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The functional significance of sex and death in phytoplankton differentiation

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A microscopic alga with a big agenda

European scientists are investigating the genetics of a phytoplankton that acts as a carbon sink and produces a climate-active gas. Unravelling what genes are responsible for survival of the sexual phase promises to unlock the secret of repopulation and formation of new blooms.

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A tiny alga, Emiliania huxleyi (E. huxleyi), with a complex life cycle plays a big role in the biogeochemistry of our oceans. As a significant part of the world's carbon sink, calcareous plates form the armour of the cell surface of the diploid life-cycle phase that is characterised by milky white blooms. E. huxleyi also produces dimethyl sulphide (DMS) which acts as a nucleus for cloud condensation and may therefore play a crucial role in global homeostasis. The other phase in the life cycle, the sexual or haploid stage produces gametes or sex cells completely resistant to viral attack and which can virtually wipe out the bloom. With European research awareness of the importance for climate regulation, the EU-funded project 'The functional significance of sex and death in phytoplankton differentiation' (Funsex-Dephynd) aimed to study the differences between the sexual and asexual phases to shed light on stress responses and exponential growth of this marine phytoplankton. Using deep Sanger sequencing, the project team estimated that there were some 20,000 expressed genes in the bloom phase of E. huxleyi, and half of these were likely to be differentially expressed in the sexual phase. The scientists also identified all-important genes relating to calcification in the diploid phase. Highly specific in the haploid stage are expression of genes that result in flagella so sex cells in some strains are motile. Funsex-Dephynd also investigated the effects of phosphorus (P) and nitrogen (N) starvation on both phases of the life cycle using microarrays. Haploid sex cells are more tolerant of low P than diploid cells and the scientists identified the changes in gene expression responsible for the differences. Genome-wide comparisons also revealed large differences between different strains of E. huxleyi – 70 strains in warmer waters were found to have lost the ability to form flagella. Strains in more temperate climates maintained the full life cycle. the Joint Genome Institute has recently completed an analysis of the whole sequence of E. huxleyi and post-genomic research is important to analyse gene function and its applications. Results of the project will serve to strengthen European research on phytoplankton, crucial for climate regulation.

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