Effects of nitrogen and phosphorus on the endosymbiont load of Rhopalodia gibba and Epithemia turgida (Bacillariophyceae)

Monocolonal cultures of R. gibba were exposed to three levels of nitrate-nitrogen. Nutrient-diffusing substrates were used in a lake environment to create nine microhabitats of varying N and P ratios for natural populations of R. gibba and E. turgida. The number of endosymbionts per diatom increased as ambient N became limiting; mean endosymbiont volume increased as N increased. The mean endosymbiont surface area:volume ratio decreased with inceasing N. Total endosymbiont volume per diatom (the product of the number of endosymbionts per diatom and their individual biovolumes) did not have a simple response to increasing N. Phosphorous limitation uncoupled the relationship between endosymbiont load and N. Flexibility of the endosymbiont load can reduce the metabolic cost to the diatom if the endosymbionts are dependent on the diatom for a resource. -from Authors