Chain or sphere? Perspectives on colony shapes and sizes in microalgae

Abstract

Abstract Some microalgal species can increase their collective size by forming colonies; notable examples are chained colonies in diatoms and Scenedesmus sp., and spherical colonies in Phaeocystis globosa. For a given cell-specific growth rate, chain formation increases collective length quickly to fend off ciliates, but not against tube- and pallium-feeding heterotrophic dinoflagellates or metazoan grazers with ability to manipulate chains to aid ingestion. Sphere increases in volume relatively slowly but would be difficult to manipulate even for metazoan grazers. Diffusive nutrient supply to a chained colony would be a fixed proportion of that to solitary cells, regardless of chain length, whereas cells within a spherical colony would experience increasing nutrient limitation with increasing colony size. One hemisphere of a spherical colony would inevitably receive less irradiance, creating an auto-light limitation. Experimental data showed that light decreased substantially as it passed through a P. globosa colony, and the optical density of the colony increased linearly with colony diameter. However, neither in situ nutrient nor light limitation alone can explain an order-of-magnitude difference in colony size between the European and the Asian P. globosa populations. Instead, some evidence of different expression of gene(s) involved in colony formation and enlargement suggests genomic variations among the different populations.