Lack of control of nitrite assimilation by ammonium in an oceanic picocyanobacterium, Synechococcus sp. strain WH 8103

Abstract

In cyanobacteria, the transcriptional activator NtcA is involved in global nitrogen control and, in the absence of ammonium, regulates the expression of genes involved in the assimilation of alternative nitrogen sources. The oceanic picocyanobacterium Synechococcus sp. strain WH 8103 harbors a copy of ntcA, but in the present study, we show that unlike other marine cyanobacteria that have been investigated, this strain is capable of coassimilating nitrite when grown in the presence of ammonium. Transcript levels for the genes encoding the nitrate/nitrite-bispecific permease NrtP and nitrate reductase (NarB) were substantially down-regulated by ammonium, whereas the abundances of nitrite reductase (NirA) transcripts were similar in nitrite- and ammonium-grown cells. The growth of Synechococcus sp. strain WH 8103 in medium containing both ammonium and nitrite resulted in only minor changes in the expression profile in comparison to that of nitrite-grown cells with the exception that the gene encoding the high-affinity ammonium transporter Amt1 was down-regulated to the levels seen in ammonium-grown cells. Whereas the expression of nrtP, narB, and amt1 appears to be NtcA dependent in this marine cyanobacterium, the transcription and expression of nirA appear not to be. The ability to coassimilate nitrite and reduced-nitrogen sources like ammonium may be an adaptive trait that enables oceanic strains like Synechococcus sp. strain WH 8103 to exploit the low nitrite concentrations found in oceanic surface waters that are not available to their principal and more numerous competitor, Prochlorococcus.

FIG. 1.

Abundance of transcripts from six different nitrogen assimilatory genes in Synechococcus sp. strain WH 8103 grown in media containing different nitrogen sources and determined by QRT-PCR. The C_T value is the C_T at which cDNAs were first detected during amplification. For a given gene, if the C_T is higher in one treatment than in another, this indicates that the mRNA in the starting RNA preparation was present at a lower concentration. The log₁₀ mean C_T values ± 1 standard deviation (SD) (n = 3) plotted are normalized to the mean C_T of rnpB and show how much higher (>1) or lower (<1) the C_T of a cDNA is in comparison to that determined for this constitutively expressed gene in each treatment. The primers designed and used in this study amplify all target genes with equal efficiencies.

FIG. 2.

Mean relative abundances (±1 standard deviation [SD]) (n = 3) of transcripts in cells grown in different nitrogen sources normalized to that determined for the same gene in cells grown with ammonium as the sole nitrogen source. The data show how much more abundant (n-fold) (where abundance equals >1) or, conversely, less abundant (where abundance equals <1) a particular mRNA is in each treatment. For example, where abundance equals 50, that mRNA is 50-fold more abundant than in ammonium-grown cells, whereas where the abundance is 0.5, that mRNA is 2-fold less abundant.

FIG. 3.

Growth curve of Synechococcus sp. strain WH 8103 in media containing nitrite and low concentrations of ammonium. Log₁₀ mean OD₇₅₀ values ± 1 standard deviation (SD) (open triangles), mean ammonium concentrations ± 1 SD (open circles), and mean nitrite concentrations ± 1 SD (filled circles) are shown (n = 3).

FIG. 4.

Growth curve of Synechococcus sp. strain WH 8103 in media containing nitrite and high concentrations of ammonium. Log₁₀ mean OD₇₅₀ values ± 1 standard deviation (SD) (open triangles), mean ammonium concentrations ± 1 SD (open circles), and mean nitrite concentrations ± 1 SD (filled circles) are shown (n = 3). Note the different scale for the x axis in comparison to that in Fig. 3.