Regulation of Trichodesmium Nitrogen Fixation by Combined Nitrogen and Growth Rate: A Field and Culture Study
Holl, Carolyn Marie
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Trichodesmium is a globally significant marine diazotroph responsible for supplying new nitrogen to the oligotrophic regions in which it is found. Though it has been studied for decades, our understanding of the ways in which environmental factors can affect its nitrogen fixation rate remains limited. A continuous culture of Trichodesmium was established in which steady state growth and nitrogen fixation were maintained at dilution rates ranging from 0.27 to 0.67 d-1. Our results clearly show that, as growth rate increased, biomass decreased linearly and nitrogen fixation rate increased linearly. C:N:P ratios remained constant over the range of growth rates studied, suggesting a tight coupling between macronutrient uptake and the maintenance of balanced growth at steady state. We used cultures at steady state to determine the impact of nitrate exposure and uptake on nitrogen fixation. Nitrate inhibits nitrogen fixation by up to 70% in a concentration-dependent manner at initial nitrate concentrations less than 10?? Nitrate uptake accounted for as much as 86% of total N uptake and, at initial nitrate concentrations greater than 2.5 ??more than made up for the observed inhibition of nitrogen fixation. A field study of this diazotroph shows that nitrogen fixation scales with light intensity from a maximum at 50% surface irradiance. Estimated areal nitrogen fixation rates in the Gulf of Mexico, based on vertical abundance profiles and the relationship between nitrogen fixation and surface irradiance, are comparable to measurements made in other oligotrophic regions. Stable isotopic composition of the particulate organic matter and the zooplankton confirms that Trichodesmium nitrogen and carbon are moving into the food chain and are important to higher trophic levels. As much as 60% of the zooplankton carbon was derived from Trichodesmium. Our work established that this diazotroph is ecologically important in the water column of the Gulf of Mexico, with important implications for nitrogen and carbon cycling. Findings from our field and culture studies can be added to models used to quantify the importance of Trichodesmium nitrogen fixation on an oceanic scale.
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