Assimilation of nitric oxide by Heterosigma akashiwo in the presence of other nitrogen sources
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Heterosigma akashiwo is a globally distributed harmful algal bloom species which can use nitric oxide (NO), a toxic free radical produced in anoxic sediments, as their sole nitrogen source. Heterosigma akashiwo possesses a hybrid nitrate reductase enzyme, NR2-2/2HbN, which is hypothesized to combine nitric oxide dioxygenase (NOD) activity with nitrate reductase (NR) activity to convert NO to nitrate which is then assimilated into biomass. In previous research, NR transcription in H. akashiwo was induced by nitrate and NR activity was inhibited by excess ammonium. We investigated the capacity of H. akashiwo to utilize NO in the presence of other nitrogen sources (nitrate and ammonium) in order to understand the regulation of NO assimilation. Heterosigma akashiwo (CCMP 2393) was grown in continuous culture with 100:0 μM nitrate: ammonium, 50:50 μM nitrate: ammonium, or 0:100 μM nitrate: ammonium. Subsamples of each treatment were spiked with 15N-labeled NO. Controls received 15N-labeled nitrate, 15N-labeled ammonium, or unamended media. Heterosigma akashiwo response to NO addition was evaluated for the relative expression of genes involved in nitrogen assimilation (nitrate reductase, NR, glutamine synthase, GS, and glutamate synthase, GOGAT), and assimilation of 15N-labeled nitrogen. Results showed that NO induced NR expression regardless of the presence of other nitrogen sources. Ammonium did not prevent induction of NR expression by NO. The presence and abundance of ammonium was directly related to 15NO uptake and assimilation. Results suggested that detoxification and subsequent assimilation of NO may provide H. akashiwo with a competitive advantage in areas with depletion of nitrate and ammonium and/or high concentrations of NO.
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Keywords
Heterosigma akashiwo, Nitric oxide, Nitrogen assimilation