Monitoring PSII Desiccation and Rehydration

For monitoring changes in effective quantum yields of photosystem II (ΔF/Fm′; PSII) during desiccation and subsequent rehydration, a standardized set-up was used as described in Karsten et al. 2014 . Briefly, algal filaments (c. 1–1.5 mg chlorophyll a L⁻¹) from agar plates were transferred to Whatman GF/F glass fibre filters that were moistened with 20 μL of BBM (four replicates). This was performed independently with algal cultures of three different ages (1, 6 and 15 months). Prepared filters were adjusted on perforated metal grids in a transparent 200-mL polystyrol box (d = 12 cm), which was filled with 150 mL of saturated KCl solution (Merck, Darmstadt, Germany) for setting relative air humidity (RH) inside the chamber to ~84 % (Greenspan 1977 (link)). Additionally, RH was recorded by using a PCEMSR145S-TH mini data logger (PCE Instruments, Meschede, Germany). The boxes were placed under a halogen lamp (40 μmol photons m⁻² s⁻¹ PAR) at ambient room temperature (23 ± 1 °C). A PAM 2500 was used to determine ΔF/Fm′ of PSII (Genty et al. 1989 (link); Schreiber and Bilger 1993 ) continuously during dehydration (60–120 min), whereas the PAM light probe was adjusted outside the sustained sealed chamber with a 2-mm distance to the cover lid. This resulted in a total distance from the PAM light probe to the algal sample of constant 12 mm. Subsequently after dehydration, filters were rehydrated by adding 20 μL of the standard growth medium to each algal sample and transferred to a polystyrol box containing 100 mL tap water to create a higher RH (~96 %). Measuring recovery of ΔF/Fm′ was performed analogously as described above.