Living organisms appear able to sense magnetic fields far weaker than thermal noise, and the leading explanation is quantum: a transient radical pair whose spin state, and therefore whose chemical fate, is nudged by an applied field. We asked whether MagLOV2, a flavin-based reporter expressed in E. coli, carries a measurable signature of that mechanism.
Imaging live colonies on the bacterioscope across five field strengths, we found that the sign of the mean magnetic-field effect flips from positive to negative as the field strengthens at low fields — positive near 0.5 mT, crossing zero, and clearly negative by 2.5 mT (Fig. 1). A sign change at low field is not what a simple thermal or heating artifact would produce; it is, however, the kind of non-trivial field dependence a radical-pair model predicts.
Pushing the field higher, a second experiment shows the effect is nonmonotonic and plateaus rather than growing without bound, while independently replicating the low-field sign flip (Fig. 2). Saturation at high field is a further radical-pair hallmark. Read together — and neither result asserts it alone — the two bundles support a common model: a radical-pair quantum mechanism underlies MagLOV2 magnetosensitivity.
Each figure remains a live discourse node, not a flattened image: follow a panel back to its bundle to reach the analysis commit, the curated dataset, and the lab-notebook caveats behind it.