Morphology and autofluorescence of the thallus of the golden-brown alga Hydrurus foetidus by optical and fluorescence-lifetime imaging

The chrysophyte Hydrurus foetidus belongs to the psychrophilic, rheophilic algae of cold, fast-flowing glacial and alpine streams but remains poorly characterised at the cellular level. We present a systematic optical-microscopy study of field-collected material of this organism, combining confocal imaging, quantitative morphometry, three-dimensional volumetry and fluorescence-lifetime imaging (FLIM). Confocal imaging revealed a pseudoparenchymatous architecture: elongated, longitudinally aligned cells dominate the central axis, whereas round cells occupy the peripheral branches, and the two populations differ in volume. High-magnification imaging showed a consistent apical bias in nuclear position along the cell files of the central axis. FLIM of endogenous autofluorescence yielded reproducible lifetime signatures across the green, orange and far-red emission channels; in the far-red the chlorophyll a signal resolved into a two-component mixture, and the round-cell population was bimodal in lifetime whereas the elongated population was not. All material derives from a single winter collection; the whole-thallus spatial analyses come from one intact thallus, while morphometry, volumetry and FLIM sampled cells from multiple thalli of that collection, and the data are analysed accordingly. Together, these data provide a morphological and photophysical foundation, and a label-free autofluorescence reference, for future cell-biological study of H. foetidus and of multicellular organisation among the chrysophytes.