# Giant multicellular magnetotactic prokaryotes in marine sediments

## TL;DR

Scientists discovered a new type of giant multicellular bacteria in marine sediments that use magnetic crystals to navigate, expanding our understanding of prokaryotic multicellularity.

## Contribution

The discovery of a new, much larger morphotype of multicellular magnetotactic prokaryotes and its classification into a new genus and species.

## Key findings

- A new giant multicellular magnetotactic prokaryote was found in Mediterranean sediments, containing about 130 cells per consortium.
- Each cell produces over 100 greigite magnetosomes arranged to optimize magnetic navigation.
- The new species was classified as Magnetogigantoglobus mediterraneus within the Candidatus Magnetomoraceae family.

## Abstract

Multicellular magnetotactic prokaryotes represent a unique group of obligately marine multicellular bacteria known for their ability to navigate along magnetic field lines thanks to ferrimagnetic nanocrystals. To date, two distinct spherical and ellipsoidal morphotypes have been described, typically ranging from 3 to 6 μm in diameter and comprising approximately 50 cells of the same species. Although widespread in highly reduced marine sediments, they are represented by solely three genera clustering into a monophyletic group within the Desulfobacterota. In this study, we report a third morphotype in reduced sediments of the Mediterranean Sea in Carry-le-Rouet, France, i.e. approximately 30 times more voluminous than any previously described form. Because their large size, we designated these multicellular bacteria as “giant” and explored their cell ultrastructure, ecological niche and physiology using magnetic enrichment and a combination of microscopy techniques and single-consortium genomics. Transmission electron microscopy and confocal microscopy images of several individual consortia revealed that they contain an average of 130 cells, each producing over 100 greigite magnetosomes arranged to optimize the overall magnetic moment. Phylogenomic analyses positioned giant multicellular magnetotactic prokaryotes, together with other morphotypes, in a previously undescribed genus and species within the Candidatus Magnetomoraceae family, named Magnetogigantoglobus mediterraneus. Although genetically divergent with a different ultrastructure, all multicellular magnetotactic prokaryotes seem to rely on sulfate reduction coupled to heterotrophy or autotrophy. We further discuss the significance of these findings in the context of the evolutionary history of multicellularity and magnetotaxis in prokaryotes.