Biology

Unraveling the regulation of nitrogen assimilation in the marine diatom thalassiosira pseudonana (bacillariophyceae): Diurnal variations in transcript levels for five genes involved in nitrogen assimilation

Document Type

Article

Abstract

We examined the diurnal expression of five genes encoding nitrogen-assimilating enzymes in the marine diatom Thalassiosira pseudonana (Hust.) Hasle et Heimdal following a transition from NH4+- to NO3--supplemented media. The accumulation of nia transcripts (encoding nitrate reductase, NR) following the transition to NO 3--supplemented media was similar to previously reported changes in NR abundance and activity. Nia mRNA levels varied diurnally, and the diurnal oscillations were abolished when cells were transferred to continuous light. Genes encoding chloroplastic (niiA) and cytosolic (nirB) nitrite reductases were identified in the genome of T. pseudonana. NiiA and nirB transcript levels increased within 2 h following the addition of NO 3- and varied diurnally. Patterns of diurnal variation in nia, niiA, and glnII (encoding the chloroplast-localized glutamine synthetase) mRNA abundances were similar. NirB and glnN (encoding the cytosolic-localized glutamine synthetase) mRNA levels also oscillated diurnally; however, the oscillation was out of phase with nia, niiA, and glnII. We propose that NO 3- is assimilated into organic molecules in both the chloroplast and cytosol of diatoms and that enzymes encoded by nirB and glnN contribute to the ecologically important dark assimilation of NO 3- observed in marine diatoms. As with nia, the diurnal variations in niiA, nirB, glnII, and glnN were abolished when cells were transferred to continuous light. Our results demonstrate that transcript accumulation is not circadian controlled, but, rather, changes in metabolic pools triggered by light:dark (L:D) transitions may be important in regulating the cellular mRNA levels encoding these key nitrogen assimilating enzymes. © 2009 Phycological Society of America.

Publication Title

Journal of Phycology

Publication Date

4-2009

Volume

45

Issue

2

First Page

413

Last Page

426

ISSN

0022-3646

DOI

10.1111/j.1529-8817.2009.00648.x

Keywords

Diatoms, gene expression, Glutamine synthetase, Nitrate reductase, Nitrite reductase, Nitrogen assimilation, quantitative real-time PCR

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