Identifying a new marine endosymbiosis as a model to study the origin of plastids

Endosymbioses are intimate partnerships that had profound impact on the diversification of life.

The evolution of plastids (photosynthetic organelles) by endosymbiosis led to the diversification of algae and land plants that fuel global biogeochemical processes such as carbon cycling.

Despite this importance, the steps that transformed free-living organisms into photosynthetic organelles remain poorly understood, in large parts because of the antiquity of plastid origins.

Here, we will test whether an elusive endosymbiosis in marine single cells named Meringosphaera represents a striking case of recent plastid origin.

The participants in this project have established a protocol to isolate Meringosphaera from natural populations on the Swedish West coast.

We showed that these cells belong to a large clade of exclusively heterotrophic species, notwithstanding the sustained presence of photosynthetic endosymbionts.

This result urgently calls for new data to determine whether the endosymbiont of Meringosphaera are indeed fully integrated plastids, which we will achieve using metagenomics, single-cell genomics, high-resolution microscopy and stable isotope probing.

The combination of these advanced methods will reveal the levels of cellular, genetic, and metabolic integration of the endosymbionts, as well as architectural adaptations to the intracellular environment. By determining the nature of a new endosymbiosis, we will greatly advance the understanding of organellogenesis.