:00 PM
Understanding Nitrogen Fixation in Phototrophic Diazotrophs: Insights from a Stoichiometric model (7836)
Primary Presenter: Angela Peace, Texas Tech University (a.peace@ttu.edu)
Nitrogen (N) plays a pivotal role in regulating productivity but also imposes constraints on the adaptability of ecosystems to climate change and biogeochemical cycles. Most N exists in the inert form of N2, accessible to specific groups of organisms capable of metabolizing it. Diazotrophic bacteria can introduce reactive N into the ecosystem through fixation, a process in which the nitrogenase enzyme converts N2 into more biologically available species. There are unknowns concerning what factors limit the growth of photodiazotrophs or more specifically limit N fixation. We developed a model to explore population and nutrient dynamics of phototrophic diazotrophs. It tracks biomass, in terms of carbon and energetic ATP, and the organismal content of several essential nutrients (N, phosphorus, iron, and molybdenum). It includes several forms of N; ammonium, nitrate, nitrogenase, and other forms of proteins, which allows us to explore stoichiometric constraints on C fixation, biosynthesis, nutrient uptake, and N fixation rates. The model examines how these factors interact and predicts when any given potentially limiting factor takes prominence over others. Our stoichiometric approach widens the types of questions that models can help address, such as: When organisms invest in producing nitrogenase? What environmental conditions, such as N:P ratios and concentrations, oxygen, and light levels, support N2 fixation? What is the fate of newly fixed N? It is a promising and novel approach that betters our understanding of phototrophic diazotrophs, N2 fixation, and N cycling.
03:15 PM
Raising the curtain on the ecology and biogeochemical significance of chemotrophic nitrogen fixation (8330)
Primary Presenter: James Cotner, University of Minnesota (cotne002@umn.edu)
Nitrogen fixation by phototrophic cyanobacteria has long been recognized as a potentially significant source of N to aquatic ecosystems, but much less is known about the importance of N-fixation by chemotrophs, despite the fact that these organisms were among the first to fix N in the primitive oceans and continue to be prevalent in marine and freshwater systems today. Here, we discuss the evolution and ecology of these organisms and their significance to N-dynamics. We argue that different redox conditions and syntrophies lead to unique regulation controls relative to cyanobacteria. Specifically, we speculate that highly reduced conditions coupled with interdependent syntrophic couplings may lead to N-fixation under seemingly replete nitrogen conditions. The ecological and biogeochemical significance of N-fixation by these ‘behind-the-curtain’ metabolisms needs further study and should be a priority of future research.
SS14 - Exploring Nitrogen Fixation Along the Freshwater-Marine Continuum; A Joint ASLO-SFS Endeavor
Description
Time: 12:30 PM
Date: 4/6/2024
Room: Lecture Hall