Photoacclimation of xanthophyll pigments in Isochrysis galbana
(Prymnesiophyceae)
Obata, Mitsuko and Taguchi, Satoru
Soka University, 1-236 Tangi-Cho, Hachioji, Tokyo 192-8577, Japan
Mail address: [email protected]
INTRODUCTION
Photoacclimation which is a physiological acclimation to change in PFD is an important factor
determining variability in the growth of algae, because natural light environments are inherently
variable from tropics to high latitude (Falkowski 1984, Palmisano et al. 1985). Photoacclimation is
generally considered to act according to rules that optimize photosynthesis within constraint by the
light condition. The optimization in photosynthesis involves a trade-off among maximizing
photosynthesis per unit of the photosynthetic apparatus, maximizing the photosynthesis per incident
photon under limiting PFD, and minimizing the damage that can be arise from excess PFD (Raven and
Geider 2003). Therefore, algae can adjust photosynthetic light-harvesting and photoprotective process
by varying pigment content and composition (MacIntre et al. 2002). Photosynthetic pigments are
increased for absorbing more light energy under lower PFD, while photoprotective pigments are
increased to protect the photosynthetic apparatus from photo-oxidative damage under higher PFD.
The DDcycle of chlorophyll c (Chl c) containing algae converts the epoxidated xanthophyll, DD into
the de-epoxidated xanthophyll, DT under high PFD, and DT into DD under low PFD or darkness
(Hager and Stransky 1970, Yamamoto 1985). Generally, DD can transfer excitation energy to Chl a and
play a role in the light energy acquisition as photosynthetic pigments, while DT can absorb excitation
energy from Chl a and play a role in the thermal dissipation as photoprotective pigments
(Demming-Adams and Adams 1992, Olaizola and Yamamoto 1994, Olaizola et al. 1994, Frank and
Cogdell 1996, Lavaud et al. 2004). A response of the DD and DT can be considered to study the
balance between light harvesting and photoprotective ability. However, a part of DD may be remained
nonconvertible (Lohr and Wilhelm 2001). This suggested that all variables related with DD-cycle
should be estimated based on the functional DD and DT. The reversible conversion of functional DD
into DT occurs on light and dark transition (Hager 1980, Yamamoto 1985). The functional DD and DT
can be detected under light and dark transition.
We define two parameters of DD-cycle which can be applied as the index of light harvesting and
photoprotective ability. The former is Energy Acquisition Efficiency (EAE) defined the slope of the
relationship between relative contents of functional DD and relative contents of functional DD+DT to
either cell density and Chl a concentration. The latter is Thermal Dissipation Efficiency (TDE) defined
the slope of the relationship between relative contents of functional DT and relative contents of
functional DD+DT to either cell density and Chl a concentration. Since the relationship between these
parameters may indicate the balance of light harvesting and photoprotective ability in the algae, those
responses to the acclimated PFD may be expected to be associated with light condition which
determines the relationship between light energy supply and algal energy demand.
Based on the relationship between growth rate of algae and the acclimated PFD (MacIntyre et al. 2002),
light conditions can be categorized for two types; light limited (LL) and light saturated (LS) condition.
Under LL condition, the growth rate depends on the acclimated PFD and algal energy demand is
overwhelmed the light energy supply. Under LS condition, the growth rate is saturated to their
maximum and the light energy supply is enough or excess for the light energy demand. We
hypothesized that the occurrence of maximum EAE and minimum TDE under LL conditions and the
decrease of EAE and increase of TDE with PFD under LS condition might be observed.
The photoresponse of DD and DT was examined in Isochrysis galbana (Prymnesiophytes) which was
previously acclimated to four different PFDs under dark and light transitions. We observed the
consistent repetition in the photoresponse of these pigments. The photoacclimation in the relationship
between EAE and TDE can be considered as the adjustment of balance between light-harvesting and
photoprotective ability to PFD for I. galbana.
MATERIALS AND METHODS
Prymnesiophyceae Isochrysis galbana Parke were obtained from the North East Pacific Culture
Collection at the University of British Columbia, Canada. Isochrysis galbana was grown in a
continuous culture in enriched f/2 seawater medium (Guillard and Ryther 1962) at 25 degrees and light
intensity of 45, 250, 425 and 1370 micro-mol photons m-2 s-1 provided by cool-white fluorescent
lamps with a 12h light and 12h dark cycle. The steady state of growth rates was established at 0.3 day-1
by controlling the dilution rate provided by a peristaric pump. The light intensity was determined by a
scalar quantum sensor. Subsamples were collected every 3 hours for 72 hours for the cell density and
the analysis of DD and DT concentrations (Suzuki et al. 1993, Head and Horne 1993). Energy
acquisition efficiency (EAE) and thermal dissipation efficiency (TDE) were estimated by the following
methods; a slope of regression analysis between cellular DD contents (DDcell) and (DD+DT)cell or
between Chl a specific DD (DDChl a) and Chl a specific DD+DT ([DD+DT]Chl a), and a slope of
regression analysis between cellular DT contents (DTcell) and (DD+DT)cell or between Chl a specific
DT (DTChl a) and (DD+DT)Chl a, respectively. Linear regression analysis was performed with Model II
(Laws and Archies 1981).
RESULTS
A highly significant relationship (p<0.001) between (DD+DT)cell and DDcell was observed during light
phase except for light phase at 1370 mol photons m-2 s-1 (Fig.1). The slope ranged from unity at the
lowest PFD to 0.3 at the highest PFD (Table 1). A significant relationship between (DD+DT)cell and
2
0.20
0.05
0.15
0.04
DTcell
DDcell
DTcell (p<0.05) was also observed during light phase (Fig.1). Their slopes increased with PFD from
almost zero at lowest PFD to 0.8 at the highest PFD (Table 1).
0.10
0.05
0.03
0.02
0.01
0
0
0
0.05
0.1
0.15
(DD+DT)cell
0.2
0
0.05
0.1
0.15
(DD+DT)cell
0.2
Fig. 1. Relationships between cellular DD+DT content and cellular DD or DT contents in light
phase at 45 (circles), 250 (squares), 425 (triangles), and 1370 (diamonds) mol photons m-2 s-1.
Broken lines indicate a least square fit to the data (Table 1).
Table 1. Statistical analysis of the relationships between DD+DT and DD or DT on the basis of cell
and Chl a during light phase. Asteriscs indicates p>0.05.
Cell specific
PFD
Slope
Intercept
Chl a specific
n
2
r
Slope
2
Intercept
n
r
- 0.004
12
0.99
DDChl a
DDcell
45
0.981
- 0.0006
12
0.99
1.01
250
0.882
0.0057
12
0.99
0.947
0.002
12
0.97
425
0.611
0.0223
12
0.94
0.191
0.152
12
0.26*
1370
0.293
0.0301
13
0.28*
0.231
0.374
13
0.37
DTChl a
DTcell
45
0.021
0.0002
12
0.34
0.004
0.002
12
0.02*
250
0.117
- 0.0055
12
0.90
0.069
- 0.004
12
0.14*
425
0.408
- 0.0241
12
0.88
1370
0.713
- 0.0314
13
0.71
0.822
0.800
- 0.155
- 0.408
12
13
0.88
0.86
Similar relationships were also obtained for Chl a specific values (Fig.2). The slopes for the
relationship between Chl a specific DD (DDChl a) and DD+DT ([DD+DT]Chl a) contents were about
unity during light phases at the two lowest PFDs (Table 1). The slopes for the relationship between Chl
a specific DT content (DTChl a) and (DD+DT)Chl a were 0.8 during the light phases at the two highest
PFDs.
3
1.0
0.8
0.8
DTChl a
DDChl a
1.0
0.6
0.4
0.2
0.6
0.4
0.2
0.0
0.0
0.5
0.0
1.5
1.0
(DD+DT)Chl a
2.0
0.0
0.5
2.0
1.5
1.0
(DD+DT)Chl a
Fig. 2. Relationships between Chl a specific DD+DT and Chl a specific DD or DT in light phase. See
Fig. 1 for symbols. Broken lines indicate a least square fit to the data (Table 1).
A slope of the relationship between DD and DD+DT (EAE) decreased with PFD while one between
DT and DD+DT (TDE) increased with PFD in either cases of cellular or Chl a consideration (Fig.3).
These relationships were almost linear based on cellular values while they were sigmoid based on Chl
a values. The former crossed at 746 mol photons m-2 s-1 while the latter crossed at 349 mol photons
m-2 s-1.
EAE and TDE
1.2
(B)
(A)
1
0.8
0.6
0.4
0.2
0
0
500
1000
PFD
1500
0
500
1000
1500
PFD
Fig. 3. Light dependence of EAE (open symbols) and TDE (closed symbols) on the basis of cell (A)
and Chl a (B). See Fig. 1 for symbols.
DISCUSSION
The linear functions of EAE and TDE with the acclimated PFD on the basis of cell density may
suggest that the balance between light harvesting and photoprotective ability at the level of the
DD-cycle pigments can be adjustable with the acclimated PFD. The sigmoid functions of the EAE and
TDE with the acclimated PFD on the basis of Chl a concentration may depend on the light condition
which determines the balance between light energy supply and demand.
4
The relationship between the growth rate of algae and the acclimated PFD varies widely between
species (MacIntyre et al. 2002). The growth rate of I. galbana became saturated at 200 ~ 300 mol
photons m-2 s-1 and any reduction of growth rate was not observed up to 600 mol photons m-2 s-1
(Jorkiel and York 1984, Falkowski et al. 1985). This may suggest that a range of LL condition may
correspond to PFDs below 200 mol photons m-2 s-1 and a range of LS condition may correspond to
PFDs beyond 200 mol photons m-2 s-1 for I. galbana in the present study.
Under LL condition when energy supply was insufficient for algal energy demand, the maximum EAE
and minimum TDE either cases of cellular or Chl a consideration were occurred. Algae might have to
enhance the light harvesting ability. In contrast, EAE decreases with the acclimated PFD and reaches to
the minimum while TDE increases with the acclimated PFD either cases of cellular or Chl a
consideration. Under this condition, algae do not require more light energy and may enhance the
performance of dissipation of excitation energy rather than absorption of photons. As growth becomes
light saturated, cells absorb a smaller fraction of the light and increase the ability to dissipate excess
absorbed energy (MacIntyre et al. 2002). The results obtained in the present study agree with the
consequence of the photoacclimation in the relative abundance of photosynthetic and photoprotecitve
pigments in the previous studies (e.g. Stolte et al. 2000).
The position of crossover in the responses of EAE and TDE to the acclimated PFD was different
between the basis of cell and Chl a. This results in the response of Chl a specific EAE and TDE to the
acclimated PFD may take account of photoacclimation of Chl a, which transfers excitation energy to
reaction center of photosystem (Scheer 2003). In addition, the position of crossover in the sigmoid
responses of EAE and TDE based on Chl a concentration to the acclimated PFD obtained in the
present study was located close to “hinge point” which was observed for the relationship between
growth rate and acclimated PFD in microalgae (MacIntyre et al. 2002). This suggests that the
photoacclimation strategy of algae may shift from light harvesting to photoprotection at this point. A
better understanding of the photoacclimation strategy in photosynthetic apparatus is required to be
taken into account of the relationship between light energy supply and demand of algae.
ACKNOWLEDGEMENTS
Technical assistance was provided by A. Mizobuchi and M. Maeda. This research was funded partly by
a grant-in-aid from the Ministry of Education and Science #16580161 to ST.
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