T churilova, Z Fnenko - Light absorption and maximum quantum yield of photosynthesis during autumn phytoplankton bloom in the western black sea - страница 1

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МОРСЬКИЙ ЕКОЛОГІЧНИЙ ЖУРНАЛ

УДК 581.526.325:581.132(262.5)

T. Churilova1, PhD, senior scientist, Z. Finenko1, prof., head of department, S. Tugrul2, PhD, head of department

1 А. О. Kovalevsky Institute of Biology of the Southern Seas, National Academy of Sciences of Ukraine,

Sevastopol, Ukraine

2 Middle East Technical University - Institute of Marine Sciences, Erdemli - Mersin, Turkey

LIGHT ABSORPTION AND MAXIMUM QUANTUM YIELD OF PHOTOSYNTHESIS DURING AUTUMN PHYTOPLANKTON BLOOM IN THE WESTERN BLACK SEA

In the frame of GEF/UNDP Black Sea Ecosystem Recovery Project the effect of nutrient availability on phytoplank-ton light absorption and maximum quantum yield of carbon fixation ())max) has been investigated. It has been found out that chlorophyll a concentration (Chl a) varied from 0.3 to 10 mg m 3 increasing from deep-water region to shelf waters. At the shallow stations, where upper mixed layer spread deeper the bottom of euphotic zone, the homogenous pigment distribution was observed. In the deep-water region the Chl a profiles showed the presence of a deep Chl a maximum (DCM) below a seasonal density cline. The functional characteristics of phytoplankton showed depth-dependent variation. The phytoplankton from DCM was characterized 20 % lower values of spectral averaged Chl-a

- specific light absorption aph (0.016±0.0025 m2 (mgChl)-1) and more than in two times higher values of )max

(0.070±0.012 molC (mol quanta)-1) compared with the surface phytoplankton avb (0.021±0.0035 m2 (mgChl)1) and

)max (0.030±0.0078 molC (mol quanta)-1). From offshore to shallow stations the increasing of surface Chl a concen-

=*

tration was accompanied with slight decrease of a ph (on ~20 %) and significant increase of )max up to its theoretical limit - 0.1 molC (mol quanta)-1, which were caused by an increase in nutrient supply across the shelf. Key words: Chlorophyll a, phytoplankton, light absorption, maximum quantum yield, nutrients.

Productivity in the Black Sea is known to vary strong in time and space, what is caused by variation in the main environmental factors such as nutrients, radiance and temperature. The spatial productivity heterogeneity is related to a differ­ence in nutritional status between shelf and deep-waters, which is more pronounced in the western part of the sea. The productivity values in the north-western shelf waters differ on three orders from deep-water region because of the input of nutrients by three big rivers flowing in. It results in that a strong nutrient gradient is observed in the direction from offshore to shore zones [7]. Waters nutrient status controls intracellular pigment con­centration, relative abundance of accessory light harvesting pigments and phytoplankton taxonomic and size structure, consequently, the nutrient con­© T. Churilova, Z. Finenko, S. Tugrul, 2008 centrations effects on phytoplankton chlorophyll a - specific absorption [1, 25]. The maximum quan­tum yield of photosynthesis depends on nutrient concentration [1, 5], because the photochemical activity of photosystem reaction centers is de­pressed by nutrient limitation [11, 28]. The Chl a -specific absorption coefficient and especially maximum quantum yield of photosynthesis are found to be more sensitive to nutrients and could be the indicators of physiological state of phyto-plankton community. However, for the Black Sea there are rather limited amount of maximum quan­tum yield of photosynthesis measurements. Only using of recent-day method of phytoplankton ab­sorption measurement [3, 4] allowed to quantum yield estimation from efficiency of photosynthesis (initial    slope    of   photosynthesis    light -

75

Т. Churilova, Z. Finenko, S. Tugrul

curve) [9]. Two-year monitoring of bio-optical characteristics of surface layer in central western part of the Black Sea showed a relationship of ab­sorption coefficients with chlorophyll a concentra­tions and revealed seasonal variability in chla-specific phytoplankton absorption coefficients [4]. The depth-dependent variability in phytoplankton absorption features and its relationships with nu­trient concentrations have not been still investi­gated, although such data would be important for estimation of physiological characteristic within euphotic zone.

The investigation aimed to estimate the variability in phytoplankton light absorption coef­ficient and maximum quantum yield of photosyn­

Legl

414

27.5 28 28.5

thesis during the autumn phytoplankton bloom and their relationships with nutrient availability.

Methods. The investigation of pigment concentration, photosynthesis-irradiance relation­ships (P-E) and phytoplankton light absorption (aph) were curried out on a few cross shelf tran­sects from Ukrainian, Bulgarian, Rumanian and Turkish coasts to the deep-waters region (Fig. 1) from 20 September to 15 October 2005 during a cruise on R/V "Vladimir Parshin". Sampling was generally performed at from dawn to midday.

Leg 2

28 30

Fig. 1 Location of the biological stations in September (Leg 1) and October 2005 (Leg 2) Рис. 1 Схема биологических станций в сентябре (этап 1) и октябре 2005 (этап 2)

Pigment concentrations. The sampling for pigment concentrations and light absorption meas­urements were done at 39 stations shown on scheme of stations (Fig. 1). Chl a and Phaeo con­centrations were analyzed by the fluorometric method [13]. The fluorometer was calibrated using solution of purified chlorophyll a (Chl a) which concentrations were determined with spectropho-tometer using the equations recommended by [14].

Water samples for pigment measurements were collected using 5-L bottles of CTD cassette at the 7 - 10 depths chosen based on a fluorescence pro­file to describe correctly vertical pigment distribu­tion. Water samples (0.2 - 0.5 L) were gently fil­tered on 25 mm Whatman GF/F filters under ~0.2 atm. vacuum. Filters were folded in half twice and wrapped in aluminum foil, labeled, and stored in liquid nitrogen (to avoid formation of degradation

products) until shore analysis. After removal from liquid nitrogen, the pigments were extracted by placing the filters in 5.0 ml 90 % acetone. The samples were allowed to extract overnight in 90 % acetone in the dark refrigerator (~ 6oC). The sam­ples were vortexed to break cell walls, and spun down in a centrifuge for 5 min to remove cellular debris and glass fibers and then measured before and after acidification. The sample was acidified with 2 drops of 1.2 M HCl. All procedures were conducted under subdued light in order to prevent photodegradation of pigments.

Particulate and phytoplankton light ab­sorption. The sample processing was performed respecting the ocean optics protocols recom­mended for satellite ocean color sensor validation [6]. Water samples (0.5 - 1.0 L) were gently fil­tered on 25 mm Whatman GF/F filters under ~0.2 atm. vacuum. Filters were placed in special plastic capsule and then stored in liquid nitrogen until analysis in the laboratory. The particulate absorp­tion was measured by method [27] in modification [21]. The optical density of the filters was meas­ured relatively to a seawater reference saturated blank filter with dual beam spectrophotometer (SPECORD-M40, Carl Zeis Yena). All spectra were offset to zero optical density at 750 nm, and corrected for the path-length amplification effects as described in [20]. The total particle absorption (ap( A)) was separated into phytoplankton (aph( A)) and non-algal particles absorption (aNAP(A)) by the methanol extraction method [15]. The measurement made before methanol ex­traction provided ap( A) while aNAP( A) was ob­tained after the extraction. aph( A ) was derived from the difference between ap( A) and aNAP( A). To calculate Chl a-specific coefficient (aph (A))

phytoplankton absorption coefficient aph( A ) was divided by a sum of Chl a concentration with phaeopigment concentration (TChl a).

Maximum quantum yield of carbon fixa­tion. The photosynthesis - irradiance (P-I) curves and phytoplankton light absorption spectra were

Морський екологічний журнал, № 3, Т. VII. 2008

used to calculate maximum quantum yield of car­bon fixation ((|)max). P-I experiments were carried out at 23 stations (23 experiments - for surface samples (where photosynthetic available radiance, PAR = 100%) and 7 ones - for deep samples (~1 % PAR). The depths to which 1 % of surface ir-radiance penetrated were determined based on PAR profile measured by PAR sensor attached to CTD-rosette. The method of P-I curve measure­ments and the results were described in [9].

The maximum quantum yield of carbon fixation ()max) was determined by the formula:

)max= k aB/ aph , (1)

where aB is the efficiency of photosynthesis, normalized to unity of Chl a concentration (initial slope of P-I curve); k is the coefficient for dimen­sional consistency of this equation, aph is the

amount of absorbed quanta, which is normalized to unity of incident quanta in visible diapason of wavelengths from 400 to 700 nm (photosyntheti-cally available radiation):

700

l aph (A)dAx Q(A)dA

= 400_ (2)

700

l Q(A)dA

400

Spectral distribution of the light energy in the experiments (Q(A)) was determined as the product of the energy spectrum of the lamp (EL(A)) and light transmission spectrum of 10 % water solution of CuSO4 - T(A): Q(A)=El(A)xT(A) (3)

Nutrient concentration. Nitrate, nitrite, phosphate, and silicate concentration were meas­ured on the board of R/V "Vladimir Parshin" just after the sampling with the autoanalyser by the standard techniques [26].

Results and discussion. The surface chlo­rophyll a concentrations varied from 0.3 to 10 mg m-3 increasing from deep-water region to shelf waters. Vertical chlorophyll a profiles were ob­tained   based   on   fluorescence   profiles and

correlation between chlorophyll a concentration

and fluorescence of chlorophyll a (Flu):

Chl = 0.00154xFlu, n=101, r[1]= 0.92 (4)

The chlorophyll a concentration, phyto-plankton absorption coefficient (aph(A)) and maximum quantum yield of photosynthesis (| max) varied in a wide range along the transects from offshore to shore regions and with depths.

Depth-dependent variation of aph(A)_and )maxin deep-waters region. In the investigated area euphotic zone was deeper than mixed upper layer. Photosynthetically available radiance attenuated up to 1 % of surface PAR value within 35 - 58 m layer (Fig. 2). Seasonal density stratification lo­cated at 14 - 22 m depth (Fig. 2).

A

0 20 3 40 3 60 80 100

10    12    14    16 CTt

0 4 8 121620 T

T

Chl a

1.25 2.5

~lIIIr

0      2      4      6 NO2+NO3

C

00

r-

5(

St. 30

0 m 10 m 20 m

B

10     12     14     16 <*t

0   4   8  12 16 20 T

0

20

Є 40-

.0 он

Q

60 80 100

T

Chl a

012

D

oo

5(

Q 1-

St. 30

30 m 40 m 50 m

400   500   600   700 800 Wavelengths, nm

Fig. 2 Typical for the deep-waters region verti­cal profiles of chlorophyll a concentration (Chl a, mg m-3), temperature (T, 0C), nitrate and nitrite concentration (NO3+NO2, цМ) and relative density (CTt, kg m-3) - A, B (Hori­zontal line shows depth of 1% PAR, ▲ - the depths of P-I experiments); Phytoplankton absorption spectra, normalized on coefficient at 678 nm (aph(A)/ aph(678)) - C, D.

Рис. 2 Типичное для глубоководного района вертикальное распреде­ление концентрации хлорофилла а (Chl a, мг м-3), температуры (Т, 0С), концентрации нитратов и нитритов (NO3+NO2, цМ) и относительной плотности (at, кг м-3) (Горизонтальной лини­ей отмечена глубина 1% ФАР, ▲ - глубины экс­периментов по опреде­лению световых зави­симостей фотосинтеза) - А, В; Спектры погло­щения света фитопланк­тоном, нормализован­ные на величину при 678 нм (aph(A)/aph(678)) -

C, D.

The stratification was found not to be strong, which was typical for autumn. The maxi­mum density gradient was equal 0.03 - 0.06 (kg m-3) m-1. The density-cline divided the euphotic

0

3

2

0

Light absorption and maximum yield of photosynthesis .

zone into two quasi-isolated layers with different environmental factors. Upper mixed layer was characterized by higher temperature and light in­tensity than the layer below the thermo-cline. It resulted in pronounced depth dependent variability in phytoplankton pigment concentrations and light phytoplankton absorption spectra shapes within the euphotic zone. The surface Chl a concentration varied from 0.33 to 0.85 mg m-3. In the upper mixed layer the Chl a concentration distribution was rather uniform. In the deeper layer located below the seasonal density cline Chl a maximum was observed (Fig. 2).

To estimate spectra shape variation the ab­sorption spectra were normalized to red peak ab­sorption value. The phytoplankton light absorption within the upper mixed layer was characterized by identical spectra shapes (Fig. 2C, 2D). The spec­tral distribution of absorption coefficient depends on phytoplankton pigment composition [24]. Therefore similarity in spectra shape reflects that pigment composition was similar within the mixed layer and the species composition and cell size structure of phytoplankton community is unlikely to vary within this layer. In the deeper (30 - 60 m) layer the shape of absorption spectra markedly changed (Fig. 2C, 2D). Near the bottom of eu-photic zone in the absorption spectra local maxi­mum at ~ 550 nm became visible. It is well known

Parameters

Surface (n=7)

1 - 5 % PAR (n=8)

Chl a

0.55 31%)

1.4 85%)

a*ph(678)

0.0181 15%)

0.0172 14%)

R

2.70 17%)

2.15 10%)

*

aph

0.0205 20%)

0.0157 21%)

))max

0.030 26%)

0.070 18%)

that this capability to absorb green wavelengths (~550 nm; Fig. 2C, 2D) is owing to phycobilins, which are pigments - markers of cyanobacteria [22]. The domination of cyanobacteria in phyto-plankton in the layer below the seasonal thermo-cline seems to be reasonable. Within the euphotic zone spectral features of irradiance change with depth, and mainly green light penetrates down to the euphotic zone bottom [19]. Cyanobacteria due to the phycobilins absorb more effectively green light than other taxons.

Chl-a   specific   absorption coefficients

(aph {a) ) decreased with depth, especially in the

blue part of spectrum. It resulted in the decrease of blue (at 438 nm) to red (at 678 nm) peak ratio, R

(Fig. 2C, 2D). The mean values of aph

(678) and R

varied from 0.0181 (±15%) m2 (mg Chl)-1 and 2.7 (±17%) at the surface to 0.0172 (±14%) m2 (mg Chl)-1 and 2.15 (±10%) at the 1% PAR depth (Ta­ble 1).

As result of variation in both specific ab­sorption coefficient and spectrum shape the spec-

=*

trally averaged Chl a - specific coefficient (aph), decreased within the euphotic zone from 0.0205 (±20 %) to 0.0157 (±21 %) m2 (mg Chl)-1.

Table 1 Mean values and standard deviation (in a bracket) of some biooptical parameters in the deep-waters region.

Табл. 1 Среднее значение и стандартное отклоне­ние (в скобках) биооптических параметров в глубо­ководной части моря

Comments: Chl a - chlorophyll a concentration, mg m-3; a *ph (678) - chlorophyll a - specific absorption coefficient at

red peak, m2 (mgChl)-1; R - blue to red peaks ratio in absorption spectra; a ph - spectrally averaged chlorophyll a-specific coefficient, m2 (mg Chl)-1 and )max - maxim quantum yield of carbon fixation, mol C (mol quanta)-1 Примечание: Chl a - концентрация хлорофилла а, мг м-3; a ph (678) - нормированный на хлорофилл а коэффи­циент поглощения света фитопланктоном в красном максимуме, м2 (мгОи)-1; R - отношение коэффициентов поглощения в синем и красном максимумах; a ph - средняя по спектру величина нормированного на хлоро­филл а коэффициента поглощения света фитопланктоном, м2 (м^Ы)-1 и )max - максимальный квантовый вы­ход фотосинтеза, мольC (моль квантов)-1

Морський екологічний журнал, № 3, Т. VII. 2008 79

Chl a- specific absorption coefficients de­creased slightly with depth within the euphotic layer. It could be explained by low pigment pack­aging degree in phytoplankton cells of surface layer and relatively high Chl a- specific coeffi­cients at the bottom of euphotic zone due to domi­nation of cyanobacteria, which is known [22] to be characterized by relatively high aph (A) in blue

part of spectrum due to higher accessory pigment to Chl a ratio than in other phytoplankton taxon

groups. The variations in aph (A) values are caused

pigment composition and intracellular pigment packaging [23]. Recent investigations [2, 17] showed that package effects were responsible for up to a 62 % reduction in the chla-specific absorp­tion coefficients at the blue part of spectra, particularly for populations dominated by larger phytoplankton. On the other hand variations in pigment composition due to change of phytoplankton taxonomic structure were also found to have smaller impact (10 - 28 %) on variations in total absorption.

The pigment packaging degree is well known to depend on intracellular pigment concen­tration and cell size [23]. The pigment concentra­tion in cells is changed to acclimate to main envi­ronmental factors as irradiance, nutrients and tem­perature. The physiological acclimation of plank-tonic algae to a decrease of irradiance and to an increase of nutrients concentration is accompanied with an increasing of intracellular chlorophyll to organic carbon ratio [10], which results in reduc­tion of a*ph (A) values. The size-dependence of ab­sorption was shown to be a pronounced character­istic of phytoplankton under low light conditions, but under high light conditions, size-dependence of absorption at 440 nm weakens due to the effects of absorption by photoprotective pigments in the blue region of the spectrum [11]. These results explain what might cause the rather vertical ho­mogeneity in aph (A) obtained in our study.

This vertical stability of a*ph (A) allowed

combining data from all depths to get relationships between light absorption coefficients and TChl a concentrations described by power functions: for the blue peak

aph(440) = 0.0451 x094 , n=39, r2=0.83 (5) and for the red peak

aph(678) =0.0181 x092 , n= 39, r2=0.91 (6) High values of power coefficients (close to 1) show, that Chla-specific coefficients were practically constant for that range of pigment con­centrations (from 0.16 to 2.3 mg m-3). The ob­tained relationships differ (by higher power coef­ficients) from results of two years long monitoring of surface phytoplankton absorption characteris­tics [4]. During that bio-optical monitoring the chlorophyll concentration reached maximum (about 2 mg m-3) in winter-spring bloom of dia­toms. The difference in power coefficients is likely to reflect the seasonal variations in chla-specific absorptions due to package effect resulted from photo physiological response of phytoplank-ton to different environmental factors.

The maximum quantum yield of carbon fixation increased significantly with depth in

stratified waters from 0.030 (±26 %) to 0.070

(±18 %) molC (mol quanta)-1 on average (Table 1). The phytoplankton near the bottom of euphotic zone (~ 1% PAR) acclimated to ambient environ­mental conditions was characterized by (|)max values closed to upper theoretical limits of photosynthesis quantum yield (0.1 molC (mol quanta)-1).

The low values of | max in the surface layer are caused by the high phytoplankton pigment ab-sorbance in blue part of spectra, which could be related with the presence of pigments-photoprotectors. The | max co-varied with depth at the oligothrophic site in Ocean and decreased with increasing of relative concentrations of non-photosynthetic pigments [1]. In that study was shown that the variable contribution of non-phototsynthetic absorption could explain 3-fold variation in the | max. At the same time nutrients

availability factor was responsible for 2-fold varia­tion in the | max [1]. In our study the | max was de­termined as an assimilation of organic carbon di­vided by photons absorbed by all pigments includ­ing both photosynthetic and photoprotective pig­ments. The spectral averaged absorption coeffi­cient decreased from surface to the bottom of the euphotic zone on 20 % (Table 1). This decreasing could be caused by both reduction relative content of accessory pigments (including photoprotective pigments) and increasing of pigment packaging degree. The contribution of absorption changing to the depth-dependent )max variation was equal ~ 20%. Consequently, a 2-fold variation (80 % of total 2.3-fold variation) in the | max could be caused by an effect of ambient environmental factors namely nutrient concentration and light intensity. Laboratory studies on quantum efficiency of pho­tosynthesis of planktonic algae cultures demon­strated that nutrient stress can markedly reduce an activity of photosystem reaction center and conse­quently )max [5, 16]. Value of )max declines at high irradiance in nutrient-replete cultures, because of the increasing of photoprotective pigments contri­bution to total absorption [18]. In our study high incident solar radiation and nutrients limitation (occurred in subsurface layer) could effect the photochemistry efficiency reaction centers and subsequently decrease | max. Obtained results that near the bottom of euphotic zone phytoplankton utilized the absorbed photons with yield closed to its potential maximum value allow making a con­clusion about high nutrient availability of phyto-plankton in this deep water layer. For this deep water layer the correlation between | max and ni­trate concentrations was not revealed. But values of | max decreased with increasing of a distance to the depth with maximum gradient of nitrate con­centrations. This distance indirectly characterizes dynamics of nutrient upflow from nitro-cline to the deep layer of euphotic zone. Consequently, for the deeper phytoplankton (below the seasonal thermo-cline) maximum quantum yield of carbon fixation was rather high and depended on nutrient availability namely - dynamics of nutrient upflow.

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T churilova, Z Fnenko - Light absorption and maximum quantum yield of photosynthesis during autumn phytoplankton bloom in the western black sea