Visualization of Loki- and Heimdallarchaeia (Asgardarchaeota) by fluorescence in situ hybridization and catalyzed reporter deposition (CARD-FISH)

Metagenome-assembled genomes (MAGs) of Asgardarchaeota are starting to be recovered from a variety of habitats, broadening their environmental distribution and providing access to the genetic makeup of this archaeal lineage. Despite their singular phylogenetic position at the base of the eukaryotic tree of life, the morphology of these bewildering organisms remains a mystery. In order to visualize this elusive group, we applied a combination of CARD-FISH and epifluorescence microscopy on coastal hypersaline sediment samples, using specifically designed CARD-FISH probes for Heimdallarchaeia and Lokiarchaeia lineages and provide the first visual evidence for both these groups. Here, we show that while Heimdallarchaeia are characterized by a uniform cellular morphology typified by central DNA localization, Lokiarchaeia display a plethora of shapes and sizes that likely reflect their broad phylogenetic diversity and ecological distribution.