Nitrate reductase and glutamine synthetase activity,internal N pools,and growth of<Emphasis Type="Italic"> Ulva lactuca</Emphasis>: responses to long and short-term N supply |
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Authors: | M Teichberg L R Heffner S Fox I Valiela |
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Institution: | (1) Marine Biological Laboratory, Boston University Marine Program, Woods Hole, MA 02543, USA;(2) Present address: Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882-1197, USA |
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Abstract: | Fast-growing macroalgae, including Ulva lactuca Linnaeus, respond rapidly to changes in nutrient conditions, particularly to short-term N supply. This ability to rapidly
take up and assimilate N contributes to the increasing occurrence of macroalgal blooms in heavily N loaded coastal ecosystems.
To determine whether long-term nutrient histories affect short-term responses in activity of N-assimilating enzymes, including
nitrate reductase (NRA) and glutamine synthetase activity (GSA), internal N storage, and macroalgal growth, we conducted an
in situ nitrate fertilization experiment between 7 and 22 July 2004, with fronds of U. lactuca collected from estuaries with high and low N loads in Waquoit Bay, Cape Cod, Massachusetts, USA (N 41° and W 70°). Initial
NRA, GSA, % N, δ15N, and growth of U. lactuca fronds were higher in the site where nitrate was in high supply. Differences in NRA persisted even after short-term experimental
enrichment. Differences in internal N pools, δ15N, and growth, in contrast, mirrored the changes of nutrient supply. The rate of turnover of the internal N content of U. lactuca was quite short (<2 d), and turnover of enzyme activity may have been even shorter. N isotopic fractionation by U. lactuca appeared to be of small magnitude, unlike the case of phytoplankton, and similar to that of vascular plants. δ15N was a better indicator of short-term response to external and internal nutrient supplies in U. lactuca than enzyme activity or N content, and may reliably detect rapid changes in N availability, source, and uptake and assimilation
processes. |
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