| Summary: | The physiological performance of Nostoc sp. colonies in the high Arctic was investigated based on their structure and function. To investigate the internal colony structure, a method based on vertical stacking of individual light microscopy images was tested under the conditions at a polar field station. The physiological state of sun-exposed and shaded surfaces of the colonies was assessed using variable chlorophyll fluorescence imaging under two distinct low- and high-light conditions. The 3D image of the internal structure of the colonies revealed a high number of cells in the central part of the colony. Two peaks of maximum cell density were observed, probably caused by two overlapping colony lobes or subcolonies. Light was the driving factor of photosynthetic activity, and the colony structure played a role in the rate of response to incoming light. Fluorescence imaging revealed heterogeneity of the photosynthetic activity in the colonies, with the maximum photosynthetic activity at the colony edge due to better access to nutrients. The differences between exposed and shaded surfaces were not as pronounced as was expected, either due to good photoacclimation to a broad range of light conditions, light distribution through translucent extracellular matrixes, or integration of fluorescence signals throughout the colonies. The slightly better photosynthetic performance under high light conditions may indicate photoacclimation of Nostoc sp. to a broad range of light conditions encountered in the field.
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