Experiment 1 (interspecific selectivity: effects of light and temperature)
In order to test the selectivity of P.diadema for different diatom species, Cgranii
and Cwailesii were chosen as host diatom species. To 'amplify' the expected
preference for Cwailesii, a mixed experimental culture of Cwailesii with a very
small diameter (120-125 um) and low density (30 cells ml"1) was used. The
Cgranii culture contained cells twice as large (240-250 um) with a 2.5-fold density
(75 cells ml"1). Experiments were carried out in 300 ml Erlenmeyer flasks containing
200 ml of the mixed diatom culture (one stock culture was divided into
subsamples) which had just reached the stationary phase. A culture in the stationary
phase was chosen to minimize physiological differences between treatments
due to possibly different physiological properties of diatoms during growth.