FORMATION OF BLOOMS BY THE SYMBIOTIC CILIATE MESODINIUM-RUBRUM - THE SIGNIFICANCE OF NITROGEN UPTAKE

Mesodinium rubrum, a holotrichous ciliate that harbors endosymbiotic chloroplasts, was responsible for patches of red water in the coastal upweUing ecosystem at 15°S, Peru during March-May 1977. Stations within red patches showed elevated concentrations of paniculate nitrogen, chlorophyll a and dissolved organic nitrogen (DON), and lower concentrations of nitrate compared with areas where there were no ciliate blooms. These naturally occurring populations of M.rubrum took up nitrate, ammonium and DON. Light was required for nitrate uptake and to a lesser extent for ammonium uptake. Dark uptake of ammonium occurred in all experiments. There was no effect of light on DON uptake. Possession of endosymbiotic chloroplasts enables this protozoan to use upwelled nitrate efficiently for development of large blooms and concentrate into patches in the face of relatively strong advective regimes. © 1990 Oxford University Press.