Desiccation Kinetics and Photosystem II

For the desiccation experiments, a new standardized set-up was developed to follow the kinetics of controlled dehydration and subsequent rehydration on the effective quantum yield of photosystem II (PSII) using noninvasive pulse amplitude modulation (PAM) fluorometry. All PAM measurements were done on low-light acclimated samples (35–40 μmol photons m⁻² · s⁻¹). In addition, the effect of increasing temperatures on photosynthesis and respiration was recorded using an oxygen optode.
Cells of each Interfilum strain were concentrated on four replicate Whatman GF/F glass fiber filters (Whatman, Dassel, Germany). Onto each filter, exactly 200 μL of the cell suspension (~1–2 mg chl a · L⁻¹; parallel filters for chl a concentration were determined using dimethyl formamide [DMF] as described below) was concentrated in the center as a light green spot using an Eppendorf Pipette. These moist filters were positioned on perforated metal grids (hole diameter: 1 mm; distance between holes: 1.5 mm) on top of four glass columns inside a transparent 200 mL polystyrol box, which was filled with 100 g of freshly activated silica gel (Silica Gel Orange, Carl Roth, Karlsruhe, Germany) and sealed with a transparent top lid (Fig. 1). To record the relative air humidity (RAH) conditions inside the chambers, a PCE-MSR145S-TH mini data logger for air humidity and temperature was employed (PCE Instruments, Meschede, Germany; Fig. 1). The boxes were kept under ambient room temperatures at 22°C ± 1°C and 40 μmol photons · m⁻² · s⁻¹ PAR (Osram light sources see above).
The effective quantum yield (ΔF/Fm’) of PSII was regularly determined during the dehydration period (350–470 min depending on the strain) using a pulse amplitude modulated fluorimeter (PAM 2500; Heinz Walz GmbH, Effeltrich, Germany) according to the approach of Genty et al. (1989) . $\Delta \mathrm{F}∕{\mathrm{F}}_{\mathrm{m}}^{\prime }$ was calculated as $({\mathrm{F}}_{\mathrm{m}}^{\prime }-\mathrm{F})∕{\mathrm{F}}_{\mathrm{m}}^{\prime }$ with F as the fluorescence yield of light-treated algal cells (40 μmol photons · m⁻² · s⁻¹) and ${\mathrm{F}}_{\mathrm{m}}^{\prime }$ as the maximum light-adapted fluorescence yield after employing a 800 ms saturation pulse as described by Schreiber and Bilger (1993) . The PAM light probe was positioned outside the cover lid of the boxes (always 2 mm distance) to guarantee undisturbed RAH conditions inside, i.e., all fluorescence measurements were done through the polystyrol lids (Fig. 1). The distance from the PAM light probe to the algal sample onto the glass fiber filters was always kept constant at 10 mm.
After the dehydration period, the dried glass fiber filters were transferred to a new polystyrol box which was filled with 100 mL tap water instead of silica gel to create a high humidity atmosphere (>95%). The filters were rehydrated by adding 200 μL of the standard growth medium to each filter and recovery of $({\mathrm{F}}_{\mathrm{m}}^{\prime }-\mathrm{F})∕{\mathrm{F}}_{\mathrm{m}}^{\prime }$ was followed with the same methodology as described (i.e., at 22°C ± 1°C and 40 μmol photons · m⁻² · s⁻¹).