Diatom ultrastructural diversity across controlled and natural environments

Diatoms are ubiquitous aquatic microalgae critical to our planet and were among the pioneer model organisms in cell biology because of their large and transparent cell structure. However, their robust silica cell wall, which renders diatoms impermeable to many dyes and antibodies, as well as challenging for gene editing tools, likely hindered the broader establishment of diatoms as standard model species, despite their unique cellular physiology and remarkable ecological success. Here, we demonstrate that cryo-fixation combined with ultrastructural expansion microscopy (cryo-ExM) can overcome the silica barrier across diverse diatom species spanning over 80 million years of evolutionary time. We illustrate cryo-ExM's potential to provide scalable, cost-effective volumetric imaging of diatom ultrastructure in laboratory cultures, as well as field-collected samples from the pan-European TREC expedition. We first reveal striking similarities in interphase microtubule organization across diverse diatom species by characterizing cytoskeletal arrangements throughout cell cycles and populations, uniting both pennate and centric morphologies under shared principles. We further unveil diatom photosynthetic diversity through qualitative and quantitative comparative analyses of chloroplast and pyrenoid morphologies, demonstrating that each diatom species architects unique photosynthetic machinery. Using cryo-ExM on environmental samples further exposes intricate diatom symbioses, revealing tight spatial organization of ecological interactions. This methodology makes diatoms more accessible for modern and comparative cell biology research, providing new opportunities to investigate the cellular physiology of one of Earth's most successful photosynthetic groups.