Abstract
Iodine metabolism is essential for the antioxidant defense of marine algae and in the biogeochemical cycle of iodine. Moreover, some microalgae can synthetize thyroid hormone-like compounds that are essential to sustain food webs. However, knowledge regarding iodine-related molecular processes in microalgae is still scarce. In this study, a de novo transcriptome of Tisochrysis lutea cultured under high iodide concentrations (5 mM) was assembled using both long and short reads. A database termed IsochrysisDB was established to host all genomic information. Gene expression analyses during microalgal growth showed that most of the antioxidant- (aryl, ccp, perox, sod1, sod2, sod3, apx3, ahp1) and iodide-specific deiodinase (dio) genes increased their mRNA abundance progressively until the stationary phase to cope with oxidative stress. Moreover, the increase of dio mRNA abundance in aging cultures indicated that this enzyme was also involved in senescence. Cell treatments with iodide modified the expression of perox whereas treatments with iodate changed the transcript levels of gpx1 and ccp. To test the dependence of perox on iodide, microalgae cells were treated with hydrogen peroxide (H2O2) either in presence or absence of iodide observing that several genes related to reactive oxygen species deactivation (perox, gpx1, apx2, apx3, ahp1, ahp2, sod1, sod3 and aryl) were transcriptionally activated although with some temporal differences. However, only the expression of perox was dependent on iodide levels indicating this enzyme, acquired by horizontal gene transfer, could act as a haloperoxidase. All these data indicate that T. lutea activates coordinately the expression of antioxidant genes to cope with oxidative stress. The identification of a phase-regulated deiodinase and a novel haloperoxidase provide new clues about the origin and evolution of thyroid signaling and the antioxidant role of iodine in the marine environment.
Highlights
Phytoplankton is responsible for approximately half of the atmospheric oxygen through oxygenic photosynthesis and provides food resources to sustain the vast majority of marine life (Falkowski, 2012; Chapman, 2013)
The transcriptome of T. lutea cultivated under different iodide and hydrogen peroxide conditions was sequenced using long(454) and short-reads (Illumina)
A previous transcriptome of T. lutea using cells in the retardation phase cultivated standard culture conditions in batch was reported (Carrier et al, 2014), our experimental conditions using cells in exponential phase cultivated under high iodide and peroxide treatments enriched in transcripts related with iodine metabolism
Summary
Phytoplankton is responsible for approximately half of the atmospheric oxygen through oxygenic photosynthesis and provides food resources to sustain the vast majority of marine life (Falkowski, 2012; Chapman, 2013). Aside of exogenous ROS sources, several endogenous metabolic pathways in microalgae produce ROS including hydrogen peroxide (H2O2) as a by-product of glycolate recycling and fatty acid oxidation in the peroxisome and superoxide (O2·−) during oxidative phosphorylation in the mitochondria and photosynthesis via the Mehler reaction in the chloroplast (Cirulis et al, 2013). The most important are superoxide dismutases (SODs) which remove O2·− by dismutation into H2O2, and peroxidases that catalyze the reduction of H2O2 to water. Polyphenols, pigments, ascorbate, glutathione, and some halogenated metabolites including iodide appear as the most important chemical ROS scavengers to prevent oxidative injury (Cirulis et al, 2013; Gribble, 2015)
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