January 13

Caroline Juery
Phaeocystis/Acantharia symbiosis: what are they exchanging?
Acantharian form a group of a ~150 species divided four main orders (Holacanthida, Chaunacanthida, Symphiacanthida and Arthracanthida), 18 families, about 50 genera and in 6 main clades and 13 subclades. Among these clades, some are specifically characterized by their ability to form endosymbiosis with microalgae, especially Phaeocystis. They are widely distributed in the Oceans and symbiotic clades are preferentially found in oligotrophic waters, where they represent a high percentage of carbon fixation. In marine ecosystems, photosymbiosis between heterotrophic hosts and microalgae is fueled by photosynthesis, which facilitates the transfer of organic carbon, such as sugars, to the host. However, the specific carbohydrates involved and the molecular mechanisms governing their exchange remain largely unknown, particularly in unicellular photosymbioses that dominate open-ocean environments. Through a combination of genomics, single-holobiont transcriptomics, and environmental metatranscriptomics, we explored the transportome of the marine microalga Phaeocystis in symbiosis with acantharians, focusing on sugar transporters. Genomic analyses revealed that the sugar transportome of Phaeocystis is similar to that of non-symbiotic haptophytes. However, symbiotic Phaeocystis exhibited substantial changes in transportome gene expression compared to its free-living stage, with 36% of sugar transporter genes showing differential expression. Additionally, these sugar transporter genes displayed distinct temporal expression patterns over the day. These findings provide new insights into the molecular mechanisms underpinning the ecological success of planktonic photosymbioses and lay the groundwork for further studies on transporters in photosymbiotic systems.

Filip Husnik
Foraminifera: One of the last frontiers of protist symbioses
Symbiotic relationships between foraminifera and their prokaryotic and eukaryotic partners are incredibly diverse. However, these interactions have mostly been described through microscopy and remain largely unexplored using modern molecular methods. Consequently, foraminifera (together with other Rhizaria) represent one of the last protist symbiosis frontiers. In this talk, I will outline some of our projects on these symbioses involving taxonomically diverse foraminifera from distinct environments such as coral reefs, mangroves, and the deep sea. I will then focus on algal symbioses of large benthic foraminifera from coral reefs as our main model system. Photosymbioses of LBF with microalgae can easily rival coral-dinoflagellate endosymbioses in terms of their taxonomic diversity, but whether this diversity is also reflected in their functions and adaptations to marine microhabitats is unclear. We characterized symbioses of eight genera of LBF with diatoms, dinoflagellates, red algae, and green algae using single-cell metagenomics, phylogenetics, and microscopy. We show which of these symbiotic relationships are likely more long-term, that their chloroplast genomes are similar to their free-living relatives, and discuss the interdependence of the partners. Even though these novel symbiotic pairs enhance our broader understanding of eukaryote-eukaryote interactions, further functional characterization of the individual partners is needed to fully understand if and how the different microalgal symbionts drive the host adaptations to specific environments.