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Lead Organizer: ASLO Meetings, Association for the Sciences of Limnology and Oceanography (meetings@aslo.org)
Presentations
05:30 PM
Metatranscriptome unveils the transcriptional responses of nitrogen-related genes in marine diatoms within the coastal waters of the Matsu archipelago (7744)
Primary Presenter: Lee-Kuo Kang, National Taiwan Ocean University (lkkang@mail.ntou.edu.tw)
The availability of inorganic nitrogen has long been recognized as a limiting factor for primary production in the ocean. Nevertheless, various species or groups have evolved distinct strategies to uptake and assimilate nitrate and ammonium, enabling them to adapt to environmental changes. In order to understand the nitrogen uptake characteristics of diatoms and their transcriptional responses of nitrogen-related genes, 2 research cruises were carried out in the southern East China Sea in 2018 and 2019. Microphytoplankton cell counts revealed that diatoms were the major dominant group, comprising over 90% of microphytoplankton composition. Despite the high abundance of microphytoplankton in 2018, both the potential maximum uptake rates of ammonium and nitrate for microphytoplankton were extremely low. Metatranscriptomic analyses of microphytoplankton unveiled the nitrogen status of dominant diatoms, such as Chaetoceros and Coscinodiscus, based on the gene expression of nitrogen-related genes. In 2018, only nitrogen transporter genes and 3 genes at the beginning of the nitrate assimilation pathway were classified as N-limited induced genes. The low relative transcript percentage of these genes indicated repression of nitrate uptake and assimilation. The remaining nitrogen-related genes expressed more like constitutive genes, with transcripts maintaining low levels even in ammonium addition treatments and failing to be induced to higher transcript levels under N-free treatments. This result suggested that the elevated concentrations of nitrate in surface water might be residual nitrogen, as diatoms had already acquired sufficient nitrogen, leading to the cessation of nitrate and ammonium uptake. In contrast, most of nitrogen-related genes (15/20) were classified as N-limited induced genes in 2019. Among these genes, only one nitrate transporter gene was highly expressed in both genera. The remaining genes expressed as low as observed in ammonium-addition treatments. This result implied that diatoms engaged in competition for the limited nitrogen available in their environment. These findings showed the capability of metatranscriptomic approaches to elucidate the transcriptional responses of dominant diatoms, providing insight into how these diatoms regulate their nitrogen-related genes to adapt to environmental fluctuations within natural assemblages.
05:30 PM
The contribution of archaea and bacteria to nitrification in sulfide-rich marsh sediment (7872)
Primary Presenter: Nicole Lyons, University of Miami (njl84@miami.edu)
A common limiting source of nitrogen for plants is nitrate and nitrite, generated by the process of nitrification by sediment microbial communities. In marsh ecosystems, there can be clear distinctions in sediment chemistry depending on local plant species and their associated microbial communities, even for plants in close proximity. Spartina alterniflora and Juncus roemerianus are two different marsh plants species that can grow directly adjacent to each other. In this project we went to a Louisiana salt marsh and took sediment cores from the root systems of each plant species and measured potential nitrification rates in the sediment over three days. We separated the sediment cores into 30 soil samples and treated each with either a control, sulfide, or the inhibitors PTIO and sulfathiazole. With this method we were able to look at the inhibitory effect of sulfide on nitrification and isolate either archaea or bacteria to identify their contribution to the potential nitrification rate. Analyzing nitrification in this manner shows how microbial community dynamics can change depending on the plant biota and geochemistry, even at distances within a few meters. Therefore, as plant populations shift due to sea level changes and coastal development, there may be drastic impacts on the nitrogen cycle and the availability of key nutrients for primary producers. It will be important to monitor these plant-microbe associations to analyze whether changes in plant biota, microbial communities or geochemistry have far-reaching impacts on water quality and overall ecosystem health.
05:30 PM
RAPID PHOSPHORUS REMOVAL CAN PROMOTE SUSTAINED DECREASED NITROGEN CONCENTRATIONS IN LAKES, BUT HOW? (8105)
Primary Presenter: Catherine Polik, University of Minnesota (polik020@umn.edu)
While anthropogenic impacts have historically led to increased phosphorus (P) concentrations in freshwaters, we are now beginning to reverse course. Reducing inputs and limiting internal loading of P has successfully pushed some eutrophic systems back to a clear, macrophyte dominated state. However, less focus has been given to how targeting P has impacted the nitrogen (N) cycle or whether changes in N pools could be partially responsible for maintaining improved water quality. Lake management efforts in Minnesota give us a unique opportunity to examine the ecosystem response to rapid P removal. In this study, we assess the impact of 48 whole-lake rapid P removal events (alum treatments) on the N-cycle using monitoring data and laboratory experiments. After P removal, summer epilimnetic N concentrations decreased. The magnitude of N removal was positively correlated with P removal, and N concentrations were reduced for at least 8 years after alum treatment. Hypotheses to explain the decrease in N include decreased inputs from N-fixing cyanobacteria, increased assimilation by macrophytes, and increased nitrate availability leading to increased denitrification or increased downstream export. Preliminary analyses indicate that total cyanobacterial cell counts decreased, and improved water clarity promoted macrophyte growth. Potential denitrification rates remained unchanged, and problems with variably reported detection limits made it challenging to assess N export. Ongoing work seeks to examine the relative importance of these hypotheses in explaining the observed patterns.
05:30 PM
Regional variation of net anthropogenic nitrogen and phosphorus inputs (NANI and NAPI) across the continental US: 1987-2022 (8245)
Primary Presenter: Dennis Swaney, Cornell University (dps1@cornell.edu)
Net Anthropogenic Nitrogen (N) and Phosphorus (P) inputs (NANI/NAPI) are estimated from available county-level US census and USDA ag census data and other sources. The recent update of the USDA ag census data to 2022, along with newer US census data and N deposition estimates, has allowed new estimates of corresponding NANI and NAPI values, comprising up to four terms: mineral fertilizer inputs, net food/feed inputs to a region (calculated as the balance between local crop production and livestock and human food demand) and, for NANI, atmospheric N deposition and crop N fixation. Strong regional variations can be seen, depending on the balance of crop production, livestock production, and human population density, with implications for the N&P loads of riverine and coastal waters. Both NANI and NAPI are dominated by fertilizer inputs in some of the large agricultural regions of the country, but livestock nutrient demands and corresponding manure availability also can have regional impacts. Crop N fixation is a major component of NANI in agricultural areas with no corresponding term in NAPI. NANI and NAPI in urban centers are strongly driven by net food inputs with implications for local sewage loads to urban streams; in contrast, crop-dominated agricultural regions export crop nutrients to other regions. Changes over 35 years are evident, including the increasing dominance of heavily-fertilized energy crops and regional shifts in livestock production. Regional variation in NANI:NAPI and riverine N:P has implications for managing the Nation’s rivers and coastal waters.
05:30 PM
MICROBIAL COMMUNITIES ACROSS OCEANIC DEPTHS PRODUCE STABLE DISSOLVED ORGANIC NITROGEN (8280)
Primary Presenter: Richard LaBrie, McGIll (richard.labrie@mcgill.ca)
Dissolved organic nitrogen (DON) represents the largest reservoir of fixed nitrogen (N) in the surface ocean. DON is critical for the growth and productivity of marine microorganisms, but also comprises compounds that are unreactive, representing a long-term storage of N. It has been hypothesized that more stable forms of DON (i.e., refractory) may originate from successive microbial reprocessing of bioavailable compounds, a “microbial nitrogen pump” (MNP). However, it is still unknown whether microbial communities from different oceanic depths are equally efficient in producing refractory DON. Using bottle incubations, we tested the MNP by mixing surface dissolved organic matter (DOM) with prokaryotic communities from the epi-, meso- and bathypelagic regions of the Labrador Sea, and tracked changes in DON concentration and composition over time. Our results suggest that DON accumulation was a two-step process, beginning with a conversion of inorganic N to organic N, followed by DON reprocessing into less reactive compounds. This was apparent in the short-term (1 month) and long-term (18 months) production/consumption cycles in fluorescent DOM, as well as in amino acids indices showing an accumulation of labile material before its progressive transformation info more recalcitrant DOM. This first empirical evidence of the MNP using unamended seawater showed that changes in DON composition were consistent in all treatments. Our results suggest that microbial communities across oceanic depths share similar metabolic pathways required for long-term DON sequestration.
05:30 PM
Sediment-water interface nitrogen removal and cycling in native and invasive seagrass meadows in Jobos Bay, Puerto Rico (8402)
Primary Presenter: Kayla Gonzalez-Boy, Kennesaw State University (kgonza36@students.kennesaw.edu)
Jobos Bay is a National Estuarine Research Reserve in southern Puerto Rico. The role of seagrasses on sediment-water interface nitrogen (N) transformations is unknown in Jobos Bay. Intact sediment cores from invasive (Halophila stipulacea) and native (Thalassia testudinum) seagrass meadows, and unvegetated sediments, were incubated in a continuous-flow system with and without 15N-ammonium or 15N-nitrate amendments to measure net nutrient (N and P) fluxes and denitrification/anammox, N2 fixation, and dissimilatory nitrate reduction to ammonium (DNRA) rates. Dinitrogen gas fluxes varied between net N2 fixation along an exposed mangrove shoreline (in mixed seagrasses and an adjacent bare sediment area) to net denitrification (no anammox was detected) in a semi-enclosed, mangrove-lined embayment and on the inland side of a barrier island. Net N2 fixation was most pronounced in bare sediments along the exposed mangrove shoreline during the dry season (March). Vegetated sediments along this shoreline switched to net denitrification in the wet season (July). Net denitrification was highest in the semi-enclosed bay in the dry season and at the barrier island in the wet season (July). Denitrification was mostly coupled to nitrification, with typically <15% of total N2 production comprised of 29/30N2 in 15-nitrate amended sediments. Evidence for DNRA was detected in preliminary sample analyses, but its relative importance is pending additional results. This study will help resource managers advocate for watershed conservation activities to reduce nutrient loading to the estuary.
05:30 PM
Sulphate-reducing bacteria dominate the benthic Biological Nitrogen Fixation in an upwelling-influenced coastal lagoon. (8449)
Primary Presenter: Ariadna Aldrich Rodriguez, University of Baja California UABC (ariadna.aldrich.rodriguez@gmail.com)
The Biological Nitrogen Fixation (BNF) is a microbial-mediated process converting atmospheric dinitrogen to ammonia that can play a key role in regulating nitrogen budgets in benthic environments. In this study, BNF measured with the acetylene reduction assay was investigated in vegetated (Zostera marina) and bare subtidal sediments of an upwelling-influenced coastal lagoon located on the west coast of Baja California. The study also quantified the abundance of the nitrogen fixation functional gene (nifH) and determined the relative contributions of sulphate-reducing bacteria (SRB) and cyanobacteria to the total diazotrophic activity. Rates of BNF ranged from 5.1x10-4 - 0.68 nmol N g-1 h-1 and they were significantly higher in sediments with vegetation than in bare sediments and, in general, the diazotrophic activity increased toward stations at the inner lagoon and varied with depth within the sediment. BNF rates were closely associated with the abundance of the nifH gene and were directly affected by the bioavailability of sediment organic carbon. The high contribution of SRB indicates that these benthic organisms can be a significant source of reactive N in coastal lagoons and that sediment heterogeneity shapes the diazotrophic community. These findings broaden our understanding on N-cycling and nutrient management in upwelling-influenced coastal estuarine environments with oyster culture activities.
CS16 - Nitrogen Biogeochemistry and Cycling
Description
Time: 5:30 PM
Date: 4/6/2024
Room: Madison Ballroom D