Bacterial colonization of the freshwater planktonic diatom Fragilaria crotonensis

Abstract

In 2006, the seasonal course of bacterial colonization and silification rates of a colonial diatom Fragilaria crotonensis were studied at 2 distinct depths ('surface' 0.5 m and Z(eu), the depth of 1% remaining surface photosynthetically active radiation) in the eutrophic Rimov Reservoir (Czech Republic). In the study period (July to September), Fragilaria dominated the phytoplankton and formed remarkable subsurface peaks of biomass at Z(eu). Diatoms living under light-limited conditions at Z(eu) displayed lower silification rates measured with the fluorescence probe 2-(4-pyridyl)-5-{[4-(2-dimethylaminoethyl-aminocarbamoyl)methoxy]phenyl} oxazole (PDMPO) and were significantly more colonized by bacteria than at the surface (seasonal means 7.8 and 5.0 bacteria per Fragilaria cell, respectively). In 3 additional experiments using double-stained Fragilaria cells (PDMPO and 4',6-diamidino-2-phenylindole [DAPI]), diatom cells with no PDMPO fluorescence were significantly more colonized by bacteria regardless of the depth, indicating that bacterial colonization reflects the physiological state of the diatoms. As an apparent consequence of a flood event in August, the Fragilaria assemblage was composed of 2 temporally isolated sub-populations differing markedly in the cell length, cell surface area and extent of bacterial colonization. Although both Fragilaria sub-populations showed similar silification rates, the late summer (post-flood) Fragilaria sub-population with smaller cells showed a significantly lower bacterial colonization, while the difference in bacterial colonization between the depths remained consistently significant throughout the study. Our results imply that bacterial colonization of diatom cell surfaces is modulated by both diatom growth and changes in population structure over the season.