The Effect of Photoacclimation on Photosynthetic Energy Storage Efficiency, Determined by Photoacoustics

doi:10.4236/ojms.2011.12005

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Open Journal of Marine Science, 2011, 1, 43-49

doi:10.4236/ojms.2011.12005 Published Online July 2011 (http://www.SciRP.org/journal/ojms)

The Effect of Photoacclimation on Photosynthetic Energy

Storage Efficiency, Determined by Photoacoustics

Yulia Pinchasov-Grinblat*, Razy Hoffman, Zvy Dubinsky

Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel

E-mail: [email protected]

Received May 11, 2011; revised May 23, 2011; accepted June 8, 2011

Abstract

Photosynthesis rates in phytoplankton depend on light intensity and its spectral composition, however their

relation changes with photoacclimation. During the photoacclimation process algal cells optimize their har-

vesting and utilization of available light through series of related physical, biophysical, biochemical and

physiological changes. These changes result in the ability of phytoplankton to survive under dim light when

transported to the depth of the water column and avoid photodynamic damage when exposed to the intense

radiation at the surface. Any reduction in the efficiency of light utilization results in decreased rates of pho-

tosynthesis rate and slow growth. We present here the study of changes in photosynthetic energy storage ef-

ficiency of three phytoplankton species upon photoacclimation to low and high light, as measured by photo-

acoustics. Our results illustrate the power of photoacoustics as a tool in aquatic ecology and in the physio-

logical research of phytoplankton.

Keywords: Phytoplankton, Photoacclimation, Photosynthesis, Photoacoustics

1. Introduction

Phytoplankton, like all photosynthetic organisms, depend

on the capture of light and the transformation of its en-

ergy into stored photosynthate, which subsequently fuels

all ecosystem activities. For the first step of the photo-

synthetic process, light has to be intercepted by a variety

of light absorbing substances, the photosynthetic pig-

ments. These pigments are associated with proteins,

forming light harvesting arrays, or “antennae”, which

collect excitation energy and transfer it to the reaction

centers of the two photosystems, PSI and PSII that to-

gether constitute the photosynthetic unit. These photo-

synthetic units have a given probability or cross-section

for absorbing impinging light photons.

Because of the importance of photoacclimation for

phytoplankton which is frequently transported over two

orders of magnitude changes in light intensity within

hours, this process received considerable attention in the

study of aquatic primary production [1-3]. In all the nu-

merous studies of the mechanisms of the photoacclima-

tion process in phytoplankton, a common trend of in-

crease in chlorophyll a and in other light harvesting pig-

ments as growth irradiance decreases was observed.

Photoacclimation also affects pigment ratios of phyto-

plankton, as light harvesting pigments such as the

chlorophylls, phycobilins, fucoxanthin and peridinin in-

crease under low light, whereas photoprotective carote-

noids like β carothene, astaxanthin and xanthophyll cycle

pigments show the opposite relation with ambient light,

increasing whenever exposed to potentially harmfully

high intense light [4-7]. Most photosynthetic pigments

other than chloropyll a are an integral part of the light

harvesting antennae. These photosynthetic pigments re-

spond to changes in light intensity in a similar way to

that of chlorophyll a [8]. Concomitantly, as cellular pig-

mentation increases in the course of photoacclimation to

low light, the cross section of phytoplankton invariably

decreases [8,9]. This change in cross section resulting

from the increase in mutual shading between cellular

light harvesting entities, as their density increases. It

happens on all scales, among individual pigment mole-

cules, thylakoids, chloroplasts within cells, and among

cells in culture [10]. Upon exposure to high light inten-

sity, the above described trends are reversed [11].

The photoacoustic method, allows the direct determi-

nation of the energy storage efficiency of photosynthesis

by relating the energy stored by photosynthesis to the

total light energy absorbed by the plant material [12-14].

Depending on the efficiency of the photosynthetic sys-

44 Y. PINCHASOV-GRINBLAT ET AL.

tem, a variable fraction of the absorbed light energy is

stored, thereby affecting the heat evolved and the result-

ing photoacoustic signal.

By exposing the cells to a saturating continuous back-

ground light, no storage of any of the pulse energy can

take place, whereas in the absence of such light, a maxi-

mal fraction of the pulse energy is stored by photosyn-

thesis. Thus the maximal efficiency max , is determined

as the complement of the ratios of the photoacoustic sig-

nal, generated by a weak pulse of light in the dark

(PAdark), to that obtained under strong continuous light

(PAlight).



max 1 PAdarkPAlight  (1)

The aim of this work was to examine the photoacous-

tic method developed by us [15-17] in physiological

phytoplankton research. We demonstrate this novel ap-

plication, by investigating changes in photosynthetic

energy storage efficiency occurring in a few phyto-

plankton species of different taxa, resulting from photo-

acclimation.

2. Methods and Materials

2.1. Algal Culture

The three species of marine phytoplankton studied were

a diatom, Phaeodactylum tricornutum, green alga Nan-

nochloropsis sp. and golden-brown flagellate Isochrysis

galbana. All cultures were grown in 250 mL Erlenmey-

ers flasks containing 200mL enriched artificial seawater

medium (Guillard’s F/2) [18] at 24˚ ± 0.5 C, under white

fluorescent lights at ~10 μmol q m-2 s-1 (low light (LL))

and ~500 μmol q m-2 s-1 (high light (HL)).

Cell concentration was measured with a hemacytome-

ter. Chlorophyll content was measured spectrophotomet-

rically in 90% acetone extracts using the equations of

Jeffrey and Humphrey [19].

2.2. Photoacoustics

The experimental system is shown schematically in Fig-

ure 1. The sample was placed in a 16 mm square glass

cell (PAC). The laser (L) pulse, after passing through a

pair of 1mm wide slits (S) is incident upon the suspen-

sion of algae whose pigments absorb part of the laser

light. Depending on the experimental conditions, a frac-

tion of the absorbed light pulse is stored in the products

of photosynthesis. The remainder of the absorbed light is

converted to heat producing an acoustic wave. This is

intercepted by a submersible detector (D), containing the

ceramic disc. A small portion of the laser pulse is de-

flected by a beam splitter (BS) and used to trigger the

Tektronix TDS 430A oscilloscope, where the amplified

(Amptek A-250 Preamp and Stanford Research A 560

Amp) photoacoustic signal is recorded. The signal con-

tains a noisy background and later reflections from the

walls of the vessel as well as from impedance mismatch

within the detector (for details see [17]).

By increasing the continuous background light inten-

sity from zero to saturation of photosynthesis, an in-

creasing fraction of the reaction centers is closed at any

time, and a decreasing fraction of the probe laser pulse

energy is stored. A corresponding increase in the fraction

of the pulse energy is converted to heat, which is sensed

by the photoacoustic detector. From these detector re-

sponses the photosynthetic energy-storage versus back-

ground light-intensity relationship was obtained [15-17].

3. Results and Discussion

In these experiments the photoacclimation of three algal

species Phaeodactylum tricornutum, Nannochloropsis sp

and Isochrysis galbana to low and high photon irradian-

ces was examined. In general, photoacclimation to low

light results in increased cellular absorption due to high

concentration of light-harvesting pigments. In the nu-

merous studies on the mechanism of photoacclimation in

phytoplankton, a common trend was reported of increase

in chlorophyll as growth irradiance decreases (Falkowski,

1980; 1984; Dubinsky et al. 1986; 1995, Ritz, et al. 2000).

As seen in Figure 2 and Figure 3 all three species showed

a difference in cellular chlorophyll content by ~56% in

Isochrysis galbana, 38% and 35% in Phaeodactilum tri-

cornutum and Nannochloropsis sp, respectively.

The ratios of cellular chlorophyll under the low and

high light were 1.415 for Phaeodactilum tricornutum,

1.398 for Nannochloropsis sp. and 1.984 for Isochrysis

Figure 1. Schematic of the photoacoustic setup: L: laser

(Minilite Q Switched Nd :YAG, 532 nm); S: beam shaping

slits, BS: beam splitter; PAC: photoacoustic cell with sus-

pension of algae; D: stainless-steel photoacoustic detector,

contained a 10-mm diameter resonating ceramic disc (BM

500, Sensor, Ontario, Canada); P: low-noise preamplifier

(Amptek A-250); A: low noise amplifier (SRS 560); PD:

photodiode; TR: trigger signal, B: background light source,

quartz-halogen illuminator (Cole Parmer 4971); O: oscillo-

scope (Tektronix TDS 430A); C: computer.

Y. PINCHASOV-GRINBLAT ET AL.

galbana. In addition to the changes in cellular chloro-

phyll most other plant pigments also respond to ambient

irradiance [4,23]. All light-harvesting pigments increase

under low light. These include the carotenoids fucoxan-

tin and peridinin, in addition to all chlorolhylls, phyco-

erythrin and phycocyanin [4].

The decrease of chlorophyll concentration in HL

growth conditions resulted in a parallel reduce in photo-

synthetic energy storage efficiency Figure 4.

We determined the photosynthetic energy storage un-

der different ambient irradiance levels, resulting in an

energy storage curve (Figure 5). This relationship is

similar to the photosynthesis versus irradiance, (P vs I)

curve obtained by the tedious standard measurements of

14C fixation and oxygen evolution [24-26] or the indirect

results from measurement of variable fluorescence [27].

These results are similar to the data cited in the litera-

ture, where cellular chlorophyll content increased under

low light (LL) conditions and reduced under high light

(HL) (Table 1)

Figure 2. The effect of photoacclimation to high light (500 mol·q·m–2·s–1) and low light (10 mol·q·m–2·s–1) on cellular chlo-

rophyll content for three algae.

46 Y. PINCHASOV-GRINBLAT ET AL.

Figure 3. The effect of photoacclimation to high light and low light on chlorophyll concentration measured by photoacoustics.

Table 1. The LL/HL chlorophyll ratios for different phytoplankton species cited in literature.

Algae Reference

Phaeodactilum tricornutum 1.415 Present study

Nannochloropsis sp. 1.398 Present study

Isochrysis galbana 1.984 Present study

Phaeodactilum tricornutum 1.592 Present study by photoacoustics

Nannochloropsis sp. 1.020 Present study by photoacoustics

Isochrysis galbana 2.501 Present study by photoacoustics

Thalassiosira weisflogii 2.486 Dubinsky et al. 1986

Isochrysis galbana 2.130 Dubinsky et al. 1986

Prorocentrum micans 2.351 Dubinsky et al. 1986

Symbiodinium microadriaticum 1.921 Iglesias Prieto and Trench, 1994

Isochrysis galbana 2.076 Herzig and Dubinsky, 1992

Y. PINCHASOV-GRINBLAT ET AL.

Figure 4. The effect of photoacclimation to h igh light and low light on p hotos ynthetic en ergy storage efficiency for th ree alga e.

Figure 5 shows that the photosynthetic energy storage

efficiency Isochrysis galbana at the initial time of ex-

periments resembles the usual P vs I curves, where pho-

tosynthesis increases with light intensity up to the onset

of light saturation. Thus, neither the initial slope, α, nor

the light saturation parameter Ek, change. These photo-

synthetic parameters relate to the efficiency of light har-

vesting by the pigments which seems to remain unaf-

fected. The effect of photoacclimation to high light is

however pronounced in the depression of the light satu-

rated rate of photosynthesis Pmax. After the exposure to

HL, all samples shows decrease in photosynthetic energy

storage efficiency by 48% in Isochrysis galbana, 29%

and 26% in Phaeodactilum tricornutum and Nan-

nochloropsis sp., respectively.

The results of this study are similar to those carried

out in many previous studies [8,21,22,28]. However,

these studies used a variety of measures of the photo-

synthetic activity and are thus difficult to compare di-

rectly. Many also involved difficult and labor intensive

measurements. In contrast, our measurements were all

carried out with the same methodology that is simple,

quick and direct. The complete measure of activity re-

quires only one minute. Most important, the activity

measured by the photoacoustic method is the absolute

thermodynamic efficiency of photosynthesis. Therefore,

while there is ample evidence that under acclimation to

high light there is a universal decrease in cellular pig-

mentation, photoacoustic reveals what fraction of the

absorbed light is dissipated as heat. Indeed, under high

light acclimation there is an increase in photoprotective

pigments that do not make the light they absorb available

for photochemistry but rather dissipate it as heat. Such

are some of the carotenoids like β carotene and astaxan-

thine as well as the pigments involved in the xanytho-

phyll cycle.

Y. PINCHASOV-GRINBLAT ET AL.

Figure 5. The effect of photoacclimation to high light and low light on relative photosynthetic energy storage efficiency of

Isochrysis galbana. In order to standardize our results, we converted the absolute energy storage efficiencies to relative ones,

setting the maximal storage in low light conditions as 100% photosynthetic energy storage efficiency.

Furthermore, reliable estimates of the quantum yields

of oceanic phytoplankton are essential parameters for any

modeling of global primary productivity, based on inter-

pretation of satellite images. In summary it is both of con-

siderable scientific interest, and of great applied impor-

tance for fisheries and for environmental protection to

measure changes over time, as well as over regional varia-

tion, of the efficiency of photosynthetic energy storage.

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