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Research

Current Projects

Collaborative Research: Linking iron and nitrogen sources in an oligotrophic coastal margin: Nitrogen fixation and the role of boundary fluxes

Collaborative project with at FSU, at UofM, at OSU, Dr. Tim Conway at USF, at USGS, and at Old Dominion University.
Support from:

The goals of this project are to quantify the flux and isotopic signatures of submarine groundwater discharge (SGD)-derived DON and iron to the WFS, and evaluate the bioavailability of this temporally-variable source using four seasonal near-shore campaigns sampling offshore groundwater wells, estuarine, and riverine endmembers and two cross-shelf cruises. The work will evaluate whether SGD stimulates nitrogen fixation on the WFS, and the potential for the stimulated nitrogen fixation to further modify the chemistry of DON and dissolved iron in the region. The cross-shelf cruises will investigate hypothesized periods of maximum SGD and Trichodesmium abundance (June), and reduced river discharge and SGD (February), thus comparing two distinct biogeochemical regimes. The concentrations and isotopic compositions of DON and dissolved iron, molecular composition of DON, and the concentration and composition of iron-binding ligands will be characterized. Nitrogen fixation rates and Trichodesmium spp. abundance and expression of iron stress genes will be measured. Fluxes of DON and iron from SGD and rivers will be quantified with radium isotope mass balances. The impacts of SGD on nitrogen fixation and DON/ligand production will be constrained with incubations of natural phytoplankton communities with submarine groundwater amendments. Two hypotheses will be tested: 1) SGD is the dominant source of bioavailable DON and dissolved iron on the WFS, and 2) SGD-alleviation of iron stress changes the dominant Trichodesmium species on the WFS, increases nitrogen fixation rates and modifies DON and iron composition. Overall, the work will establish connections between marine nitrogen and iron cycling and evaluate the potential for coastal inputs to modify water along the WFS before export to the Atlantic Ocean. This study will thus provide a framework to consider these boundary fluxes in oligotrophic coastal systems and the relative importance of rivers and SGD as sources of nitrogen and iron in other analogous locations, such as coastal systems in Australia, India, and Africa, where nitrogen fixation and SGD have also been documented.
The specific roles of the Chappell lab on the project are: 1) measuring nitrogen and carbon fixation rates, Trichodesmium abundance and gene expression, and chlorophyll and particulate carbon/particulate nitrogen (PN:PC) from the two research cruises and select samples from the seasonal well sampling transects; and 2) coordinating and sampling the ship-board incubation experiments associated with testing hypothesis 2 above.

Recent Projects

Exploring a pelagic N2 fixation 鈥渉otspot鈥� along the Mid-Atlantic Bight continental shelf
Supported by: Jeffress Trust Awards Program in Interdisciplinary Research

This project combines innovative quantitative and bioinformatic approaches to explore the temporal extent and biogeography of biological dinitrogen (N2) fixation in a recently discovered 鈥渉otspot鈥� of N2 fixation along the Northern Outer Banks with a special focus on interactions between N2-fixing symbionts and marine diatoms.  This work capitalizes upon recent previous sampling efforts that measured unexpected elevated rates of N2 fixation in North Carolina shelf waters in a region with elevated abundance of a diatom known to harbor N2 fixing symbionts, low surface nitrate concentrations, and reduced salinity. The project involves monthly sampling at a coastal site near the hotspot in addition to several transects offshore from this site.  Sampling will include N2 fixation rate measurements, molecular characterization of both the N2-fixer and phytoplankton communities, and additional hydrographic parameters including temperature, salinity, nutrients, and chlorophyll.  In addition to providing valuable data about both the extent (temporal and spatial) of coastal N2 fixation in a region where few measurements exist, the matrix of environmental data will be analyzed using correlative measures to help explore different potential influences on both carbon and nitrogen cycling in the region.

Quantifying the effects of variable light and iron on the nitrate assimilation isotope effect of phytoplankton

Collaborative project with (was at FSU, now at Rice) and at FSU
Support from:

Interpretation of both modern water column nitrate (NO3-) isotopic ratio (d15N) measurements generated by GEOTRACES and other cruises, as well as metrics of paleo-nutrient utilization, depend upon a mechanistic understanding of the degree to which NO3- assimilation by phytoplankton discriminates against the heavier isotope, 15NO3- (NO3- assimilation epsilon). We currently lack the ability to predict how iron and light stress impacts the NO3- assimilation epsilon. The goal of this collaborative project is to test the hypothesis that enhanced light and/or iron stress elevates the epsilon for NO3-assimilation. This hypothesis will be tested by a combination of laboratory culture work and field work on a cruise of opportunity in the Southern Ocean. Mesocosm experiments will include both increasing and alleviating light and/or iron stress on monoclonal phytoplankton cultures and in natural phytoplankton communities while measuring the response of the NO3- assimilation epsilon. Water column samples will be collected on the cruise for analysis of dissolved and size-fractionated particulate N concentration and d15N, as well as phytoplankton community composition, photophysiology and gene expression markers of iron and light stress and NO3- assimilation. In particular, the expression of iron and light stress markers will be used to evaluate the relative contribution of iron and light stress to field-based estimates of the NO3鈥� assimilation epsilon. The results from these field measurements, together with lab-based culture studies, will be used to constrain the range of the epsilon for NO3- assimilation under environmentally-relevant light and iron conditions, including the potential alleviation of iron stress as has been hypothesized to have occurred during the last glacial maximum (a.k.a. the Martin hypothesis). The Chappell lab is responsible for the molecular biological sampling and analysis.   Chappell lab Ph.D. student Sveinn Einarsson is participating on the Spring-Summer cruise of the international SCALE project:

Investigating iron-binding ligands in Southern Ocean diatom communities: The role of diatom-bacteria associations

Collaborative project with (URI) and (was at USF, now at OSU) Support from:

This is a collaborative project between the Chappell lab at Old Dominion University, the Jenkins lab at the University of Rhode Island and the Buck lab at the University of South Florida.  The project combined trace metal biogeochemistry, phytoplankton cultivation, and molecular biology to address questions regarding the production of iron-binding compounds and the role of diatom-bacterial interactions in this iron-limited region. Diatoms are an important group of photosynthetic algae whose growth fuels the food web in the Southern Ocean and play a role in balancing atmospheric carbon dioxide by sequestering the carbon used for growth to the deep ocean on long time scales as their cells sink below the surface.  Iron is an essential micronutrient for marine phytoplankton. Diatom growth in much of the Southern Ocean is limited by the available iron in the seawater, most of which is tightly bound to organic compounds. The nature of these compounds and how phytoplankton acquire iron from them is critical to understanding productivity in this region and globally.  This project tested three hypotheses concerning the production of these iron-binding compounds, limitations on the biological availability of iron even if present in high concentrations, and the roles of diatom-associated bacteria in these processes.  Experiments involved supplying nutrients at varying nutrient ratios to natural phytoplankton assemblages to determine how diatoms and their associated bacteria respond to different conditions as well as laboratory based experiments with recent diatom isolates from the Southern Ocean.  While the experiments are complete, the analysis continues as publications are under revision.  Stay tuned!

OCE-RIG: Developing molecular bioassays for evaluating iron status of environmentally relevant diatoms

Support from:

This project focused on an important group of photosynthetic algae, the diatoms, and how iron availability modulates their growth. Diatoms are important organisms at the base of the marine food web able to balance atmospheric carbon dioxide by sequestering the carbon they use for growth in the deep ocean when their cells sink.  The project combined trace metal biogeochemistry, phytoplankton cultivation, and molecular biology to address questions about the bioavailability of various iron forms to environmentally relevant diatoms. Iron is an essential micronutrient for marine phytoplankton, including diatoms, as it is required for the structure and function of proteins essential to diatom growth. Iron concentrations can be low in many areas of the ocean and exists in different forms classified by size and chemical characteristics. While knowledge of the distribution of iron has increased significantly as a result of the ongoing GEOTRACES program, a mechanistic and quantitative understanding of the bioavailability of various forms of iron remains elusive. Dr. Chappell has previously developed a calibrated molecular bioassay capable of indicating when a specific oceanic diatom was experiencing iron stress. This assay has been used to evaluate iron stress in oceanic field samples and in incubation samples in response to additions of different iron fractions. The goal of this project was to develop additional calibrated molecular assays for iron stress in other environmentally relevant diatoms from both coastal and oceanic habitats and use them to assess the biological availability of iron from varying sources. The project supported analysis of diatom communities in samples from the California Current System using high-throughput sequencing.  The experimental plan included culture experiments monitoring growth, photosynthetic physiology, and gene expression of iron stress response genes. Results from this project will provide tools to answer fundamental questions about the bioavailability of various forms of iron to important phytoplankton. These data will help us understand the relationship between iron and an important group of organisms at the base of the food web, which will prove valuable to climate and food web modelers.