Original Article

Subject Category: Geomicrobiology and microbial contributions to geochemical cycles

The ISME Journal (2012) 6, 629–637; doi:10.1038/ismej.2011.121; published online 22 September 2011

Decoupling of ammonium regulation and ntcA transcription in the diazotrophic marine cyanobacterium Trichodesmium sp. IMS101

Anton F Post1,2, Branko Rihtman3 and Qingfeng Wang2

  1. 1The Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, Marine Biological Laboratory, Woods Hole, MA, USA
  2. 2The ‘H. Steinitz’ Marine Biology Laboratory, Interuniversity Institute for Marine Science, Eilat, Israel
  3. 3Life Sciences Institute, Hebrew University at Givat Ram, Jerusalem, Israel

Correspondence: AF Post, The Josephine Bay Paul Center for Comparative Genomics and Molecular Evolution, Marine Biological Laboratory, 7 MBL Street, Woods Hole, MA 02543, USA. E-mail: apost@mbl.edu

Received 13 May 2011; Revised 3 August 2011; Accepted 3 August 2011; Published online 22 September 2011.

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Abstract

Nitrogen (N) physiology in the marine cyanobacterium Trichodesmium IMS101 was studied along with transcript accumulation of the N-regulatory gene ntcA and of two of its target genes: napA (nitrate assimilation) and nifH (N2 fixation). N2 fixation was impaired in the presence of nitrite, nitrate and urea. Strain IMS101 was capable of growth on these combined N sources at <2μM but growth rates declined at elevated concentrations. Assimilation of nitrate and urea was impaired in the presence of ammonium. Whereas ecologically relevant N concentrations (2–20μM) suppressed growth and assimilation, much higher concentrations were required to affect transcript levels. Transcripts of nifH accumulated under nitrogen-fixing conditions; these transcript levels were maintained in the presence of nitrate (100μM) and ammonium (20μM). However, nifH transcript levels were below detection at ammonium concentrations >20μM. napA mRNA was found at low levels in both N2-fixing and ammonium-utilizing filaments, and it accumulated in filaments grown with nitrate. The positive effect of nitrate on napA transcription was abolished by ammonium additions of >200μM. This effect was restored upon addition of the glutamine synthetase inhibitor L-methionin-DL-sulfoximine. Surprisingly, ntcA transcript levels remained high in the presence of ammonium, even at elevated concentrations. These findings indicate that ammonium repression is decoupled from transcriptional activation of ntcA in Trichodesmium IMS101.

Keywords:

cyanobacteria; Trichodesmium; N assimilation; nitrogen fixation; ntcA