Culturing

The marine diatom Thalassiosira weissflogii Grunow (strain CCMP 1010) was cultured in sterile filtered natural North Sea water (Schleswig-Holstein, Germany) or Baltic Sea water (Schleswig-Holstein, Germany). The medium was enriched with f/4 concentrations of macro- and micronutrients (nitrate, phosphate, silicic acid, trace metal mixture, vitamin mixture (Guillard and Ryther, 1962). All experiments were performed in sterile 2.1 L Schott Duran glass bottles. These bottles were made of borosilicate glass (filters UV radiation <310 nm) except for the quartz glass bottles (pure silica without UV radiation filter) used in the UV experiment. The cultures were either incubated in climate chambers with 400 –700 nm radiation or 10 cm below water level at low tide in Kiel Fjord in May 2011. Water temperature and light irradiance data were obtained from the weather station maintained by the GEOMAR institute in Kiel, Germany. Growth conditions for the various treatments, i.e. salinity, pH, temperature, and irradiance are given in Table 1. pH values (reported on free scale) were measured  with separate glass and reference electrodes (Metrohm) and calculated with equation 3 from DOE 2007 chapter 6b (Dickson et al., 2007) as described in (Bach et al., 2012). Cultures were inoculated with densities of 20 cells ml-1.  Cell densities and equivalent spherical diameters were determined with a Coulter Counter (Beckman Coulter) at the beginning and the end of the experiment, respectively. When incubations ended, cells were filtered on 47 mm diameter, 5 um mesh size Nucleopore Track-Etch Membrane filters (Whatman) and frozen at -18 deg C immediately after filtration.