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.