2001, 2009; Moore et al. 2003). Although the FRRF was recalibrated by the manufacturer into the low sensitivity mode (0–150 μg chl a l−1) the biomass (as in the growth conditions) was still too high, leading to saturation of the fluorescence signals. We, therefore, used neutral density filters (grey tinted polycarbonate films), shielding

the photomultiplier light intake path of the apparatus to obtain suitable detection ranges (see Fig. 1 for a schematic drawing of the experimental set-up). The data were fitted using the software provided by the manufacturer. Samples were kept in 50-ml culture vessels, under airtight conditions at constant stirring at room temperature (20–22°C). A cooling jacket was placed against the culture vessel and was facing the light source. A manually controlled halogen light source was used for application of PF of 50–470 μmol photons m−2 s−1 buy LEE011 (FL 440 Walz GmbH, Germany). A FL

103 F short pass filter (<700 nm, Walz GmbH, Germany) was used block the near-infrared wave band. The PF was measured using a spherical (4π) quantum sensor. For differences between the multiple (e.g. PAM fluorometers) and single turnover protocols see SN-38 nmr Kromkamp and Forster (2003). Fig. 1 Schematic drawing of the FRRF experimental set-up. A 50-ml culture bottle contained the samples and was placed against the FRR fluorometer so that it received the flashlet sequences from behind (fluorometer light output), and the actinic light the front (i.e. the left side in this drawing). The photomultiplier detected chlorophyll fluorescence from below. Due to relatively high cell densities, neutral density filters shielded the light intake to avoid overload of the photomultiplier. A translucent cooling jacket was placed against the front of the sample to avoid rising temperatures due to heat emission from Progesterone the actinic (halogen) light source. The sample was stirred with the stirrer placed at the side

of the culture bottle For calculations of variable fluorescence parameters, the standard nomenclature was used (refer to, e.g. Kolber and Falkowski 1993; Kromkamp and Forster 2003; Fujiki et al. 2007). The functional absorption cross section (σPSII) describes the maximal light utilisation efficiency for photochemistry in PSII, expressed in area per quantum (Å2). The same is true for σPSII′, but for a light acclimated state. Plastic PSII energy distribution can be distinguished between the lake model, where PSII centres are energetically connected, and the single unit model, where one PSII centre receives energy from its most adjacent light harvesting complex only. The connectivity parameter p is calculated from the kinetics of fluorescence increase during a flashlet sequence and describes the fraction of energetically connected PSII. Further details and algorithm are given in the literature (Kolber and Falkowski 1993; Kolber et al. 1998).