LIGHT ABSORPTION AND QUANTUM YIELD FOR GROWTH IN FIVE SPECIES OF MARINE MACROALGA1

J. Phycol. 29, 54-63 (1993)

LIGHT ABSORPTION AND QUANTUM YIELD FOR GROWTH IN FIVE SPECIES OF MARINE MACROALGAE'

Stiig Markager, Botanical Institute, Aarhus University, Nordlandsvej 68, DK-8240 Risskov, Denmark

ABSTRACT

Light utilization eficiency in five species of marine macroalgae was measured in laboratory growth experiments (13-41 days duration) at dafferent irradiances at 7" C. All species acclimated to irradiance by changing their light absorption, resulting in a peak in light absorption between 2 and 15 ~mol-m-2.s-1. Light absorption increased with thallus-specijic chlorophyll and carbon content according to linear inverse relationships between chlorophjll content (chl . area-l) and log[transmission J and between log[carbon content, C. area-l] and log[transmission J. Quantum yields f o r light-limited growth and estimated gross photosynthesis were calculated based on incident and absorbed light. Quantum yield for photosynthesis based on light absorbed by pigments was high (mean = 114 mmol C . mol-' photons) and similar among the species. Quantum yield for net growth based on incident light was also high but more variable, between 22 and 75 mmol C.mo1-l photons. Dzfferences among species were mainly due to dafferences in light absorption. In conclusion, all species acclimated to low light by increasing light absorption to the maximum attainable, and growth eficiencies based on absorbed light were close to the maximum theoretically possible.

Key index words: chlorophyll content; Chondrus crispus; Fucus serratus; growth; light absorption; macroalgae; Petalonia fascia; Porphyra purpurea; quantum yield; Ulva lactuca

Light is an essential resource for all plants and often limits their distribution and growth in nature. The efficiency of light utilization is, therefore, an important parameter in plant physiology and ecology. Light utilization can be divided into two processes: (1) absorption of incident light by photosynthetically active pigments and (2) conversion of light energy to plant biomass. T h e first process is tightly coupled to pigment content, which can vary as a result of acclimatization to different irradiances (Ra- mus et al. 1976, Sand-Jensen 1988). The second process is constrained by the demand of eight photons to produce one 0, molecule, three adenosine 5'-triphosphate (ATP) molecules, and two molecules of reduced nicotinamide adenine dinucleotide phos- phate (NADP) according to the Z-scheme of photosynthesis (Hill and Bendall 1960), but the realized

Received 5 May 1992. Accepted 25 September 1992. * Present address: Institute of Life Sciences and Chemistry,

Roskilde University, Postbox 260, DK-4000 Roskilde, Denmark.

efficiencies depend on the physiology of the plant (Myers 1980).

Macroalgae with flat thalli are ideal organisms for the study of light utilization efficiency in plants because light absorption and growth measurements are easy to perform per unit of surface area and most cells are active in photosynthesis. In this experiment, five species that possess different pigment types and thallus thickness were used to measure light absorption, net growth, and respiration at different irradiances. From these measurements light absorption and quantum yield were evaluated as a function of irradiance.

Theory and background. Growth of autotrophic plants is basically a reduction of CO, to organic carbon compounds with water as the electron donor and light as the energy source. Quantum yield is a measure of the efficiency of light utilization in the process and has the unit of substrate used or product formed per photon. Convenient units are mol 0, produced or mol C used in photosynthesis or incor- porated in plant biomass per mol photons. However, besides the problems of scaling, quantum yield may be defined in various ways. An ecologically relevant process is formation of plant biomass from inorganic compounds and incident light. The quantum yield of this process is here denoted 4; (net growth based on incident light). This overall process can be divided into subprocesses (Fig. 1): (1) absorption of incident light by photosynthetically active pigments and (2) splitting of water and formation of reduced NADP and ATP. These products then act as substrate in (3) reduction of CO, to simple carbohydrates (gross photosynthesis), (4) conversion of carbohydrates to plant biomass, involving fixation of several inorganic elements (N, P, S, etc.), and (5) maintenance of cell integrity (e.g. maintaining the chemical gradients across membranes and replacing organic compounds that decay during growth). Pro- cess (5) is sometimes regarded as separate from growth but utilizes the same substrate pools (reduced NADP and ATP) as the strict growth processes (processes (3) and (4)), and the overall quantum yield for growth will, therefore, depend on maintenance processes.

Reduced NADP and ATP needed for processes (4) and (5) can be provided either directly by the light reactions or by respiration. Efficiency in terms of photons needed for production of 1 mol ATP or reduced NADP is about the same (Raven 1984), and quantum efficiency is, therefore, not affected by how ATP and reduced NADP are produced. This jus-

54

LIGHT UTILIZATION IN MACROALGAE 55

tifies incorporation of respiration in subprocess 3 (Fig. 1) as the backward process of carboxylation in photosynthesis. Mitochondria1 respiration occurs both in the light and in the dark, but the rate in the light is difficult to measure and much discussed. Some experiments indicate that mitochondria1 respiration is depressed in the light (Bidwell 1977), whereas others have shown that it proceeds at maximal speed (Weger et al. 1989, Beardall and Raven 1990). This question complicates calculation of gross photosynthesis by adding dark respiration to net growth or net photosynthesis. However, what matters for calculation of quantum yield is not the respiration rate but, rather, the demand for ATP and reduced NADP, provided that photon requirements are about the same when ATP and reduced NADP are produced directly by photosynthesis or by respiration of carbohydrates. The assumptions made here of the role of respiration in metabolism are probably valid in healthy cells at low light. At high light, pho- torespiration may become important, and in starved or senescent cells compounds other than carbohydrates can be important in respiration.

Relationships between irradiance and algal light absorption, net growth rate and respiration at 6-8 irradiances were measured in a culture facility at 7" C over 13-41 days. Special precautions were taken to obtain a well-defined light field and sensitive measurements of growth rate, respiration, and light absorption on the same individual thallus pieces to allow accurate calculations of quantum yield for each individual. Light was measured as incident light (I), light absorbed or reflected by the thallus (A), and light absorbed by pigments (Apigm). Growth was measured as the net increase in biomass and dark respiration as oxygen consumption. Both were converted to carbon units. The carbon demand from respiration was added to the net growth rate to obtain the gross photosynthetic rate. Assuming that the dark respiration rate expresses the demand for ATP and reduced NADP in processes (4) and (5) above, and that these demands are equal in the light and the dark, the quantum yield for both gross photosynthesis ($J, including only processes (2) and (3), and for net growth (@,,), including processes (2)-(5) were estimated. Thus, six different values for quantum yield based on three different light values (incident, total absorbed, pigment absorbed) and two different growth processes (net and gross) were obtained. This allowed an analysis of how net quantum yield based on incident light ($2) is controlled in different species by the subprocesses mentioned above.

MATERIALS AND METHODS

Six independent growth experiments were performed from January to August 1989 with one to three species each time. Three of the species, Ulva lactuca L., Petalonia fascia (0. F. Mull.) Kuntze, and Porphyra purpurea (L.) J. Ag., have thin foliose thalli (thallus-specific carbon content (TSC) between 0.25 and 1 mol

4.. . . . . . .

Light Absorption

i Light processes in photosyn-

: ATP, NADPH :. ---__.__--

i

/ Dark processes In photosynthe- thesis and respiration

(ATP NADPH.: ~ - - _ _ _ _ - - .--------

I

i Growth a-1

L I

FIG. 1. Schematic presentation of the five subprocesses that constitute the overall growth process in autotrophic algae. Boxes represent a process, and ellipses represent pools of compound(s). Solid ellipses = external substrate or products of the growth process; dashed ellipses = intermediate compounds in the growth process; dashed arrows = transfer of energy or compounds between subprocesses; solid arrows = transfer of substrate or end products. Numbers refer to subprocesses described in the text. A,,;, = light absorbed by photosynthetically active pigments; A, = light absorbed by structural components of the thallus; I = incident light; Ref = reflected light; T = transmitted light.

C.m-Z) where all cells can divide. The two other species, Fucus serratus L. and Chondrus crispus Stackh., have thicker leathery thalli (TSC 2-6 mol C.m-2) andapical growth. The species belong to the Chlorophyceae (Ulva lactuca), Phaeophyceae (Petalonia fascia and Fucus serratus), and Rhodophyceae (Chondrus crispus and Porphyra purpurea) and, therefore, represent a range of thallus thicknesses and pigment compositions.

Algae were collected in Arhus Bay, Denmark (salinity 207~0)~ at 0.3-1.5 m and acclimated in dim light at '7°C for 7-14 days. Then, 150 discs of 16 mm in diameter (Ulva lactuca, Petalonia fascia, and Porphya purpurm) or 2-4-cm-long apical shoots (Fucus serratus and Chondrus crispus) with similar appearance and pig- mentation were prepared of each species. Transmission, wet weight, and surface area were measured on all thalli and preex- perimentak relationships between surface area (discs) or wet weight (apical shoot) and carbon content (Carbo-Erla CHN-analyzer) were established on 20 scarified thalli. Another 20 thalli were analyzed spectrophotometrically for chlorophyll by extraction of intact thalli with N,N-dimethylformamid (Inskeep and Bloom 1985). The remaining thalli were placed at 6-8 irradiances (10-14 at each intensity) in 10-L aquaria (dimensions L:W:H 20:30:18 cm) illuminated by cool-white fluorescent tubes (Thorn 36W, 4000) from above in continuous light (experiments 1 and 2) or a 14:lO h LD cycle. The aquaria were shaded to the desired levels by neutral-density filters placed above them. Growth medium was GF/C filtered seawater enriched with 500 rg NH,-N.L-', 500

56 STIIG MARKAGER

pg NO,-N.L-I, and 100 pg PO,-P.L-'. Water circulated between the aquaria and an aerated reservoir and was renewed weekly. Total volume was 140 L, and flow rate in the aquaria was 8.4 cm.min-l corresponding to 3 L.min-l. Aquaria were made of black Perspex to avoid incoming light from the sides and bottom and to minimize inside reflection. Thalli were held in a fixed position perpendicular to the light field by a net and two strings of thin fishing line. This allowed us to follow each individual and measure absorption and surface area with minimal handling. In the first experiment, thalli floated freely, which only allowed calculation of mean values for absorption and growth parameters.

Incident photon flux density at the location of the thalli was measured biweekly with a newly calibrated Li-Cor (LI-192SA) underwater quantum sensor. Light transmission through thalli was measured every week on all individuals (as in Vermaat and Sand-Jensen 1987) placed at the same distance under the light source used to illuminate the aquaria. Surface area (one side) was calculated from diameter measurements (discs) or measured with a Li-Cor area meter (LI-3000).

Growth experiments were terminated after 13-4 1 days depending on growth rate. Dark respiration was measured as 0, consumption of each individual kept in closed bottles in the dark for 14 h at 7" C in an incubator with slow stirring. Oxygen concentrations were measured by the Winkler technique using am- perometric titration. Oxygen values were converted to carbon units assuming a respiratory quotient of 0.8 mol CO,.mol-' O,, which yields a conservative estimate of the carbon demand for respiration (Gibbs 1962, Geider and Osborne 1989). Subsequent- ly the discs were analyzed for surface area, weight, carbon, and chlorophyll content as described above.

Calculations. Initial biomass of individuals was calculated from relationships between carbon content and surface area (thin-thallus species) or fresh weight (thick-thallus species). Incident photons received by each individual were calculated from irradiance and mean thallus area (x) in the periods between measurements by

where xu is the initial thallus area and x, is the thallus area at the end of the period.

Total light attenuation (A) was calculated as 1-thallus transmission (T). Total attenuation includes absorption and reflection, which could not be separated by this technique. However, reflection is known to be low, about 5% (Luning and Dring 1985) or < 1 % (Frost-Christensen and Sand-Jensen 1992) when the light beam is perpendicular to the thallus surface. A fraction of absorption (A), independent of thallus-specific chlorophyll content (DChl), was calculated from the relationship between A and D,,, (Fig. 4). The relationship between T and D,,, is described by Eq. (2), which is analogous to Lambert-Beers law for absorption in a solution:

(2) T = e-d(OChlk+=)

where k is the molar absorption coefficient, d is the length of the light path, and a is a constant representing the absorption independent of chlorophyll. Equation (2) was rearranged to

(3) T = e-&e(--Ld)DChl or T = T,bDChl,

where To = e-& and b = e-(kd). This provides a simple relationship between the measured quantities T and D,,,. Equation (3) can be rearranged to represent absorption:

A = 1 - T = 1 - (1 - Ao)bWhl, (4) where A, is the absorption at D,,, = 0 and b describes the curvature of the line for A and Dchl. Light absorption by chlorophyll and pigments co-varying with chlorophyll was calculated as

A+, = A - A,. (5)

This is a simplified description of the light absorption process. However, a good fit to the observed data was obtained, and it is likely that APi, provides a better estimate of light available for photosynthesis than does total absorption (A).

RESULTS

Light absorption. Light absorption (A) of individuals grown at different irradiances showed the same pattern in the five macroalgal species after the experiments. Before the experiments light absorption varied randomly among individuals at different irradiances, but after experiments maximum light absorption occurred in individuals that had been exposed to intermediate irradiances (Fig. 2). The light range yielding maximum absorption was 1-2 pmol. m-2.s-1 in Porphyra purpurea, 5-10 pmol.m-2.s-1 in Ulva lactuca and Chondrus crispus, and 15-20 pmol. m-*.s-' in the two brown algae. Light absorption increased at intermediate irradiances during the course of experiments and usually declined in the dark. However, the decrease in the dark was not significant and may in part be due to dilution of pigments and not to a decrease in content per individual, because all species showed a small increase in surface area in the dark. For the two brown algae, the increase in absorption at intermediate light was small, though significant ( P < 0.05, t-test), providing a relatively constant absorption regardless of the incident irradiance. T h e other species showed a pronounced increase of absorption when exposed to intermediate light ( > O . 1 absorption unit) and, therefore, displayed a clear acclimatization. Changes in absorption per day decreased during the experiments for individuals in light.

Maximum light absorption (A,=) differed according to thallus thickness of the species. Maximum absorption was 0.94 * 0.04 (mean -+ 95% CL) for Chondrus crispus and 0.92 f 0.01 for Fucus serratus, both of which possess thick thalli. Maximum absorption was 0.78 -t 0.02 for Petalonia fascia, 0.71 * 0.02 for Ulva lactuca, and notably lower at 0.49 k 0.02 for Porphyra purpurea all of which have thin thalli. Also, the variability in maximum absorption was relatively small in three experimental trials with the same species collected at different times (i.e. 0.74-0.79 for Petalonia fascia and 0.65-0.71 for Ulva lactuca). Maximum light absorption of the five species was closely related to their TSC (Fig. 3). The increase in absorption became gradually smaller at high TSC. Thus, absorption per unit TSC was higher for thin Porphyra purpurea (1.3 m2.mol C-I) than thick Chondrus crispus (0.2 m2.mol C-'). The relationship was closely linear between log(1 - A,,,) and log(TSC) (P < 1 * 1 O-6; Fig. 3).

Light absorption of individual algal species increased with thallus-specific chlorophyll content (Dch,) for all species with thin thalli (Fig. 4). The dependency of absorption on DChl was weak for Pe- talonia fascia and even weaker for Fucus serratus and Chondrus crispus (data not shown), which showed high absorption irrespective of chlorophyll level. For Ulua

57 LIGHT UTILIZATION IN MACROALGAE

O E

O t

0 4

0 2

o a

0 6

0 4

G 0 0 2

.A

4

o a 0 fA 0 6 e (d

A

4

0 4 4

y 0 2

0 8

0 6

0 4

0 2

0 8

0 6

0 4

0 2

Ulva lactuca

Petalonia farcia

6

1 I I 1 I I I I I

Porphyra purpurea

1 I I I I I I I I I

Fucus serratus

Chondrus crispus I I I I I I I I I

0 20 40 60 80

Incident light for growth (pmol m?s-')

Mean light absorption of five species of marine macroalgae before (0) and after (0) growth experiments at different irradiances. Duration of experiments: Ulva lactuca, 13-22 days; Petalonia fascia, 17 days; Porphyra purpurea, 29 days; Fucus serratus, 28-41 days; Chondrus crispus, 14-22 days. 95% CL are given for 10-14 individuals at each irradiance after the experiment. CL not shown when within symbols.

FIG. 2.

lactuca and Porphyra purpurea, the power equation (Eq. (3)), derived from Lambert-Beers law, provided good description of absorption versus D,,,. The curvature of the line at high D,,, shows that the mar- ginal increase in absorption becomes gradually smaller as D,,, increases, presumably due to self- shading among pigments. Estimated background absorption (+95% CI) at zero chlorophyll (A, = y-axis intercept), caused by structural parts and pigments

0.8

d

2 0.6 0 v) e ld

2 4

0.4 2 w 3

0.2

0.0 0 1 2 3 4 Thallus specific carbon content (mol C m -2 )

FIG. 3. Relationship between maximum absorption (Amax) and corresponding TSC in Ulva lactuca (0), Petalonia fascia (O), Por- phjra purpurea (V), Fucus serratus (V), and Chondrus crispus (0). The line is the function A,,, = 1 - transmission (T), where T = 69.2 TSC-08555 corresponding to a linear relationship between log(T) and log(TSC) (P < 5*10-6).

not covarying with chlorophyll, was 0.034 + 0.004 for Porphyra purpurea, 0.13 -C 0.02 for Ulva lactuca, and much higher (0.56 f 0.02) for Petalonia fascia. The latter high value was probably due to brown pigments, which do not co-vary with chlorophyll (Dring 1982). This obviously makes calculation of A, invalid for Petalonia fascia. One component of A,, is reflection, which Luning and Dring (1985) found to range between 0.04 and 0.09 of incident light in different marine macroalgae. Accounting for light absorption by compounds other than pigments, our values of 0.034 for Porphyra purpurea and 0.13 for Ulva lactuca seem realistic. A conservative estimate of 0.1 for A,, was applied for the other algal species in subsequent calculations of quantum yield based on photons absorbed by pigments ($*pi").

Quantum yield. Quantum yield based on absorbed photons ($*) declined with irradiance corresponding to increasing light saturation of growth (Fig. 5) . Dif- ferences between net (4") and gross (&) quantum yield were higher at low light because of the pro- portional increase of dark respiration relative to net growth. Subsequent calculations of quantum yield at light limitation were based only on values for individuals grown at irradiances up to the level where the mean $-values declined markedly (indicated with 1 on Fig. 5) . Mean $-values at light limitation for net growth based on incident light (4;) varied between 2 2 and 75 mmol C.mol-' photons (Table 1) . The lowest values were found in experiment 1, where the specimens floated freely in the aquaria and, therefore, received light from different angles. Mean

58 STIIG MARKAGER

0.8 l’O ~

0.6

0.4

0.2

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G 0 .+ 0.8 4 a

0.6 v1

(d 0.4 e

2 M 0.2

3 0.0

0.8

0.6

0.4

0.2

0 .0

i 0 I

A,=0.56*0.02 b=0.97910.005 p=o.o0019

0 10 20 30

Thallus specific chlorophyll content

( p g Chl ~ r n - ~ )

Relationship between absorption (A) and thallus-specific chlorophyll content (DCY) for A) Ulua lnctuca, B) Porphyra purpurea, and C) Petaloniafascia. The line is the function A = 1 - transmission (T), where T = T,bDCh’ is calculated from the linear regression equation In(T) = In(T,) + D,,,ln(b) (Eq. (3)). A, = (1 - T,) corresponds to the absorption at D,,, = 0. 0 = data from Sand-Jensen 1988.

Fic. 4.

+-values based on absorbed light ($nA) were higher (52-91 mmol C.mol-I photons) and based on photons absorbed by pigments. Mean values were between 58 and 103 mmol C.mol-’ photons). Overall, no significant differences were found among species. Quantum yield increased further when calculated as gross photosynthesis. Mean values for +gAPi@” were between 104 and 1 19 mmol C.mol-’ photons for all

120

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i Ulva lactuca

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Petalonia fascia c J 1 1 1 1 1 1 1 1

Porphyra purpurea -

1 0

Fucur serratus 1

1

J I I I 1 1 1 1 1 1

0 20 40 60 80

Incident light f o r growth (pmol rn-‘s-l)

Quantum yield for growth based on absorbed light (A) of five species of marine macroalgae at different irradiances. 0 = Mean values &95% CL for net carbon growth (&,A) of 10- 14 individuals; 0 = Values for gross photosynthesis (&A) (net carbon growth plus respiration). Values below (1) were used to calculate mean quantum yield at light limitation.

FIG. 5.

species except for Porphyra purpurea, where data were only available from experiment 1. Besides the increase in mean values from +ni to +gA~i@”, the variability among experiments (excluding experiment 1) decreased from 39 mmol C.mol-I photons for +: (82% of the mean) to 13 mmol C.mol-I photons (1 1% of the mean) for $gApi@”.

T h e frequency distributions of individual +-values


TABLE 1. Quantum pield at light limitation forjive species of marine macroalgae based on net growth ($,J or gross photosynthesis (net growth plus respiratorj loss) (@J. The light level used for calculation of @ was either incident light on the thallus (i), total absorbed light (A), or light absorbed by pigments (Are,,,). Mean zlalues t CL in inmol C.inol-' photons, except in experiment 1 where CL is not prozided. Illumination was continuous in experiments 1 and 2 and followed a 14:lO h LD cycle in the others.

Net growth basis Gross growth basis Exp. no. 4,; .$,,"P'S" 4; +&A . & & , X ~

Ulva lactuca 1 2 5

3 6

Fucus serratus 4

Petalonia fascia 1

Porphya purpurea 1 Chondrus crispus 4

35 36 t 8 39 + 10

34 37 5 19 4 4 + 1 1

52 t 32 22

7 5 f 1 1

60 52 t 12 65 f 17

56 52 t 26 67 f 18 60 t 38

59 91 f 13

75 64 2 15 83 f 21

67 58 k 30 73 f 22 68 ? 43

64 103 + 14

50 49 f 4 54 + 10

52 72 2 20 63 f 12 83 f 46

28 82 + 10

83 70 * 18 92 + 17

86 100 + 28 95 ? 21 98 f 54

73 99 * 12

106 119 f 10 118 f 22

104 114 k 33 106 f 26 110 f 61

79 112 * 14

were normally distributed with mean values (kSD) of 59 k 28 mmol C.mol-' photons for 42, 95 +_ 32 for

From the preceding values, the fate of light energy reaching the thallus surface was calculated for individuals with maximal absorption assuming that 100% efficiency equals conversion of all incident light to carbon biomass with maximal quantum yield ( 1 25 mmol C.mol-l photons; Raven 1984). This constitutes a theoretical maximum that allows calculation of the "loss" of energy in different subprocesses. The dominant "loss" of incident light energy in all species was transmission, reflection, and absorption by structural components (Fig. 7A, B). About 35% of the photons (57% for Porphyra purpurea) were lost in this way for the thin species and 20% for the two thick species. Loss of energy due to suboptimal conversion of light energy to fixed carbon (4gAp'gm

below 125 mmol C-mo1-I photons; Fig. 7C) was mar- ginal for all species except for a single experiment with Porphyra purpurea. The ecologically relevant light utilization efficiency for net growth (6;) (Fig. 7E) was between 25 and 75% of the theoretical maximum equivalent to 6,i-values of 28-83 mmol C.mol-' photons (right-hand axis, Fig. 7). These values are slightly higher than the mean values in Table 1 because they were calculated for individuals with maximal absorption.

and 114 f 37 for (bgAPigm (Fig. 6).

DISCUSSION

Absorption. The five species examined here acclimated to the irradiance for growth by forming an absorption peak at intermediate irradiances. This pattern agrees with earlier results for Ulva lactuca (Sand-Jensen 1988). Absorption and pigment content (here expressed only as chlorophyll per thallus area) are interrelated so that a peak in DChl will also occur at intermediate irradiance. The flattening of the absorption-chlorophyll curve at high absorption values implies that the chlorophyll peak will be even more pronounced than the absorption peak. A peak in chlorophyll content per cell at intermediate

irradiances was also found in several species of microalgae at 20-50 hmol.m-2.s-L (PrPzelin and Sweeney 1978, Falkowski 1980) and submerged freshwater macrophytes at 3-1 1 pmol.m-*.s-* (Sand- Jensen and Madsen 1991), suggesting that it is a common phenomenon among aquatic plants. At low light, acclimatization might be very slow, and the observed low pigment content at low light might be due to incomplete acclimatization. However, the pattern persists in Ulva lactuca for 70-80 days (Sand- Jensen 1988), and the reduced changes in light absorption per day during my experiments also indicate that the pattern might hold in balanced growth.

It is generally accepted that high pigment content directs the resources of the cell toward capturing photons when light is limiting for growth (Beardall and Morris 1976, Falkowski 1980, Raven 1984), but the mechanisms regulating the increase in chlorophyll content are unknown (Falkowski 1980). How- ever, the absorption peak at intermediate light shows that the mechanism stops working at a certain low light level. Two not mutually exclusive mechanisms are proposed to explain this pattern.

The first mechanism assumes that pigment synthesis is limited by available light and degradation of pigments proceeds at a constant specific rate. The amount of pigment degraded per unit time will then increase with increasing pigment content in the cell, which will stabilize at a level where synthesis and degradation balance. This implies that the maximum possible pigment content will decline below the light level saturating for chlorophyll synthesis. The peak pattern then arises from an active regulation of chlorophyll content above the irradiance for peak absorption and a limitation of chlorophyll synthesis below this light level.

The irradiance providing maximum pigment content exceeds the light compensation point for growth (Markager and Sand Jensen 1992) except for Por- phyra purpurea. If pigment synthesis is limited by available light, as assumed here, this limitation occurs when growth still takes place, so energy is avail-

60 STIIG MARKAGER

15

10

5

0

15

10

5

0

15

10

5

0

1

A

g §

I I I. I I

A

g @ p'gm

J I I I

O O O l I I I I o r o o o o o o

c n ) N l n C D d 7 I 7 I T i w

@ (mmol C rnol-'photons) FIG. 6. Frequency distribution of individual quantum yields

for light-limited growth calculated from experiments 2-6 (Ta- ble 1).

able but not used for chlorophyll synthesis. The turnover time of pigments is not included in the proposed mechanism, but net changes in pigment content will be very slow as it approaches the equi- librium at low light.

100

d 80

5 +

4

60 4 9

B

w

40 &

L a, a 20

0

o m A B

B R r n C D E

FIG. 7. The fate of light energy incident on the thallus at the light level providing maximal absorption. A) Light transmitted through the thallus. B) Light reflected or absorbed by structural components. C) Loss due to suboptimal conversion of light energy to organic carbon, assuming an optimal efficiency of 125 mrnol C.mol-I photons. D) Light energy used in processes other than carbon fixation. E) Light energy converted to carbon biomass. On the right-hand scale, the +value for net growth (E) and gross photosynthesis (E + D) based on incident light can be read.

The second mechanism assumes that the algae actively regulate the chlorophyll content at all irradiances and optimize the chlorophyll content according to a cost-benefit model. The advantages for the algae of an increase in chlorophyll content will decrease with increasing pigment content because of self-shading and will also decrease at very low light because few extra photons are absorbed by an increase in absorption. This may lead to a maximum chlorophyll content at intermediate light where investment in new chlorophyll balances the cost of synthesis (Raven 1984). The advantages for the algae of an increase in thallus-specific chlorophyll content can be calculated in terms of extra light absorbed and the photon cost of chlorophyll. This was done at the maximum thallus-specific chlorophyll content in the three species for which data were available for absorption versus chlorophyll (Table 2). Assuming that the second mechanism is correct, and that chlorophyll synthesis proceeds until photon cost minus benefit is zero, the minimum lifetime for a chlorophyll molecule can be estimated. Raven (1 984) calculated the photon cost for synthesis of different pigments and their associated proteins from inorganic compounds. Assuming an average value of 2000 mol photons for synthesis of 1 mol chlorophyll with associated proteins, the minimum life-


time for chlorophyll pigments is 71, 75, and 89 h for Petalonia fascia, Ulva lactuca, and Porphyra purpurea, respectively, if a new chlorophyll molecule pays back the investment (in photons) during its lifetime. This is much longer than the turnover time for chlorophyll measured by Riper et al. (1979). However, as pointed out by Falkowski (1980), degradation products are perhaps used in synthesis of new chlorophyll, making synthesis much cheaper than calculated by Raven (1984). Further research is clearly needed to elucidate the regulation of pigment content in algae at very low light.

Chlorophyll content is obviously important for light absorption (Fig. 4), but the contribution of other pigments was not accounted for. In Ulva lactuca chlorophylls are the dominant pigments, and the observed close correlation between D,, and absorption is anticipated (Sand-Jensen 1988). In Por- phyra purpurea absorption and chlorophyll were also closely related, suggesting a parallel change in chlorophyll and phycobilins, which are important ac- cessory pigments in red algae. This is supported by the higher curvature of the relationship between absorption and chlorophyll for Porphyra purpurea than for Ulva lactuca. In Petalonia fascia (Fig. 4C) the relationship is weak, indicating that the brown pigments in this species have a significant influence on absorption and only a weak relationship to chlorophyll (Dring 1982).

Absorption leveled off at high D,,, for Ulva lactuca and Porphyra purpurea, suggesting an upper limit for absorption. This was also supported by the same maximal absorption in different experimental trials. A possible explanation for an upper limit to absorption, besides the regulation mechanisms proposed above, is an upper limit to the pigment content of chloroplasts. In two-layered Ulva lactuca and one-layered Porphyra purpurea, both with a fixed chloroplast number per cell, the thallus-specific chlorophyll content will be limited. The sieve effect (Rabinowitch 1951, Britz and Briggs 1987) means that even with complete absorption in the chloroplasts part of the light will pass through the thallus between the chloroplasts.

Quantum yield. Quantum yield of gross photosynthesis based on light absorbed by pigments (@gApigm)

should approach the theoretical limit of 125 mmol C.mol-' photons (Raven 1984). Individual values ranged from 60 to 225 mmol C.mol-' photons, but these values were normally distributed (Fig. 6) as expected for values subject to random experimental errors. Mean values for each experiment were close to, but below, the theoretical limit and similar to the highest quantum yield previously reported. BjSrkman and Demmig (1 987) found a mean +-value of 106 mmol O,.mol-' photons for gross photosynthesis in 37 species of terrestrial vascular C,-plants. These measurements were corrected for reflection but not for absorption by nonphotosynthetic components. This correction would bring their values close to those reported here. For Ulva rotundata

TABLE 2. Calculation of the margmal increase of fractional light absorption and number of photons absorbed at maximal thallus-specific chlorophyll content (DChl) in Ulva lactuca, Petalonia fascia, and Por- phyra purpurea. Also, the minimum lijetimefor a chlorophyll molecule if chlorophyll synthesis is favorable is calculated as explained in the text. To, b, and maximum Dchl were obtained from Figure 4, and irradiance of maximum Dchl was obtained from Figure 2. dA/d(D,,) is thefirst derivatizv of Eq. (4), and dIA Jd(DchI) is the increase in absorbed photons per increase in D,,,.

Parametei

TO 0.87 0.44 0.966 0.961 0.979 0.888 b

dA/d(Dcd (Cm2.fig-') 0.0133 0.0056 0.0280

dI,/d(Dchl) (firnol.s-'.g-' Chl) 6.65 7.02 5.60 Min. turnover time (h) 75 71 89

Maximum DChl (fig-cm-2) 24 24 12

Light of max D,,, (fimol.m-2.s-1) 5 12.5 2

@-values measured as gross photosynthesis of absorbed light were between 80 and 110 mmol 0,. mol-I photons in the morning but declined throughout the day to about 50 mmol O,.mol-l photons (Henley et al. 199 1). Luning and Dring ( 1 985) found similar values for different macroalgae species, and Sand-Jensen (1988) found a lower value (51 mmol O,.mol-' photons) for Ulva lactuca. These values are within or somewhat below the values for cbgA in Table 1 (70-100 mmol O,.mol-l photons). Ng and Bass- ham (1 968) found &values of 68-1 10 mmol C.mol-' photons for net photosynthesis in Chlorella, and Welschmeyer and Lorenzen (1981) found values of 96 mmol C- mol-' photons (14C-mea~~rement~) for Thalassiosira pseudonana. Similar values were found for Phaeodactylum tricornutum for photosynthesis (100-150 mmol O,-mol-l photons) and net growth (80 mmol C.mol-I photons) by Geider eta]. (1985, 1986). This comparison shows that @-values vary depending on techniques used and assumptions made, but usually they are about 10-2096 below the theoretical upper value. Mean values are expected to be somewhat below the theoretical upper maximum, because any individual "not up to par" will draw the value down. Values considerably below the theoretical value may occur due to photoinhibition (Henley et al. 199 1) or seasonal effects (Sand-Jensen 1988, Henley et al. 1991).

Quantum yield for gross photosynthesis based on pigment-absorbed light (C$~*P~P) reported here de- pends on calculation of A, and on the assumption that gross photosynthesis is obtained by adding dark respiration to net growth. Any overestimation of A, will reduce quantum yields to between @A and @A~i@"

(Table 1). Increased accuracy of @ A p i p could be obtained, particularly for brown algae, by establishing a relationship between absorption and total pigment content, thereby obtaining a better estimate for A,. According to definitions of quantum yields in the introduction, dark respiration represents the demand for ATP and reduced NADP for all processes other than carbon-fixation. The question is, therefore, whether respiration in the dark (measured here

62 STIIG MARKAGER

as mean values over 14-16 h) is representative for the demand of ATP and reduced NADP in the light. Based on the assumption that growth processes run at maximum rates in the light and gradually decrease in the dark (Markager et al. 1992), the dark respiration measured here is likely to be a low 24-h estimate, thus yielding a conservative estimate of gross I quantum yield.

Experimental quantum yields based on incident light are tightly coupled to the present light climate. In the field the spectral composition of light is variable but often more favorable for absorption in green and brown algae than the dominant green light emission from the cool-white fluorescent tubes used here. Experimental thalli were kept in an optimal position perpendicular to incoming light. With other angles between light and thallus, incident light will decrease per unit surface area of the thallus. T h e light utilization efficiency based on the down- welling irradiance in the field may, therefore, be lower. However, this may be compensated for by reflection from the underlying substrate, which may increase the fraction of incident light from above that is absorbed by the algal thallus. Although it is difficult to evaluate the light utilization efficiency in the field, the irradiance at the depth limit of marine macroalgae is so low (Agegian and Abbott 1985, Littler et al. 1985) that the +values obtained in this experiment are also likely to apply to field conditions (Markager and Sand-Jensen 1992).

Excretion of organic carbon is known to take place in macroalgae (Sieburth 1969), but it was not accounted for in this experiment. Excretion will increase the reported &values. Because q&*piP-values are close to the theoretical maximum, carbon exu- dates must be insignificant at low light. The opposite process, heterotrophic use of organic carbon, was observed in Ulva lactuca at high concentrations of glucose and acetate (Markager and Sand-Jensen 1990). However, a possible effect of heterotrophic uptake will probably only be significant at very low light (< 1 ~mol.rn-*.s-’) and high external substrate concentrations (Markager and Sand-Jensen 1990) rarely encountered in the field.

Calculation of the fate of light energy (Fig. 7) allows comparison of energy loss processes among species. For the thin species, light transmission is the dominant loss of incident energy, emphasizing the importance of pigment content and absorption for light utilization. “Loss,” however, may not be the proper term, because high transmission and low TSC are coupled (Fig. 3) and TSC determines the specific growth rate for a particular net carbon fixation per unit area. Low TSC is, therefore, advantageous at low light because thin species have the highest absorption per unit biomass. The loss of light energy by reflection, absorption by structural components, and suboptimal efficiency of conversion of light to carbon biomass (Fig. 7B, C) may be regarded as true losses. These losses constitute about 20% of incident

light energy and are approximately the same for all species. Respiration, or demand for ATP and reduced NADP besides what is needed for carbon fixation, constitutes 15-30% in green and brown algae but only 5% in red algae. Whether or not low respiratory demands are a general feature of red algae deserves further examination, considering that they often penetrate to the greatest water depths (Sears and Cooper 1978, Dring 1981, Mathieson et al.

In conclusion, the experimental evidence for macroalgae is consistent with the theoretical upper quantum yield of 125 mmol C-mol-I photons for fixation of inorganic carbon to simple carbohydrates. The five species tested here all approached this value during growth at low light irrespective of their different pigment systems. Differences in light utilization were mainly associated with the ability to absorb light, which, in turn, was tightly coupled to TSC. The processes leading to losses of absorbed light energy were small and of minor importance for the variability of light utilization efficiency among the five species.

198 1).

This work was supported by the Danish Natural Science Research Council by grant 11-7139. I thank T. Lieu, H. Nielsen, and S. Siune for technical assistance; T. Madsen, B. Olesen, H. Schierup, and K . Skovhus for valuable discussions; H. Frost-Christensen for improving the manuscript; and L. Mathiesen for help with col- lection and identification of the species. I am in great debt to Kaj Sand-Jensen for his help throughout this study.

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LIGHT ABSORPTION AND QUANTUM YIELD FOR GROWTH IN FIVE SPECIES OF MARINE MACROALGA1

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