An enigmatic widespread one cellular organism “steals” algal chloroplasts

Micrographs of the centrohelid Meringosphaera and its stolen plastids (kleptoplasts). Left: Secondary electron micrograph (SEM) showing the central capsule (mint green) and undulating spines (purple). Note the numerous bulges in the central capsule from the kleptoplasts. Right: An overlay of confocal laser scanning microscope images highlights the positive hybridisation for the Meringosphaera hosts (green), a general stain for nuclear DNA (blue), and natural autofluorescence of the kleptoplasts (red). Images courtesy of co-authors V. V. Zlatogursky (left) and M. SØrensen (right).

A collaborative research project led by former Postdoctoral Scholar Megan SØrensen used a variety of culture independent techniques to investigate the poorly studied centrohelid Meringosphaera. Meringosphaera are small eukaryotic plankton, approximately 10 um in cell diameter, that are not inherently photosynthetic, yet many report the presence of internal auto-fluorescent ‘green bodies’. Using genetic material generated from single cells (or single amplified genomes, SAGs) collected from several locations of the west coast of Sweden, SØrensen and colleagues began to unravel the mystery of the origin and integration of Meringosphaera’s plastids.

Postdoctoral scholar Megan Sørensen whom led the research and is now a EMBO/Marie Curie Fellow at Heinrich Heine University Düsseldorf. Photo: Heinrich Heine University Düsseldorf

The research team recovered six full plastid genomes of which two distinct subgroups were identified that were congruent with the identity of the host cells. In other words, two closely related “sister” Meringosphaera hosts take up two closely related “sister” plastids from other algal cells in the environment. The two symbiotic pairs were monitored in monthly water collections by specific assays for both partners genes. Combined results showed evidence that plastids were detected only sometimes, and likely the relationship is a kleptoplastidic one. Importantly, several genes from the stolen plastids and other algae were found in the Meringosphaera host genome suggesting that the first steps of genetically integrating the plastids are underway. 

To date, a few similar cases of kleptoplastidy have been reported in other eukaryotic groups that have previously possessed plastids (e.g., dinoflagellates). Thus, Meringosphaera represents a new system to study the origin, stabilization, and establishment of plastids. 

This scientific work was funded by the collaborative postdoc project grant linked to SciLifeLab. It was a state-financed strategic fund (SFO) awarded to Rachel A Foster at the Department of Ecology, Environment, and Plant Sciences of Stockholm University and Fabien Burki at Department of Organismal Biology of Uppsala University of SciLife Lab. Field samples were collected by the Swedish Meteorological and Hydrological Institute (SMHI). 

Rachel A Foster reflects, “Planktonic symbioses are so fascinating and understudied, largely because many, including Meringosphaera, are not easily isolated or available in culture. But thanks to SMHI for helping us with sampleing, and the efforts of Megan and our collaborators at UU, we could start to unravel the role of kleptoplastidy in organelle establishment. Our results are exciting and have opened more questions to follow up on.”

More information

The article “A novel kleptoplastidic symbiosis revealed in the marine centrohelid Meringosphaera with evidence of genetic integration” is published in the scientific journal Current Biology. 
DOI: 10.1016/j.cub.2023.07.017.