EMILIANIA HUXLEYI(PRYMNESIOPHYCEAE): NITROGEN-METABOLISM GENES AND THEIR EXPRESSION IN RESPONSE TO EXTERNAL NITROGEN SOURCES

Abstract

The availability and composition of dissolved nitrogen in ocean waters are factors that influence species composition in natural phytoplankton communities. The same factors affect the ratio of organic to inorganic carbon incorporation in calcifying species, such as the coccolithophore Emiliania huxleyi (Lohman) W. W. Hay et H. Mohler. E. huxleyi has been shown to thrive on various nitrogen sources, including dissolved organic nitrogen. Nevertheless, assimilation of dissolved nitrogen under nitrogen-replete and -limited conditions is not well understood in this ecologically important species. In this study, the complete amino acid sequences for three functional genes involved in nitrogen metabolism in E. huxleyi were identified: a putative formamidase, a glutamine synthetase (GSII family), and assimilatory nitrate reductase. Expression patterns of the three enzymes in cells grown on inorganic as well as organic nitrogen sources indicated reduced expression levels of nitrate reductase when cells were grown on NH 4 + and a reduced expression level of the putative formamidase when growth was on NO 3) . The data reported here suggest the presence of a nitrogen preference hierarchy in E. huxleyi. In addition, the gene encoding for a phosphate repressible phosphate permease was more highly expressed in cells growing on formamide than in cells growing on inorganic nitrogen sources. This finding suggests a coupling between phosphate and nitrogen metabolism, which might give this species a competitive advantage in nutrient-depleted environments. The potential of using expression of genes investigated here as indicators of specific nitrogenmetabolism strategies of E. huxleyi in natural populations of phytoplankton is discussed.

Metatranscriptome analyses indicate resource partitioning between diatoms in the field

“…Specific genes in this set included, but were not limited to, those genes associated with N assimilation (e.g., glutamate dehydrogenase, glutamine synthase, nitrate reductase), dissolved organic N utilization (e.g., urease, aminopeptidase, amino acid transport system), P scavenging (e.g., phosphate transporter, NPT), and DOP utilization (e.g., phosphatases) (Dataset 2). A number of these genes have been shown to be N-or P-responsive in transcriptional studies with cultures of the diatom T. pseudonana (56,61), and transporters and enzymes for the processing of organic N or P, as observed here, are well known to be RR in many phytoplankton (56,(62)(63)(64). Overall, these genes demonstrated patterns of regulation in situ (Fig.…”

Comparative metatranscriptomics identifies molecular bases for the physiological responses of phytoplankton to varying iron availability

“…Of particular interest is that after iron enrichment, no haptophyte or chlorophyte transcripts were detected for any genes involved in nitrate transport or reduction (Fig. 4) despite the presence of all genes necessary to carry out these processes within their genomes (33,34). This result suggests that both groups are unresponsive to the large concentrations of available nitrate following iron enrichment, perhaps because their newly acquired iron is partitioned for other uses such as a more efficient photosynthetic electron transfer.…”

Transcriptomic response of the toxic prymnesiophyte Prymnesium parvum (N. Carter) to phosphorus and nitrogen starvation

“…Examples include the chlorophyte Chlamydomonas (Moseley et al, 2006;Wykoff et al, 1999), the prasinophyte Micromonas (McDonald et al, 2010), the dinoflagellate Alexandrium minutum (Yang et al, 2011), various diatoms (Brown et al, 2009;Parker and Armbrust, 2005), as well as the prymnesiophyte E. huxleyi (Bruhn et al, 2010;Dyhrman et al, 2006;Riegman et al, 2000).…”