Visible light excited ratiometric-GECIs for long-term in-cellulo monitoring of calcium signals.

•Visible light excited ratiometric-GECIs (VR-GECIs) enabled long-term tracking of organellar Ca2+ levels in living cells.•mt-riG6m, a GCaMP6m-based VR-GECI for mitochondrial Ca2+ signals, outperformed current GECIs by having a larger dynamic range and bigger responses.•VR-GECIs could be used to monitor intra-cellular or intra-organellar Ca2+ signals regardless of changes in cell shapes and indicator expression levels.•ER luminal Ca2+ levels in mammalian cells remained relatively stable during mitosis.