SBIS, a new orange fluorescent vital probe for the 4D imaging of brown algal cells

Living cells of brown algae are difficult to observe in 3D because pigments such as fucoxanthin and chlorophyll diffract light. Furthermore, at the beginning of their life, brown algae develop slowly in seawater. To gain insight into the 3D shape and size of brown algal cells during embryogenesis, we designed a fluorescence probe that efficiently and selectively labels the plasma membrane. Styryl benzoindoleninium sulfonate (SBIS) is a bright orange fluorogenic probe that is soluble and virtually non-emissive in seawater and is activated upon binding to the plasma membrane. Unlike Calcofluor White, SBIS enables observation of cells at thicknesses of up to 25 µm. More importantly, SBIS allows three-dimensional observation of the cells in the growing uniseriate filaments of Ectocarpus sp., the polystichous filaments of Sphacelaria rigidula and the cellular monolayered lamina of Saccharina latissima over periods of up to seven days. Altogether, these properties allow visualization of the entire cell contours in living brown algae, making the study of early development at the cellular level in 4D now possible in these marine organisms.