Environmental Adaptation in Nostoc sphaeroides: Spherical Multicellularity, Molecular Signalling and Implications for...

Nostoc sphaeroides is a diazotrophic cyanobacterium that forms macroscopic spherical colonies in seasonally flooded paddy fields. Studies of its environmental physiology, colony development and cultivation have largely proceeded independently. This review integrates strain-resolved genomic, transcriptomic, physiological and ecological evidence to examine spherical multicellularity as a possible adaptation to a variable habitat. Developmental transitions between motile hormogonia and vegetative, heterocyst-forming filaments link dispersal with establishment, while an extracellular polymeric substance (EPS) matrix promotes water retention, ion binding and formation of an associated cyanosphere. Colony enlargement may improve persistence but also produces gradients of light, gases and nutrients that can limit carbon acquisition. Visible light, low-dose ultraviolet radiation, nitrogen form, phosphate, calcium, hydration and agrochemicals influence photosynthetic electron flow, carbon-concentrating mechanisms, repair and developmental investment. The NsHik33-NsRpaB pathway provides a causal connection between environmental sensing and ultraviolet acclimation, whereas many proposed colony-level functions remain supported mainly by omics data or evidence from model cyanobacteria. Important research priorities include spatial chemical mapping, defined cyanosphere communities, multiplex genetics and experiments under fluctuating environmental conditions. Together, the evidence identifies N. sphaeroides as a promising organism for studying how cyanobacterial developmental multicellularity promotes persistence in variable environments and may inform cultivation practices.