Physiological Characterization and Morphogenesis of an Edible Cyanobacterium, Nostoc sphaeroides Kützing, in Liquid Suspension Culture

Abstract

Nostoc sphaeroides Kützing, a cyanobacterium valued for its nutritional properties, is commonly used as both food and herbal medicine. Despite its significance, limited information exists on the morphological formation of its spherical colonies and the cultivation factors influencing their structure. This study investigates the formation of N. sphaeroides spherical colonies from single trichomes, providing detailed morphological and cellular characterizations. N. sphaeroides was identified by its scattered, straight trichomes, the formation of heterocytes and akinetes, and the development of spherical macrocolonies. Cultured in BG11 medium, N. sphaeroides showed significant morphological variation and biomass growth. The optimal growth conditions were observed at 50 rpm agitation, with incubation at 25±2 °C under a 16:8 light-dark cycle and a light intensity of 150 μmol photons m^-²·s^-¹.  Under these conditions, the colony concentration, fresh weight, and dry weight reached their peaks on the 40^th day, reaching 15.672 g·L^-1 for fresh weight and 0.610 g·L^-1 for dry weight. Key developmental stages were noted in the batch culture: aggregated cell masses by the 7^th day, asteriated colonies between the 13^th and 15^th days, 300–350 µm microcolonies by the 21^st day, and 9–13 mm spherical macrocolonies by the 30^th day. As the colonies expanded beyond 13 mm after the 30^th day, they differentiated into distinct layers, eventually forming discoid or flat colonies upon rupture. These findings provide valuable insights for the scale-up and field cultivation of N. sphaeroides.