Nostoc pruniforme is a freshwater cyanobacterium forming large spherical colonies of up to several centimeters in diameter. The size and shape result in low surface area to volume (SA/V) ratios that potentially put severe constraints on resource acquisition. In the present study we have specifically examined how N. pruniforme copes with the acquisition of dissolved inorganic carbon (DIC) for photosynthesis. The results showed that N. pruniforme elevates its functional SA/V considerably by locating the cyanobacterial trichomes primarily in the outer shell, while the inner mucus of polysaccharides contains declining densities of trichomes with increasing colony size. N. pruniforme is a very efficient bicarbonate (HCO₃^-) user and, in addition, actively accumulates large pools of DIC that can support net photosynthesis for >22 h without a supply of external DIC. Both the efficient HCO₃^- utilization and the large internal DIC pools greatly reduce N. pruniforme dependence on the immediate availability and species of external DIC. The location of the trichomes in the shell, the HCO₃^- utilization and the internal DIC pools mean that colony size only has a minor effect on photosynthetic rates as a function of external DIC availability.