DIEL PATTERNS IN LIGHT ABSORPTION OF PHYTOPLANKTON:
PACKAGE EFFECTS AND PIGMENTATION
植物プランクトンの光吸収の日周性: パッケージ効果と色素組成
00D5501 大井 信明
指導教員 田口 哲
SYNOPSIS
プリムネシウム藻綱 Isochrysis galbana を 12 時間の明暗周期下で連続培養し、光吸収特性は弱光下で暗期
の終わりで最大に、明期の終わりで最小になる日周性を示し、強光下では 12 時間変位した周期性を示し
た。光吸収特性の日周性は、弱光下では成長により細胞直径が明期に増加することでパッケージ効果が増
加し、強光下では明期に光保護色素が増加し、光合成色素が減少し色素組成が日周変化することに起因し
て生じていた。北西太平洋域の自然光下でピコプランクトン群集の観察からも 12 時間変位した光吸収特
性の日周性が得られていた。本研究により光吸収特性の日周性が存在し、光放射量により日周性が 12 時
間変位することが明らかになり、ピコプランクトンが優占する外洋域においては光学特性を予測するため
には光吸収特性の日周性を考慮する必要があることが示された。
Keywords: absorption, diel patterns, irradiance levels, package effects, pigmentation, picoplankton
INTRODUCTION
Reynolds 1993, Stramski et al. 1995).
However, the
There is increasing interest in using optical measure-
effect of irradiance levels on the diel patterns in the a * (λ)
ments, such as ocean color and beam attenuation, to pro-
have not been considered as a controlling variable.
vide convenient estimates of oceanic biomass and pro-
Knowledge of the diel patterns in a * (λ) is significant on
ductivity on large scales. Algorithms to interpret opti-
optical properties of phytoplankton and could contribute
cal measurements in terms of biogeochemical processes
to interpretations of absorption properties and estimates
are based on the fact that the color of the ocean is largely
of primary production from the irradiance levels which
determined by the optical properties of the suspended
are encountered from the surface to deep layer of eu-
particles, particularly the phytoplakton and their prod-
photic zone.
ucts.
Understanding the variability in the optical prop-
The absorption coefficient is directly related to the
erties among the particles in the ocean can lead to a bet-
dimensionless efficiency factor for absorption (Qa ), i.e.
ter interpretation of these bulk measurements and thus
the ratio of the energy absorbed within a mean cell (rep-
better estimates of large-scale production. Diel patterns
resentative of the population) to the energy incident on
in optical properties of surface phytoplankton has been
its geometrical cross section. These efficiency factors
recently recognized as important factor or contributor for
can be reconstructed from the cell characteristics,
the estimation of daily primary production.
namely the intracellular concentrations of the various
Diel patterns in the attenuation, scattering and ab-
pigments and the mean cell size, using the anomalous
sorption coefficients have been observed in various parts
diffraction approximation, and then converted into a * (λ)
of the ocean, but diel patterns in the chlorophyll a spe-
coefficients. Based solely on the pigment composition
*
cific absorption coefficients, a (λ), has been less ob-
and the unpackaged absorption coefficients of individual
served. The coefficient of variation (CV) characteriz-
pigments, the reconstruction of a * proposed in this study,
ing the overall magnitude of diel patterns in a * (673) was
via the Qa factors, accounts for the package effects, if
at 8.7 % for the diatom Thalassiosira pseudonana and
any.
2 % for the cyanobacterium Synechococcus (clone
whether the observed absorption properties are fully
WH8103) over a natural day-night cycle (Stramski &
consistent with, and predictable from cell characteristics.
Such reconstruction allows us to determine
This reconstruction is also aimed at determining the re-
how the package effects contribute to the diel patterns in
spective contributions of the factors responsible for
a * (λ) by examining the intracellular concentration of
changes in the absorption properties of phytoplankton
pigments and cell size; (II) to describe the diel pattern in
cells at low and high irradiance levels .
light absorption properties of I. galbana under high-light
Bricaud et al. (1999) concluded that autotrophic pro-
condition (1500 µmol photons · m-2 · s-1 ; HL) to deter-
karyote Prochlorococcus marinus of picoplankton un-
mine how the pigmentation contribute to the diel pat-
*
dergo significant changes in their a (λ) in response to
terns in a* (λ); and (III) to describe the diel patterns in
light variations. In a large number of marine systems,
light absorption properties of picoplankton under the
autotrophic prokaryotes and cyanobacteria collectively
daily cycle of varying irradiance in the western subarctic
known as picoplankton are responsible for a majority of
Pacific to determine how intracellular chlorophyll a
the primary production.
concentration and cell size contribute to the diel patterns
Variations of picoplankton
occurring at the daily scale are more relevant than those
in a * (λ).
at monthly, seasonal, or annual scales in oligotrophic
oceans. In particular, chlorophyll fluorescence of pi-
I. DIEL PATTERNS IN LIGHT ABSORPTION AND
coplankton exhibits clear diel periodicity: under high
PACKAGE EFFECTS OF ISOCHRYSIS GALBANA
irradiance levels, minima occur at midday, and maxima
(PRYMNESIOPHYCEAE)
occur at night, while patterns are inverted under low
The package effects on chlorophyll a specific absorption
irradiance levels.
coefficients [a * (λ), m2 · mg chl a –1 ] were examined in
In the present study the prymnesiophyceae Isochrysis
chemostat culture of the prymnesiophyceae Isochrysis
galbana was chosen to conduct a continuous culture
galbana (Parke) under 12 h light : 12 h dark cycle at
experiment to study on diel patterns in the absorption
low-light (75 µmol photons · m-2 · s -1 ; LL) and me-
properties. This species has division patterns typical of
dium-light (500 µmol photons · m-2 · s -1 ; ML) conditions.
many phytoplankton, i.e., cell division occurs during the
Other associated measurements such as pigment comp o-
night. Similarly sized nanoplankton cells are known to
sition, cell density and diameter were also made at the
contribute significantly to the bulk optical properties of
two light regimes every 2 h for 2 days to confirm the
open ocean waters.
periodicity.
*
A distinct diel pattern was observed for the
*
The variability in a (λ) results from the combination
a (λ) with maxima near dawn and minima near dusk.
influences of the pigment package effects that vary with
The magnitude of diel variation in a * (440) was 15 % at
cell size and intracellular pigment content (e.g. Morel &
LL and 22 % at HL. Pronounced diel patterns were
Bricaud 1981) and the pigmentation that is controlled by
observed for cell size with minima near dawn and
photoprotective and photosynthetic pigments (e.g. Sosik
maxima near dusk. The magnitude of diel variation in
& Mitchell 1994). The contribution of package effects
cell size was 9.3 % at LL and 21 % at ML.
*
The ab-
or pigmentation to variation in a (λ) could be dependent
sorption efficiency factors [Qa (440)] were determined
upon irradiance levels.
by reconstruction using intracellular concentrations of
The aims of this paper are
therefore (I) to describe the diel patterns in light absorp-
pigments and cell size.
tion properties of unicellular I. galbana under low-light
distinct diel pattern, with minima near dawn and
-2
-1
The Qa (440) also showed a
(75 µmol photons · m · s ; LL) and medium-light (500
maxima near dusk. The diel patterns in a* (λ) and Qa
µmol photons · m-2 · s-1 ; ML) conditions to determine
(λ) were primarily caused by changes in cell size due to
growth, although there was some influence from diel
levels .
patterns in the intracellular pigment concentrations.
III. LIGHT ABSORPTION DYNAMICS OF PI-
The results presented here indicated that diel pattern in
COPLANKTON IN THE WESTERN SUBARCTIC
a * (λ) was an important component of the optical char-
PACIFIC
acterization of phytoplankton.
Diel patterns in the chlorophyll a specific absorption
coefficient of surface picoplankton [a * pico (λ), m2 · mg
II. DIEL PATTERNS IN LIGHT ABSORPTION AND
chl a –1 ] were studied under daily cycle of light in the
PIGMENTATION OF ISOCHRYSIS GALBANA
western subarctic Pacific. All data were normalized by
(PRYMNESIOPHYCEAE)
dividing the anomaly from daily averaged a * pico with one
Diel pattern in chlorophyll (chl) a specific absorption
*
2
–1
standard deviation. Significant diel patterns were ob-
coefficients [a (λ), m · mg chl a ] was examined to
served for the normalized a * pico with maximum during
study the effect of pigmentation on a * (λ) of the prymn e-
day and minimum during night in daily high irradiance
siophyceae Isochrysis galbana (Parke) in chemostat
levels (HL) and vice versa in daily low irradiance levels
culture under 12 h light : 12 h dark cycle at high-light
(LL) (p < 0.05). The absorption efficiency factors at
-2
-1
condition (1500 µmol photons m s ; HL) .
Photosyn-
red absorption peak, Qa (675), were determined by re-
thetic and photoprotective pigment as well as cell den-
construction with intracellular chlorophyll a concentra-
sity and diameter were also determined at two light re-
tion and cell size.
gimes every 3 h for 2 days to confirm the periodicity of
showed two significant diel patterns with maximum
*
a (λ).
*
A distinct diel pattern was observed for a (λ)
The normalized Qa (675) also
during day and minimum during night in HL and vice
The diel patterns in a *pico (675) and Qa
with maxima near dusk and minima near dawn. The
versa in LL.
magnitude of diel variation in a * (440) or a * (674) was
(675) were primarily caused by changes in intracellular
50 % and 24 %, respectively. To reconstruct the ab-
chlorophyll a concentration due to photoadaptation in
sorption coefficient, a rec (λ), the absorption efficiency
HL and LL, although there was some influence of pig-
factors [Qa (674)] were determined using intracellular
mentation in blue to red ratio [a * pico (443) / a * pico (675)]
chl a concentration, cell size, and chl a specific absorp-
in HL. This study proposed that the 12h shifted diel
tion coefficient after solubilization on Triton X [a * TX
patterns in a * (λ) might occur for a wide range of algal
(λ)]. Contribution of variation in the intracellular chl a
species due to the 12h shifted diel patterns in intracellu-
concentration and cell size to the variation of a * rec (λ)
lar chlorophyll a concentration depending on high and
*
was less than that in a
(λ). Therefore any changes
low irradiance levels . The results presented here have
in a * TX (λ) or a * (λ) are due only to changes in the pig-
important consequences for the interpretation of diel
mentation. Diel pattern in the absorption ratio of pho-
patterns in optical properties observed in the open ocean.
TX
toprotective pigments to chl a was observed more significant than that in the absorption ratio of other photo-
CONCLUSIONS
synthetic pigments to chl a. The diel patterns in a * (λ)
The 12h shifted diel patterns in a* (λ) were observed for
were primarily caused by changes in pigmentation of
the laboratory culture experiment with prymnesiophy-
photoprotective pigments due to high irradiance levels.
ceae Isochrysis galbana with maxima near dawn and
These results indicated that phytoplankton exhibited 12
minima near dusk in low-light conditions and vice versa
h shifted diel patterns in a * (λ) depending on irradiance
in high-light condition over a light-dark cycle. The diel
patterns in a * (λ) were caused by changes in cell size due
a specific absorption coefficients, a*rec (λ), can be esti-
to growth at low-light conditions and changes in pig-
mated by dividing the reconstructed absorption coeffi-
mentation
photoprotec-
cients, a rec (λ), by [Chl a]. The [Chl a] can be esti-
tive-pigments due to photoadaptation in high-light con-
mated by the cell density, N / V, the intracellular chl a
dition. Because the diel patterns in cell size and pig-
concentration, ci, Chl a , and the cell volume of spherical
of
photosynthetic-
and
mentation of I. galbana were similar to major constitu-
cell of diameter, d:
ent of phytoplankton communities, the results presented
[Chl a] = N / V ⋅ ci, Chl a ⋅ (p / 6) d 3
here seemed to be important for interpreting diel patterns
in a * (λ) from the surface layer to deep layers of euphotic
zone. Similar 12h shifted diel patterns in a * pico (λ) depended on irradiance levels were observed under daily
(1).
The a rec (λ) can be estimated by the N / V, the efficiency
factors for absorption, Qa (λ), and the geometric
cross-section of spherical cell of diameter, d:
cycle of light in the western subarctic Pacific. These
diel patterns are associated with significant diel patterns
a rec (λ) = N / V ⋅ Qa (λ) ⋅ (p / 4) d 2
(2).
in intracellular chlorophyll a concentration. Therefore,
varying particle concentrations will be a further factor
The Qa (λ) for a single cell, that is, the ratio of absorbed
responsible for diel patterns in the bulk optical properties
to impinged energy on the geometrical cross section of
of the surface water. In summa ry, the results presented
the cell, can be estimated as:
here will have significant consequences for the interpretation of diel patterns in optical properties observed in
the open ocean.
REFERENCE
Bricaud, A., Allali, K., Morel, A., Marie, D., Veldhuis, M. J.
W., Partensky, F. & Vaulot, D. 1999. Mar. Ecol. Prog. Ser.
188: 21-32.
Morel, A. & Bricaud, A. 1981. Deep-Sea Res. 28: 1375-93.
Sosik, H. M. & Mitchell B. G. 1991. Limnol. Oceanogr. 36:
Qa (λ) =
1 + 2 exp [-ρ’ (λ)] / ρ’ (λ) + 2 {exp [-ρ’ (λ)] – 1} / ρ’ (λ) 2 (3)
where ρ’ (λ) is the dimensionless optical thickness of the
particles. The ρ’ (λ) is defined as the product of three
quantities; the specific absorption coefficients, a * i, p (λ)
of pigment p, the intracellular concentrations of the
various pigments, ci, p , and the cell diameter, d, that is,
ρ’ (λ) = ∑ a * i, p (λ) ⋅ ci, p ⋅ d
(4)
910-21.
Stramski, D. & Reynolds, R. A. 1993. Limnol. Oceanogr. 38:
1347-64.
Stramski, D., Shalapyorok, A. & Reynolds, R. A. 1995. J.
where ci, p is the ratio of the concentration of this pigment within the suspension, Cp , to the product of cell
volume and cell density:
Geophys. Res. 100: 13295-307.
ci, p = Cp [(p / 6) d 3 ⋅ N / V]-1
(5).
APPENDIX
Reconstruction of absorption efficiency factors from cell
The a * i, p (λ) spectra were estimated by scaling the ab-
characteristics
sorption spectra of individual pigments measured (in
Cells are assumed as spherical and homogeneous with
relative values) by HPLC to the weight-specific absorp-
respect to the refractive index, and the anomalous dif-
tion coefficients, and the shifting the positions of
fraction approximation is made. The reconstructed chl
maxima in solvents to match the in vivo positions.