Magnetotactic bacteria: concepts, conundrums, and insights from a novel in situ approach using digital holographic microscopy (DHM)

Magnetotactic bacteria (MTB) are a diverse group of highly motile Gram-negative microorganisms with the common ability to orient along magnetic field lines, a behavior known as magnetotaxis. Ubiquitous in aquatic sediment environments, MTB are often microaerophilic and abundant at the oxic/anoxic interface. Magnetic field sensing is accomplished using intracellular, membrane-encased, iron-containing minerals known as magnetosomes. The chemistry, morphology and arrangement of magnetosomes differs substantially among different MTB. Although magnetic field sensing mechanisms, genetic bases and protein functions have been elucidated in select model organisms such as the Magnetospirillum strains and Desulfovibrio RS-1, not all findings are applicable to diverse clades of MTB. As the number of identified species has increased, it has become evident that many of the characteristics and mechanisms once presumed to be prototypical of MTB are in fact not universal. Here we present a general overview of the current state of MTB research for readers outside of the realm of prokaryotic research, focusing on recent discoveries, knowledge gaps and future directions. In addition, we report new insights acquired using holographic technology to observe and quantify microbial responses in magnetic fields that are earth-strength or weaker, providing a new ecophysiological approach to in situ MTB research. ; © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2022. Received: 19 May 2021 / Revised: 9 December 2021 / Accepted: 11 December 2021. The authors would like to thank Radu Popa, Tingting Yang and Leila Mahrokh for assistance in obtaining magnetotactic bacteria and TEM expertise. We also would like to thank the staff of the Los Angeles County Arboretum for their help and support in our research. This work was funded in part by grants from the Air Force Office of Scientific Research to KHN, [FA9550-14-1-0208 and FA9550-20-1-0399 to K.J.L.], and the National Science Foundation ...