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

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05:30 PM

OPTICAL INDICATORS OF PHYTOPLANKTON COMMUNITY STRUCTURE/ FUNCTION NUTRIENT LIMITATION EXPERIMENTS (7787)

Primary Presenter: Jaylen Powell, Baylor University (jaylen_powell1@baylor.edu)

Both nitrogen (N) and phosphorus (P) can limit the growth of aquatic flora, but how the relative proportion of these elements affect their ecophysiology remains understudied. Eutrophic lakes that are P rich and N poor should select for phytoplankton that fix atmospheric N and have predictable pigment and toxin production rates. In this study, we conducted a nutrient enrichment bioassay on phytoplankton sampled from a long-term mesocosm eutrophication experiment at the University of Mississippi Field Station in which N dosing was recently halted. We conducted a microcosm experiment in which we fertilized subsamples of water from the mesocosms with +N, +P, or +N+P. A total of 144 experimental bottles (3 replicate control and 3 replicates per treatment across 12 mesocosms) were incubated for five days in late July and sampled for particulate phosphorus (PP), chlorophyll-A (Chl-A), and Carbon-Nitrogen (CN), as well as, various optical properties including  Effective Quantum Yield (EQY), Electron Transport Rate (ETR), and Maximum Quantum Field Yield (MQFY). The response of these optical properties relative to phytoplankton biomass and stoichiometry may give us insight on how phytoplankton community structure/function changes with limiting nutrients.

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05:30 PM

THE EFFECTS OF VARIABLE NUTRIENT STOICHIOMETRY ON BIOGEOCHEMICAL PROCESSES IN SHALLOW LAKE MESOCOSMS (7788)

Primary Presenter: Alexa Hoke, Baylor University (alex_hoke1@baylor.edu)

Anthropogenic nutrient loading into freshwater systems can have significant impacts on ecosystem function. The application of fertilizer in agricultural settings can increase the relative amount of nitrogen (N) compared to phosphorus (P) entering freshwater systems during runoff events. How these potential stoichiometric imbalances influence microbial communities and associated biogeochemical processes remains poorly studied. We assessed the impact of stoichiometric extremes on the sediment-water interface during a fertilization experiment conducted in twelve experimental limnocorrals located in three ponds at the University of Mississippi Field Station in Abbeville, Mississippi. During the sampling seasons of 2019, 2020, and 2021, the limnocorrals were fertilized to target N:P (molar) ratios of 2.2, 16, 55, and 110. These open-bottomed mesocosms allowed for biogeochemical exchange with the sediments. In August of 2021, sediment cores were collected from each limnocorral as well as the surrounding ponds. Sediment cores were segmented and analyzed for total N, C, and P above the clay layer, as well as organic and inorganic C. Despite the differences in treatment dosing, preliminary results indicate that total N, C, and P in the sediments did not differ between treatments. Sediment DNA extractions will be used for whole genome metagenomics. Extracted DNA will also be used to quantify nifH gene abundance in the sediment to confirm previous sediment core flux data that suggests nitrogen fixation occurs in sediments from the 2.2 and 16 N:P treatments.

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05:30 PM

INVESTIGATING THE LINKAGE BETWEEN METHANOGENESIS AND NITROGEN FIXATION IN FRESHWATER TEMPERATE WETLANDS (7848)

Primary Presenter: Lien Tran, Natural Resources Research Institute (Tran1390@d.umn.edu)

Freshwater wetlands play an influential role in the carbon cycle by sequestering and storing carbon, while also being the largest source of natural methane. The carbon cycle can be linked to the nitrogen cycle as some methanogens harbor nifH, a key gene for nitrogen fixation. Limited research, however, has examined the performance of nitrogen fixation by methanogens. This study aims to investigate the linkage between methanogenesis and nitrogen fixation activities by correlation analysis and nifH amplicon sequencing in different geochemical conditions. Sediments were collected from four wetlands with varying characteristics of high iron, sulfate, organic carbon, and low phosphorus levels. Microcosm experiments were conducted to evaluate methane production and nitrogen activities with wetland sediments as a function of different substrates: a mixture of hydrogen and carbon dioxide (4:1), acetate, trimethylamine, or cellulose at 25℃. Wetland sediments supplemented with trimethylamine showed greater nitrogen activities. For the organic-rich carbon sediment supplemented with cellulose, a significant correlation (correlation coefficient = 0.99; p-value < 0.05) between methane production and nitrogen fixation was observed. Currently, we are characterizing nif-harboring microbial communities to identify nitrogen-fixing methanogens enriched during the incubation. Ultimately, this study will improve our understanding of how methanogens may be interconnected with the nitrogen cycle through nitrogen fixation, and how this process may be linked to methane flux in freshwater wetlands. 

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