Abstract
The diversity of selenoproteins raises the question of why so many life forms require selenium. Selenoproteins are found in bacteria, archaea, and many eukaryotes. In photosynthetic microorganisms, the essential requirement for selenium has been reported in 33 species belonging to six phyla, although its biochemical significance is still unclear. According to genome databases, 20 species are defined as selenoprotein-producing organisms, including five photosynthetic organisms. In a marine coccolithophorid, Emiliania huxleyi (Haptophyta), we recently found unique characteristics of selenium utilization and novel selenoproteins using 75Se-tracer experiments. In E. huxleyi, selenite, not selenate, is the main substrate used and its uptake is driven by an ATP-dependent high-affinity, active transport system. Selenite is immediately metabolized to low-molecular mass compounds and partly converted to at least six selenoproteins, named EhSEP1–6. The most (EhSEP2) and second-most abundant selenoproteins (EhSEP1) are disulfide isomerase (PDI) homologous protein and thioredoxin reductase (TR) 1, respectively. Involvement of selenium in PDI is unique in this organism, while TR1 is also found in other organisms. In this review, we summarize physiological, biochemical, and molecular aspects of selenium utilization by microalgae and discuss their strategy of selenium utilization.
Highlights
The diversity of selenoproteins raises the question of why so many life forms require selenium
When Se was added to the culture medium, growth was stimulated in the diatom Thalassiosira pseudonana [3], Chysochromulina breviturrita in Haptophyceae [4], the dinoflagellates Gymnodinium catenatum and Alexandrium minutum [5,6], and other algae [7,8,9,10,11,12,13,14,15,16] (Table 1)
Se is an essential element in E. huxleyi cells, and we identified Se compounds accumulated in the cells by using a radio-labeling technique with 75Se-selenite. 75Se-labeled metabolites were analyzed using thinlayer chromatography (TLC) for the development of metabolites and radio-luminography for the detection of radioactive compounds
Summary
An oceanic, unicellular haptophycean calcifying alga, is an abundant coccolithophorid known to fix a large amount of carbon and produce a huge biomass during blooms. When Se was added to the culture medium, growth was stimulated in the diatom Thalassiosira pseudonana [3], Chysochromulina breviturrita in Haptophyceae [4], the dinoflagellates Gymnodinium catenatum and Alexandrium minutum [5,6], and other algae [7,8,9,10,11,12,13,14,15,16] (Table 1). Such a growth-stimulating effect is greater for selenite than selenate ions [17].
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