Phaeodactylum tricornutum exhibits cytosolic Ca²⁺ ([Ca²⁺]_cyt) elevations in response to rapid cooling. A, Eight representative fluorescence ratio traces (F/F₀, blue lines) of P. tricornutum cells expressing R-GECO1 representing changes in cytosolic Ca²⁺. Cells were perfused with ASW of different temperatures to cause rapid temperature shifts (black line). Cold shock 30°C–12°C, heat shock 12°C–30°C or control 22°C–22°C. B, False color images of a PtR1 cell exhibiting a [Ca²⁺]_cyt elevation in response to cold shock. The temperature decrease begins at t = 0 s. Note that the [Ca²⁺]_cyt elevations initiate at the tips of the cell and spread toward the central region. Left part indicates a differential interference contrast (DIC) image overlaid with chlorophyll autofluorescence. Bar represents 10 µm.