Targeted volume Correlative Light and Electron Microscopy of an environmental marine microorganism

Abstract

Photosynthetic microalgae are responsible for an important fraction of CO2 fixation and O2 production on Earth. Three-dimensional ultrastructural characterization of these organisms in their natural environment can contribute to a deeper understanding of their cell biology. However, the low throughput of volume electron microscopy (vEM) methods, along with the complexity and heterogeneity of environmental samples, pose great technical challenges. In the present study, we used a workflow based on a specific EM sample preparation, compatible with both light and vEM imaging in order to target one cell among a complex natural community. This method revealed the 3D subcellular landscape of a photosynthetic dinoflagellate with quantitative characterization of multiple organelles. We could show that this cell contains a single convoluted chloroplast and the arrangement of the flagellar apparatus with its associated photosensitive elements. Moreover, we observed chromatin features that could shed light on how transcriptional activity takes place in organisms where chromosomes are permanently condensed. Together with providing insights in dinoflagellates biology, this proof-of-principle study illustrates an efficient tool for the targeted ultrastructural analysis of environmental microorganisms in heterogeneous mixes.

Diatom pyrenoids are encased in a protein shell that enables efficient CO2fixation

“…Even in our cryo-ET comparison between two diatom species, we noted substantial differences in PyShell architecture, in particular at the pyrenoid ends (Figures 2A-D, G-H; S3A,E). Capturing the variations in PyShell architecture across evolution will require extensive cryo-ET of diverse algae, including in non-model species sampled directly from the environment (Mocaer et al, 2023). S1.…”