CZECH POLAR REPORTS 6 (1): 87-95, 2016
———
Received October 2, 2015, accepted November 8, 2015.
*
Corresponding author: Miloš Barták <mbartak@sci.muni.cz>
#
Recent address: Department of Botany, Faculty of Science, Charles University, Benátská 2,
128 01 Praha 2, Czech Republic.
Acknowledgements: The authors thank the project CzechPolar (the Czech Ministry of Education,
Sports and Youth) for providing field facilities and infrastructure for the research reported in this
study.
87
Dehydration-induced responses of primary photosynthetic
processes and spectral reflectance indices in Antarctic Nostoc
commune
Miloš Barták*
, Jana Hazdrová, Kateřina Skácelová#
, Josef Hájek
Department of Experimental Biology, Laboratory of Photosynthetic Processes, Faculty
of Science, Masaryk University, University Campus – Bohunice, Kamenice 5, 625 00
Brno, Czech Republic
Abstract
In this study, we investigated the relationship between relative water content (RWC) of
N. commune colonies recorded during gradual dehydration and (i) normalized difference
vegetation index (NDVI), (ii) photochemical reflectance index (PRI), and (iii) primary
photochemical processes of photosynthesis, effective quantum yield of photosynthetic
processes (PSII) in photosystem II particular. PRI increased from -0.05 to 0.02 with
RWC decrease from 100% (full hydration) to 0% (dry state). NDVI showed somewhat
curvilinear relationship with desiccation with minimum value of 0.25 found at 10%
RWC. Negative effect of suprasaturation of N. commune colony with water on effective
quantum yield (PSII) was found at RWC range 80-100%. At the RWC range, PSII
reached only 50 % of maximum found at RWC of 30%. In general, desiccation-response
curve of showed polyphasic character with three main phases (phase I – constant PSII
values, phase II – an increase with desiccation at RWC 80-30%, and phase III –
sigmoidal decrease with desiccation at RWC 0-30%). Non-photochemical quenching
(qN) of absorbed light energy showed triphasic dependence on RWC as well. qN showed
constant values in the phase I, an increase (phase II), and constant values at severe
dehydration (phase III).
Key words: PRI, NDVI, Antarctic, James Ross Island, saturation effect, colony
DOI: 10.5817/CPR2016-1-9
M. BARTÁK et al.
88
Introduction
Nostoc commune is frequently found in
terrestrial and semi-aquatic Antarctic habitats.
It is reported from maritime Antarctica
(Victoria et al. 2013) as well as from
continental Antarctica (e.g. Novis et Smissen
2006). Generally, the ability of N. commune
to fix atmospheric nitrogen and, to
resist desiccation may explain its dominance
in many terrestrial habitats in Antarctica.
In Antarctic ecosystems, such as
margins of freshwater terrestrial lakes,
temporal ponds, streams and seepages, rich
in melt water during austral summer Nostoc
commune typically forms colonies. Among
Nostoc species forming sheetlike colonies,
N. commune, and closely related N. flagelliforme
are reported (Sand-Jensen 2014).
At James Ross Island, Antarctica, N. commune
colonies are quite abundant in shallow
streams and seepages (Skácelová et al.
2013, Komárek et al. 2015).
N. commune colonies form inactive
crusts when dry, but rapidly changes to
gelatinous structures when wet (e.g. Novis
et al. 2007). Major part of colonies are
composed of the extracellular matrix – up
to 60% of dry mass is reported (Hill et al.
1997). Matrix forms upper and lower surface
of the colony as well as an intracolonial
matter filling the space in between
individual filaments of N. commune cells.
The matrix is formed primarily of several
polysaccharides (Rossi et Philippis 2015)
that have a high viscosity and molecular
weight (Sand-Jensen 2014). In general,
N. commune matrix contains also monosaccharides,
uronic acid, deoxy-sugars, pyruvate,
acetate and peptides (Pereira et al.
2009). Physico-chemical properties of extracellular
matrix may vary according to
number and arrangement of exopolysaccharides.
However, quantitative data on
polysacharides in extracellular matrix are
still insuffient for functional analysis of
N. commune during wet-dry cycles. However,
1-4-linked xylogalactoglucan backbone
with D-ribofuranose and 3-0-[(R)-
1-carboxyethyl]-D-glucuronic acidpendant
groups are reported from desiccation tolerant
N. commune (Helm et al. 2000).
N. commune is typical by a high tolerance
to a great variety of environmental
stressors including desiccation (Satoh et al.
2002, Tamaru et al. 2005). The species is
considered extremely tolerant to long-term
desiccation, some biomolecules may stay
active even when rewetted after hundred
years in dry state (Shirkey et al. 2000).
During desiccation, several photoprotective
mechanisms are activated in Nostoc.
Among them, zeaxanthin, but also nostoxanthin
and caloxanthin, and ketocarotenoids
echinenone were detected in Nostoc
sp. (Schagerl et Müller 2006) as well as
carotenoids (Potts et al. 1987) and beta
carotene that represents a strong photoprotective
quenchers.
Photosynthetic processes in N. commune
collonies from polar regions have been
investigated by a variety of chlorophyll
fluorescence techniques in response to
light and temperature (Kosugi et al. 2010),
UV-B radiation (Estêvāo 2015).
In this paper, we focused on changes in
primary photosynthetic processes monitored
by chlorophyll fluorescence parameters
during desiccation in N. commune
colony. We hypothesized that, similarly to
cyanolichens (Lange et al. 1996), water
suprasaturation effect resulting in inhibition
of photosynthetic processes could be
detected at high thallus hydration. We,
therefore, also hypothesized that optimum
hydration of N. commune colony would be
less than full hydration, i.e. under 100% of
relative water content. Simultaneously, we
focused on changes in spectral reflectance
indices NDVI and PRI. Similarly to the
study of Yamano et al. (2006) who studied
soil crusts. The letter one is associated with
activation of xanthophyll-cycle pigments
and studied in higher plants frequently. In
lichens and cryptogamic polar autotrophs
forming microbiological crusts, number of
FUNCTIONAL CHANGES IN DESSICATING NOSTOC
89
studies is limited. Recently, an interest in
spectral reflectance properties of polar soil
crust has been increased (see e.g. Walker
et al. 2012), as well as ecophysiological
characteristics, such as e.g. water holding
capacity. The same trend is apparent also
in the research of temperate zone ecosystems
(Gypser et al. 2016). However, knowledge
on physiological background of PRI
changes in response to hydration status is
still insufficient polar autotrophs.
Material and Methods
The colonies of Nostoc commune were
collected in James Ross Island, Antarctica,
from the long-term research plot located
close to Czech Antarctic station J. G. Mendel
(63° 48′ 02″ S, 57° 52′ 57″ E). Sampling
plot is typical by patchy moss cover
(more than 50% of total plot area) with
several lichen species (Barták et al.
2015b). The plot is wet at least for some
part of austral summer season. It is supplied
by melt water form two temporal
streams originating in neighbouring annual
snow field. Austral summer microclimate
is typical by air temperature ranging from
-10°C (monthlymean of September) to 6°C
(monthly mean of February) – measured at
30 cm above surface (Láska et al. 2011).
Austral summer surface temperature (absolute
daily minimum, maximum) ranges
from -7.1to12.0°C (Barták et Váczi 2014),
and close-to-ground relative air humidity
is found within the range of 80-100%
(Láska et al. 2011). Dehydration response
curves of PRI and NDVI were measured at
J. G. Mendel station laboratory, while chlorophyll
fluorescence parameterswere measured
after transfer to Brno, Czech Re-
public.
Dehydration and spectral indices measurements
Measurements of spectral reflectance
indices and photosynthetic measurements
were done in samples desiccating from
fully hydrated to fully dry state in a laboratory
(Mendel station). During the desiccation,
room temperature was kept constant
(18°C, 40% RH) and lichen thalli
were left in open Petri dishes to desiccate
naturally desiccation. Relative water content
(RWC) was evaluated before each
single individual PRI, NDVI measurements
usinga gravimetric method. Samples
were weighted on an analytical RADWAG
scale (XA 60/220/X), and RWC calculated
according to the equation:
RWC (%) = [(FM - DM)/(FW - DM)] * 100 Eqn. 1
where, FM, is the actual fresh mass
(weight) of a sample, DM is the mass of
fully dry sample, and FW, is a mass of
fully wet sample. From fully wet (RWC=
100%) to dry (RWC=0–10%) state of lichen
thalli, NDVI and PRI were measured
at specific hydration status. Normalized
difference vegetation index (NDVI) was
measured by PlantPen NDVI 300 (Photon
System Instruments, Czech Republic). Photochemical
reflectance index (PRI) was
measured by a PlantPen PRI 200 (Photon
System Instruments, Czech Republic). Both
instruments use the below-specified spectral
reflectance for calculation of the indices
using the below equations 2, and 3.
The NDVI, PRI data were then plotted
against particular RWCs of individual
species that was measured simultaneously.
M. BARTÁK et al.
90
NDVI = (R740 – R660)/(R740 + R660) Eqn. 2
PRI = (R531 – R570)/(R531+ R570) Eqn. 3
Chlorophyll fluorescence
Repetitive measurements of effective
quantum yield (PSII) of photosystem II in
Nostoc sp. started from fully wet state and
ended in dry state. Samples were allowed
to desiccate at room temperature (t=22°C,
EEL laboratory, Brno, Czech Republic)
and low irradiance (20 mol m-2
s-1
). During
gradual slow-rate desiccation (according
to sample size 7-14 h from fully wet to
dry state), actual fresh mass (weight) FM
was evaluated (Mettler AS100, Germany)
as well as several chlorophyll fluorescence
parameters using a PAM-2000 fluorometer
(H.Walz, Germany): variable fluorescence
at steady state (FS). The measurements
were taken simultaneously so that the response
of particular chlorophyll fluorescence
parameters (see below) to RWC
(Eqn. 1) could be evaluated. Light-adapted
chlorophyll fluorescence comprised signals
(FS, FM´) and parameters (PSII, qN). They
were measured each 20 min. until dry state
was reached.
Results
In N. commune, NDVI showed a curvilinear
relationship to RWC with desiccation
(see Fig. 1). Maximum values were
found in the range of 40–70% RWC. Minimum
NDVI of 0.12 (individual measurement)
was found at low RWC (0%). PRI
values exhibited an increase with desiccation
(within a range from -0.02 to 0.01)
with maximum found at RWC of 10%.
Fig. 1. Courses of PRI (left) and NDVI (right) in Nostoc sp. colony as dependent on relative water
contents (RWC, %) in a colony.
Dehydration-response curve of PSII
had polyphasic character with 3 main
phases (Fig. 2). At RWC range 80–100%
RWC (phase I), PSII was more or less
constant (0.15). With gradual dehydration
(phase II) from RWC of 80 to 30%, in-
FUNCTIONAL CHANGES IN DESSICATING NOSTOC
91
creased and showed a maximum of 0.27 at
RWC of 30%. Then, with further dehydration
(phase III, RWC 0–30%), N. commune
colony showed a S-shaped decline
in PSII with 0 found at RWC of about 5%
RWC. Phase I with limited PSII could be
related to reduction of the diffusive flux of
dissolved inorganic carbon to the colony
thanks to boundary layer thickness (SandJensen
2014).
Fig. 2. Effect of gradual dehydration of Nostoc sp. colony (expressed as relative water content RWC
decrease from 100-0%) on effective quantum yield of PS II.
Variable fluorescence at steady state
(FS) declined with desiccation in a polyphasic
manner (see Fig. 3). It showed,
however, constantly high values (about
0.730 mV) at RWC range of 79–100%,
and, at final phase of dehydration (RWC
0–17%), constantlylow values (0.160 mV).
Non-photochemical quenching (qN) showed
constant value at the RWC range (79–
100%), followed by an increase towards
maximum (RWC 17–79% corresponding
to phase II in PSII). Then, maxium qN of
about 0.94 was maintained during following
desiccation to fully desiccated state
(from 17 to 0% RWC). Non-photochemical
quenching (qN) of absorbed light energy
showed constant value of about 0.7 at
full hydration, then increased with following
dehydration (RWC range of 79–
100%), and reached its maximum at RWC
of 17% indicating that capacity of protective
mechanisms in photosynthetic apparatus
was fully exploited at the RWC.
M. BARTÁK et al.
92
Fig. 3. Dehydration - response curves of steady-state chlorophyll fluorescence (FS, left), and nonphotochemical
quenching (qN, right). Decrease in relative water content (RWC ) from full
hydration (RWC=100%) to desiccated state (RWC=0%) leads to a polyphasic decrease in FS, and
an increase in qN.
Discussion
A polyphasic curve of PSII to RWC relationship
reflects several limitations of
photosynthetic processes in N. commune
colony. Initial phase of desiccation (WP
range 100 to 80%), at which PSII values
are constant but far below their maximum,
is attributed to photosynthesis exploiting
carbon available inside Nostoc cells thanks
to carbon concentrating mechanisms exclusively.
At this RWC range no atmospheric
carbon contribute to photosynthetic
processes thank to a physical barrier of
water-saturated colony for CO2 diffusion.
Such phenomenon is called suprasaturation
(i.e. the photosynthetizing poikilohydric
organism contains more water than necessary
to saturate photosynthesis) and well
described for lichens (see e.g. Lange et al.
1996, 2001). In the following phase (RWC
from 80 to 30%), the atmospheric CO2 diffusion
into the colony is enhanced and an
increased PSII can be explained as a coaction
of increased CO2 intake and fixation
by the colony (when considering an equilibrium
between photochemical and biochemical
processes). The fall of PSII in
the last RWC range (WP decreasing from
30to0%) is associated dehydration-decline
in photosynthesis in Nostoc cells.
Our results showing a steep PSII decline
at RWC below 20% suggest that
even a partially hydrated Nostoc commune
colony is capable to maintain high rates of
photosynthetic processes in desiccating
colonies. This conclusion might be supported
by the data presented by Kvíderová
et al. (2011), who reported unchanged
FV/FM and PSII for at least 6 h from the
start of desiccation. Similar rate is reported
for rehydration, Gupta et Kashyap (1995)
showed that water uptake by desiccated
thalli of N. commune occurring in Schirmacher
Oasis (Antarctica) took about
5 hours for the thalli to absorb water to a
saturating level. In the field, however, desiccation
rate and gradual loss of photosynthetic
activity is even slower than reported
by Kvíderová et al. (2011) because
of contact of the colonies with wet surfaces
such as e.g. soil, mosses. Such contact
allows to slow down desiccation and
prolong the time of photosynthetic activity.
In the field, size of N. commune colony
may play a role since a faster desiccation
FUNCTIONAL CHANGES IN DESSICATING NOSTOC
93
in small-area colonies is reported (Gao et
Ai 2004). It has been demonstrated earlier
that Nostoc colonies decrease their mean
diameter from 150–250 µm to 50–100 µm
during desiccation from wet to dry state
(Belnap et Gardner 1993). Apart of colony
size, a 3-D complexityof colony may affect
the rate of desiccation as well. Typically,
surface to volume ratio and morphological
structures (Esseen et al. 2015) may affect
water holding capacity and the rate of des-
iccation.
Decrease of FS and an increase in qN is
a general phenomenon found in desiccating
poikilohydric autotrophs. Such phenomenon
is a mechanism providing effective
protection of photosynthetic apparatus.
For terrestrial cyanobacteria, numerous
pigments, carotenoids in particular (Vincent
et al. 1993, Lakatos et al. 2001), and
other quenchers, such as e.g. inhibition of
state 2to state 1 transition (Trnková et Barták
2016, accepted), are involved into the
protection during desiccation, which, together
with fast reactivation of photosynthetic
pigments (Abed et al. 2014) and lipids
resynthesis after wetting(Taranto et. al.
1993), represent a strategy to cope with
repeated drought stress. Specifically for
N. commune, these mechanisms make the
species extremely desiccation and photoinhibition
tolerant (Fukuda et al. 2008).
Optimum RWC at which maximum
NDVI was reached ranged 40-60%. The
RWC range is well comparable to the values
reported in our earlier studies (similarly
to Barták et al. (2015a), Trnková et
Barták 2016, accepted). NDVI index might
be affected by phycocynine since spectral
reflectances between 610 and 650 nm are
included into PC index (see e.g. Kutser et
al. 2006). PRI exhibited negative values at
RWC range 40-100%. It means that wellhydrated
Nostoc sp. colony has lower reflection
at 570 than 531 nm. Possible explanation
is an interaction with absorption
spectra of phycobilisomes (phycoerythrin,
phycocyanin) that overlap at the wavelength
about 570 nm (Singh et al. 2015).
Similarly, recent study of Li et al. (2015)
indicated a decrease in reflectance 570 nm
in cyanobacterial community and attributed
the decrease to absorption by phyco-
erythrin.
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