An Efficient Method for the Plating of Haploid and Diploid Emiliania huxleyi on Solid Medium1

Characterization of Delta-7 Alkenone Desaturase in Haptophyte Gephyrocapsa huxleyi Through Heterologous Expression in Tisochrysis lutea

The marine haptophyte Gephyrocapsa huxleyi is an ecologically and geochemically important phytoplankton due to its contribution to the global carbon cycle and its ability to biosynthesize certain alkenones. These alkenones are long-chain alkyl ketones with two to four trans-type double bonds. The genes encoding alkenone desaturase in G. huxleyi have not been experimentally characterized so far, partly due to the difficulty of inducing genetic transformation in G. huxleyi. Therefore, we introduced the putative alkenone delta-7 desaturase of G. huxleyi (designated "DesT") to the transformable and alkenone-producing haptophyte Tisochrysis lutea. We found two types of coding sequences for DesT, which are probably derived from the expression products of different alleles, and designated them "DesT-1" and "DesT-2." The ratio of C37:3 to C37:2 methyl alkenone in the DesT-1 transformant was significantly higher than that in the mock strain that expressed only the hygromycin resistance gene, suggesting that DesT-1 was an alkenone delta-7 desaturase in G. huxleyi. In the protein structure, a tunnel where a substrate alkenone penetrates was predicted to be located around the histidine box of DesT, and hydrophilic and hydrophobic amino acids were respectively located at the proximal (near side to the histidine box) and distal ends of the tunnel. This is the first study to conduct experimental characterization of the alkenone metabolism-related gene in G. huxleyi. The heterologous expression system using T. lutea paves the way for further characterization of the alkenone metabolism-related genes in less transformable haptophytes.

Optimized transgene expression in the red alga Porphyridium purpureum and efficient recombinant protein secretion into the culture medium

Microalgae represent a promising but yet underexplored production platform for biotechnology. The vast majority of studies on recombinant protein expression in algae have been conducted in a single species, the green alga Chlamydomonas reinhardtii. However, due to epigenetic silencing, transgene expression in Chlamydomonas is often inefficient. Here we have investigated parameters that govern efficient transgene expression in the red microalga Porphyridium purpureum. Porphyridium is unique in that the introduced transformation vectors are episomally maintained as autonomously replicating plasmids in the nucleus. We show that full codon optimization to the preferred codon usage in the Porphyridium genome confers superior transgene expression, not only at the level of protein accumulation, but also at the level of mRNA accumulation, indicating that high translation rates increase mRNA stability. Our optimized expression constructs resulted in YFP accumulation to unprecedented levels of up to 5% of the total soluble protein. We also designed expression cassettes that target foreign proteins to the secretory pathway and lead to efficient protein secretion into the culture medium, thus simplifying recombinant protein harvest and purification. Our study paves the way to the exploration of red microalgae as expression hosts in molecular farming for recombinant proteins and metabolites.

Cell-Penetrating Peptide Delivery of Nucleic Acid Cargo to Emiliania huxleyi, a Calcifying Marine Coccolithophore

Coccolithophores are a group of unicellular marine phytoplankton that exhibit a prolific capacity for carbon conversion and are critical to ocean biogeochemistry. A fundamental understanding of coccolithophore biomineralization has been limited, in part, by the lack of genetic and molecular tools to investigate the organisms. In particular, it has proven to be difficult to deliver macromolecules across the coccosphere-membrane complex. To overcome this barrier, we employed cell-penetrating peptides (CPP) in the Emiliania huxleyi coccolithophores. We evaluated three established CPPs (TAT, R9, and KFF) and designed a CPP that incorporates a high proline content identified in the protein transduction domain of EhV060, an E. huxleyi virus lectin protein. To measure the delivery performance, we covalently linked CPPs to synthetic peptide nucleic acids (PNA) and attached a fluorescein marker. CPP-PNA-FITC complexes were efficiently delivered across the coccosphere-membrane complex to the cytoplasm of E. huxleyi cells. Characterization of E. huxleyi demonstrates that CPP-PNA are nontoxic and reveals specific effects of CPP-PNA on cell biology and calcification. Direct delivery and characterization of synthetic nucleic acids represent a step forward in synthetic biology to explore coccolithophore biomineralization.

Gephyrocapsa huxleyi (Emiliania huxleyi) as a model system for coccolithophore biology

Coccolithophores are the most abundant calcifying organisms in modern oceans and are important primary producers in many marine ecosystems. Their ability to generate a cellular covering of calcium carbonate plates (coccoliths) plays a major role in marine biogeochemistry and the global carbon cycle. Coccolithophores also play an important role in sulfur cycling through the production of the climate-active gas dimethyl sulfide. The primary model organism for coccolithophore research is Emiliania huxleyi, now named Gephyrocapsa huxleyi. G. huxleyi has a cosmopolitan distribution, occupying coastal and oceanic environments across the globe, and is the most abundant coccolithophore in modern oceans. Research in G. huxleyi has identified many aspects of coccolithophore biology, from cell biology to ecological interactions. In this perspective, we summarize the key advances made using G. huxleyi and examine the emerging tools for research in this model organism. We discuss the key steps that need to be taken by the research community to advance G. huxleyi as a model organism and the suitability of other species as models for specific aspects of coccolithophore biology.