Habitat of origin is predictive of vegetative desiccation tolerance exhibited by the Tetradesmus species
Our current understanding of vegetative desiccation tolerance is primarily based on the research of desiccation tolerant land plants, also called resurrection plants, and their streptophyte algal relatives (Farrant et al. 2009; Holzinger et al. 2011), or studies of individual lichenized algae (Banchi et al. 2018). Physiological responses to desiccation of desiccation-tolerant green algae and mosses appears to have much in common (e.g., Proctor & Smirnoff 2000; Proctor et al. 2007; Koster et al. 2010; Karsten et al. 2016; Pierangelini et al. 2017, 2019), including rapid (within minutes after rehydration) recovery even from lengthy and severe desiccation events.
A study by Gray et al. (2007) examined responses to desiccation and rehydration in a wide range of aquatic and desert green algae. They showed dramatic differences in the behavior of species adapted to these habitats; and that only desert algae were capable of recovering their full photosynthetic capacity after being dry for a period of 1–30 days. Conditions under which desiccation and rehydration occur influence the response of the algae. For example, desiccation in the dark resulted in the recovery of more species (Gray et al. 2007). In Scenedesmaceae (a family including Tetradesmus ), algae isolated from desert soil crusts recover photosynthetic activity three minutes after rehydration and to a much higher degree than the aquatic species (Cardon et al. 2018). They also demonstrated variation in recovery among the desert algae. However, species included in this study belonged to multiple genera and only short-term rehydration (3 min) was recorded, whereas our experiment aimed to investigate closely related species and we measured ΦPSII for ~12 h after rehydration.
We first tested the hypothesis that the investigated desert species are capable of recovering from desiccation events compared to the aquatic species. All species studied here lost their photosynthetic capacity upon desiccation. Upon rehydration, desert algae showed an immediate recovery, with 50–80% of initial photosynthetic activity being recovered in 10–30 min, followed by a period of slower change in which the hydrated levels of photosynthesis were reached in 24–48 h. However, fast and severe desiccation at 5% RH was damaging even for these species (after short-term recovery of the photosynthetic activity immediately upon rehydration, photosynthetic capacity invariably again declined over time). Similar responses characterize desiccation-tolerant mosses (e.g., Proctor et al. 2007) and algae (Gray et al. 2007, Karsten et al. 2016, Cardon et al. 2018).
In our experiments the severity of desiccation is correlated with the time it takes to reach the final humidity level. The duration of water loss from hydrated samples ranged between 4–16 h, which may potentially be experienced by cells in fundamentally different ways. Slow desiccation potentially allows cells to detect the signal and activate desiccation-induced protective pathways, whereas to survive rapid desiccation they must have these protective compounds accumulated prior to desiccation. The difference between these processes is demonstrated by comparisons of desiccation-tolerant angiosperms and bryophytes. Angiosperms can only survive slow desiccation and demonstrate activation of protective pathways during desiccation (Farrant 2000, Alpert 2006, Farrant et al. 2009). Bryophytes, on the other hand, can recover after rapid desiccation events due to constitutive production of osmolytes and other protective compounds during the hydrated state (Oliver et al. 2011). Interestingly, accumulation of protective compounds may depend on the growth conditions and on the age of plants under the stress. For example, a differential expression study showed that older cells of streptophyte algae were less stressed by desiccation compared to younger ones (Rippin et al. 2017).
Our measurements of ΦPSII suggest that the rate of decrease in photosynthesis is similar in all three desiccation modes, but that the different desiccation modes varied in the timing of the onset of the decrease in photosynthesis. In other words, the treatments differed in how long ΦPSII remains stable at the beginning of the experiment following the initiation of the desiccation treatment. We suggest that the cells were initially surrounded by a liquid layer of culture medium, which is attached to cells by surface tension forces. Cells start losing water only after this water film is absorbed by the desiccant.
The modes of desiccation (final humidity level and duration to reach it) have an impact on the desiccation tolerance phenotype of desertTetradesmus (Fig 4). The strongest desiccation (5% RH, 4-5 h) inflicted damage even to terrestrial algae. Despite a short partial recovery of ΦPSII after 10 min of rehydration in terrestrial algae was observed, in all of the studied desert strains the ΦPSII subsequently dropped to zero. A short-term initial recovery of ΦPSII with a subsequent decline of the signal was also reported for other green algae exposed to desiccation (e.g., Pierangelini et al. 2019). A lack of capacity of desert algae to fully recover from this harsh treatment is likely related to the difference between our experiment design and what the cells experience in nature. Desert soil crusts were shown to remain humid for longer than the surrounding bare sand (Belnap & Lange 2001), which prolongs the hydration time and slows down desiccation. Despite of how extreme our treatment was, clear differences in response to it between the aquatic and desert species is apparent and indicates underlying physiological differences. Gentler modes of desiccation (RH 65% and 80% reached in 8 or 12 h, respectively) resulted in complete or almost complete recovery in all desert species. However, only one species (T. bajacalifornicus ) recovered its maximum photosynthetic yield within 10 min of rehydration under both of these conditions. Among desert species, variation in recovery was higher for the strains of T. deserticola and T. adustus . Some fully recovered within 10 min when desiccated at RH 80%. Others required more time. Algae desiccation at RH 65% resulted in recovery of 45–75% of the maximum in the first 10 min of rehydration, and did not reach the maximum even 12 h later. The desiccation/rehydration profiles demonstrate variation in desiccation phenotype in this genus. The re-decline of photosynthetic activity after short-term recovery upon rehydration in the rapid and severe desiccation experiment shows the importance of extended data collection to completely understand the response of a species to desiccation and rehydration. Variation in the desiccation phenotypes among the desert Tetradesmus, belonging to separate clades within the genus, may indicate differences in the mechanisms of vegetative desiccation tolerance in these species and opens the possibility for future development of the genus as a model group for investigation of evolution of this complex trait.
To investigate variation among the strains of Tetradesmus species we included multiple strains (2–4) for the species, where possible. In most of the cases the within-species variation was minimal and different strains of a species clustered together (Fig. 4). The only exception was UTEX 393, a strain of the aquatic species T. obliquus , which recovered 75% photosynthetic activity after 12 h of rehydration following the mildest desiccation of our treatments (80% RH, 12 h). This result is similar to the findings of Cardon et al. (2018) during desiccation of the same strain. Consistent recovery of UTEX 393 from mild desiccation suggests the presence of protective pathways even in the generally desiccation-sensitive strains. The genes responsible for these pathways are likely shared by all species of Tetradesmusand may prove key to discovery of the mechanisms of desiccation-tolerance in the desert species.
TEM does not reveal changes in cell ultrastructure during desiccation and rehydration .
Our observations of cell ultrastructure in two selectedTetradesmus species in different hydration states are drastically different from what can be found in the literature (e.g., Domozych et al. 2003; Proctor et al. 2007). We did not detect cell plasmolysis (i.e., decrease in cell volume and retraction of the protoplast from the cell wall caused by a loss of water). The cell wall retained the same thickness and none of its layers expanded. An undulation of the cell wall reported for Klebsormidiuim crenulatum (Holzinger et al. 2011) was not observed; slightly wrinkled cell walls were seen across all treatments and in both species, which may indicate an artifact of sample preparation. No specific products were seen accumulating in studied cells, and the plastoglobuli detected in the chloroplast do not show distinct patterns when cells of each species are compared across different treatments and with the control. Despite the apparent intact nature of cells of T. obliquus, they do not recover ΦPSII after desiccation and therefore must be damaged in a way that is not manifested in the ultrastructural morphology of cells.