Diverse algae possess the ability to recover from extreme desiccation without forming specialized resting structures. Green algal genera such as Tetradesmus (Sphaeropleales, Chlorophyceae) contain temperate terrestrial, desert, and aquatic species, providing an opportunity to compare physiological traits associated with the transition to land in closely related taxa. We subjected six species from distinct habitats to three dehydration treatments varying in relative humidity (RH 5%, 65%, 80%) followed by short- and long-term rehydration. We tested the capacity of the algae to recover from dehydration using the effective quantum yield of photosystem II as a proxy for physiological activity. The degree of recovery was dependent both on the habitat of origin and the dehydration scenario, with terrestrial, but not aquatic species, recovering from dehydration. Distinct strains of each species responded similarly to dehydration and rehydration, with the exception of one aquatic strain that recovered from the mildest dehydration treatment. Cell ultrastructure was uniformly maintained in both aquatic and desert species during dehydration and rehydration, but staining with an amphiphilic styryl dye indicated damage to the plasma membrane from osmotically-induced water loss in the aquatic species. These analyses demonstrate that terrestrial Tetradesmus possess a vegetative desiccation tolerance phenotype, making these species ideal for comparative omics studies.

YII data June 2018, YII data June 2020

Desiccation experiments were carried out using desiccation chambers previously described by Karsten et al. (2014). Different levels of relative humidity (RH) in the chamber were achieved by adding one of three different desiccants to the chambers: 100 g of CaSO4 (W. A. Hammond DRIERITE Co. LTD, Xenia, OH, USA) achieving ~5 % RH, a solution of 33g LiCl in 100 ml of dH2O for RH 65%, and a saturated solution of KCl in dH2O (100 ml) for RH 80%.

Algal cell suspensions (50 μl, approximately 150,000 cells corresponding to chlorophyll concentration 30–40 mg ml-1) were placed onto glass fiber filters (Whatman, Maidstone, United Kingdom) in replicates of four. Three filters at a time were then positioned on a perforated metal grid inside a desiccation chamber containing the appropriate desiccant. RH levels in the chambers were monitored using PCEMSR145S-TH mini data logger (PCE Instruments, Meschede, Germany). The chambers were kept under dim light of ~10 μmol photons m-2s-1 at 22˚C. Measurements of ΔF/Fm’ of PSII (FPSII) were taken using a PAM 2500 chlorophyll fluorometer (Heinz Walz GmbH, Effeltrich, Germany) for T. obliquus, T. deserticola, T. adustus, and T. bajacalifornicus (the light probe was adjusted outside of desiccation chamber in 12 mm distance from the algae samples). Strains of T. dissociatus and T. sp. “raciborskii" (CCAP 276/35) were acquired later. Experiments involving these species, along with a previously measured aquatic and terrestrial strain, followed the same protocol except that their chlorophyll fluorescence was recorded using a Junior PAM (Heinz Walz GmbH, Effeltrich, Germany).

Measurements were taken every 10 min during the desiccation period (PAM settings: measuring light 3, saturation pulse 6, actinic light 3). The desiccation was assumed complete when the mean of measured ΔF/Fm’ for the algae on all filters reached zero. The samples were then rehydrated by adding 50 μL of the KSM growth medium to each replicate on the filter and the desiccant in a chamber was replaced with 100 ml of tap water to achieve higher RH (96%) after which the measurements were resumed at the same intervals.

YII data June 2018 (described in the article as original data): was collected using a PAM 2500 chlorophyll fluorometer (Heinz Walz GmbH, Effeltrich, Germany)

YII data June 2020: was collected using a Junior PAM (Heinz Walz GmbH, Effeltrich, Germany)

2020FluorescenceR.Rproj: R script used for data visualization and cluster analysis

Tetradesmus_concat_alignment.nex: alignment of 3 genes used in phylogenetic analysis (rbcl, tufA, ITS2)