Variability in the provision and function of mucilage in phytoplankton: facultative responses to the environment

Phytoplanktic cells and coenobia vary in mucilaginous investment. Mostly this is taxon-specific but the thickness and texture of the mucilage itself is variable and responsive to environmental variation. This paper considers the role and function of mucilage provision in the phytoplankton, and it explores the drivers of inter- and intra-specific variability. Benefits considered include density reduction, dynamic streamlining, as well as suggested roles in the sequestration and storage of nutrients. Metabolic self-regulation, as a way of surviving in nutrient-poor water, is also evaluated, including the maintenance of a weakly oxidative environment for nitrogen fixers and the exclusion of toxic metals. The possibility that mucilage resists ingestion by grazers is also re-examined, as is the alleged resistance to digestion. The review concludes that there is no single, unambiguous function of mucilage. Not all of the several that have been suggested may be considered to be credible or authenticable. On the other hand, there is good empirical verification for the buoyant properties, for the generation of a reducing microenvironment, and for deterrence of grazers. However, none is considered to be an original purpose, which may have been as a means to balance cell stoichiometry.