P versus Ed curves

In the dark there is of course no photosynthesis and aquatic plants exhibit a net consumption of O2 and liberation of CO2, due to cellular respiration. As the light intensity is gradually increased from zero, some photo-synthetic O2 production and CO2 consumption takes place, but at very low intensities shows up as a diminution in the rate of O2 consumption rather than a net liberation of O2, i.e. there is significant gross photosynthesis but no net photosynthesis. An irradiance value (Ec) is eventually reached at which photosynthetic oxygen liberation just equals respiratory oxygen consumption: this is the light compensation point. Beyond this point liberation exceeds consumption and net photosynthesis is achieved. The typical pattern of behaviour from here on is that P increases linearly with Ed up to a certain value. The graph then begins to curve over and eventually levels off. In the range of irradiance values where P does not vary with Ed, photosynthesis is said to be light saturated, P now having the value Pm, the maximum photosynthetic rate. The biomass-specific maximum photosynthetc rate, P*m is sometimes referred to as the photosynthetic capacity. With further increase in irradiance, P begins to decrease again, a phenomenon referred to as photoinhibition. Figure 10.1 shows two typical P versus Ed curves, one for marine phytoplankton,1159 the other for a mixed population of freshwater diatoms.91 P versus Ed curves for macrophytes have a similar shape but often do not show

1mt> tsod kkin i'raninrcc li-ntf n^-lons HAH'sn nctTEisccl

Fig. 10.1 Relative specific photosynthetic rate (P/Pm) of phytoplankton as a function of irradiance (Ed). Marine phytoplankton from the mid-point of the euphotic zone in the Sargasso Sea (redrawn from data of Ryther and Menzel, 1959) ---•—. Freshwater diatoms (mainly Asterionella formosa and Fragilaria crotonensis) in Lake Windermere, England (redrawn from data of Belay, 1981) ---o---. Appropriate conversion factors have been used to convert the authors' original Ed values to mmol photons PAR m~2 s~1

photoinhibition in full sunlight. Figure 10.2 shows the variation of P with irradiance in four freshwater macrophyte species,1394 and in green, brown and red marine algal species.691

Because of the gradual onset of saturation as Ed increases, it is difficult to pinpoint the irradiance value at which photosynthesis is just saturated. A more easily measured parameter by means of which the onset of saturation may be characterized1334 is the irradiance (Ek) at which the maximum rate, Pm would be reached if P were to continue to increase linearly with Ed. The value of Ek is that value of Ed corresponding to the point of intersection between the extrapolated linear part of the curve and the horizontal line at Pm. This is illustrated on the idealized P versus Ed curve in Fig. 10.3. It can be seen that the slope, a, of the linear part of the curve is equal to Pm/Ek: a is the rate of photosynthesis per unit biomass

Fig. 10.1 Relative specific photosynthetic rate (P/Pm) of phytoplankton as a function of irradiance (Ed). Marine phytoplankton from the mid-point of the euphotic zone in the Sargasso Sea (redrawn from data of Ryther and Menzel, 1959) ---•—. Freshwater diatoms (mainly Asterionella formosa and Fragilaria crotonensis) in Lake Windermere, England (redrawn from data of Belay, 1981) ---o---. Appropriate conversion factors have been used to convert the authors' original Ed values to mmol photons PAR m~2 s~1

¿1 L

K

F E ! 1 ■ 1 ■ 1

' *———_ ■ 1 1

L i,- I rr«Jimotr Efj fnrbirrBrv ni 1 s

L i,- I rr«Jimotr Efj fnrbirrBrv ni 1 s

Fig. 10.3 Idealized curve of specific photosynthetic rate (P) as a function of irradiance (Ed) illustrating the maximum photosynthetic rate, Pm, and the saturation onset parameter, Ek. The variation of P/Ed (a measure of the efficiency of utilization of incident light) with irradiance value is also indicated ( ).

per unit of incident irradiance, and so is a measure of the efficiency with which the biomass utilizes light, at low intensities, to fix CO2.

The light intensity required to saturate photosynthesis, and the compensation point, vary markedly from one species to another. Furthermore, as we shall see in a later section, these parameters also depend on the CO2 concentration and the temperature. Thus, if measurements of the photosynthetic response as a function of light are to be ecologically meaningful, they must be measured under conditions approximating those in the aquatic ecosystem. In the case of measurements on phyto-plankton the effect of photoinhibition, which develops to a greater extent

Caption for Fig. 10.2 Photosynthetic rates of aquatic macrophytes as a function of irradiance of PAR (a) Freshwater macrophytes from lakes in Florida, USA (by permission, from Van, Haller and Bowes (1976) Plant Physiology, 58, 761-8. The rates are limited by low, but typical in situ, CO2 concentration (0.42mgl-1). Temperature is 30°C. L.S. = irradiance required for light saturation. L.C.P. = irradiance at the light compensation point. ^Vmax = irradiance required for half-maximal photosynthetic rate. (b) Green, brown and red multicellular algae from the western Baltic Sea (plotted from data of King and Schramm, 1976). The samples of Ulothrix speciosa (eulittoral green), Scytosiphon lomentaria (eulittoral brown) and Phycodrys rubens (sublittoral red) were collected in the spring and the measurements were carried out at 10 ° C. The published irradiance values have been converted from mWcm~2 to mmolphotonsPARm^s-1.

during incubation in stationary bottles than in freely circulating cells (see later), is to lead to underestimation, both of the irradiance required for light saturation and of the maximum photosynthetic rate.867 Thus, for phytoplankton, P versus Ed data obtained in relatively short incubations are to be preferred: there is inevitably considerable uncertainty associated with results obtained from incubations of many hours' duration, such as are required for unproductive oceanic waters. The ability of aquatic plants, including phytoplankton, to utilize light of any given intensity can be highly dependent on the light climate to which they were exposed during growth: it is therefore preferable, for ecological interpretation, if P versus Ed curves are measured on naturally occurring, rather than laboratory-grown, plant material. Table 10.1 presents a selection of published data on the irradiance values that have been observed to correspond to light compensation, onset of light saturation and saturation, in a range of naturally occurring aquatic plants. The data do not permit any firm generalization with respect to differences in light saturation values between one algal class and another. This is partly because of the inherent difficulty in identifying the saturation irradiance in a P versus Ed curve. Comparisons would be facilitated if the practice of always recording the more easily identified parameter, Ek, corresponding to onset of saturation, was generally adopted. One permissible generalization is that for any given algal species the light compensation point is lower in the winter or spring than in the summer or autumn: whether this is due simply to the differences in temperature or whether other factors are involved remains uncertain.691 Within some species the greater the depth from which the sample was taken, the lower the irradiance required to saturate photo-synthesis:1101 this is true of phytoplankton as well as macroalgae.1297 Adaptation to different ambient light levels is discussed more fully in Chapter 12. Within the phytoplankton there is evidence that the dinofla-gellates have higher respiration rates, and therefore higher light compensation points, than the diatoms:382,1258 this may be due to the energy required to sustain motility in the former group.

In the measurement of P versus Ed curves by standard procedures, the photosynthetic system - angiosperm leaves, phytoplankton suspensions, macrophyte thalli - is given enough time, at least some minutes but often much longer, to settle down at each new irradiance value used. With the advent of fluorometric methods of measuring electron transport rates in photosystem II, such as PAM fluorometry, it has become possible to measure photosynthetic response as a function of irradiance very much more rapidly, e.g. carrying out measurements of ETR at a series of eight

Table 10.1 Irradiance values required for satura tion and light compensation of photosynthesis in various aquatic plants. Only data obtained for naturally occurring plant material, measured in natural water or its equivalent, have been used. Irradiance values published in other units have where necessary been converted to mol quanta of PAR m~2 s(where 1.0 mol quanta = 6.02 x 1017 quanta) with the use of appropriate conversion factors. In many cases saturation onset, E^. or compensation irradiance values, where not published, have had to be estimated from authors' data; the uncertainty is particularly great in estimates of the irradiance required to saturate photosynthesis.

Table 10.1 Irradiance values required for satura tion and light compensation of photosynthesis in various aquatic plants. Only data obtained for naturally occurring plant material, measured in natural water or its equivalent, have been used. Irradiance values published in other units have where necessary been converted to mol quanta of PAR m~2 s(where 1.0 mol quanta = 6.02 x 1017 quanta) with the use of appropriate conversion factors. In many cases saturation onset, E^. or compensation irradiance values, where not published, have had to be estimated from authors' data; the uncertainty is particularly great in estimates of the irradiance required to saturate photosynthesis.

Species or plant type

Location, season

Temperature °C

At saturation

At onset of saturation Ek

At compensation point

Refei

Freshwater algae

Diatoms

Asterionella formosa

L. Windermere,

5

-

28

-

1333

England. Spring

10

-

50

-

Melosira italica

L. Windermere, England.

5

-

16

-

1333

Winter

Blue-green algae

Microcystis etc.

L. George, Uganda

27-34

-

135-323

-

431

Arthrospira fusiformis

L. Bogoria, Kenya

28

-

41

-

1003

L. Nakuru, Kenya

24

-

38

-

1003

L. Elmentaita, Kenya

23

-

128

-

1003

Oscillatoria sp.

L. Neagh, Ireland. Spring

9

14

49

-

641

L. Neagh, Ireland. Summer

15

203

64

-

641

Green

Cladophora glomerata

Green Bay, L. Michigan,

25-27

345-1125

-

44-104

USA, July-Aug

Freshwater macrophytes

Hydrilla verticillata Ceratophyllum demersum Myriophyllum spicatum Cabomba caroliniana Myriophyllum brasiliense Vallisneria americana Nuphar japonicum Floating leaf Submerged leaf Deep-water characean species Chara fibrosa Chara globularis Chara corallina Marine microalgae Oceanic phytoplankton 0 m Oceanic phytoplankton

10m 80 m

Oceanic picoplankton 10 m 100 m

NE Tropical Atlantic Ocean Phytoplankton

Lakes, Fla., USA Lakes, Fla., USA Lakes, Fla., USA Lakes, Fla., USA Orange L., Fla., USA Lake, Wise., USA. Summer Japan

L. Coleridge, New Zealand

Pacific Ocean (3° S) Pacific Ocean, off Japan. Summer

Coral Sea, Oct-Nov

Eutrophic site Mesotrophic site Near surface Mid-point euphotic

250-300 - 42-45 1166

72-245

1267

23.5 ±2.3 8.3 ± 1.6 24.1 ±4.3 4.8 ±1.1 22.3 ±4.9 1.5 ±0.2

1332

~240 150-160

Species or plant type

Location, season

Temperature At At onset of compensation

°C saturation saturation Ek point Reference

North Atlantic Ocean

Continental shelf phytoplankton Coastal phytoplankton

(1-10 m) Coastal phytoplankton (0 m) 3 Feb 15 July

31 Oct

Coastal phytoplankton lm

32 m

Surf zone phytoplankton

Oligotrophic site Near surface Mid-point euphotic Bottom of euphotic Azores Front, ~34° N Surface

Deep chlorophyll maximum (~100 m) Bransfield Strait, Antarctica,

Dec-March Nova Scotia, Canada, all year

Baltic, Denmark

S. California Bight, USA

Algoa Bay, Sth Africa

0-15

17 12

16 12 15

22.5

-300

400 1200 800

700 1000

206 28

200 500 300

254 42 300 450

581, 915

1063

1298

1252 198

Estuarine phytoplankton

Prorocentrum mariae-lebouriae

Surface mixed layer (0.5 m) Subpycnocline (15 m) Intertidal benthic diatoms Sea-ice algae (pennate diatoms) Sea-ice algae (pennate diatoms) Platelet ice diatoms

Benthic diatoms (26 m) Subtidal microphytobenthos

Seagrasses

Cymodocea nodosa Halophila stipulacea Posidonia oceanica Phyllospadix torreyi

Zostera marina Zostera angustifolia Halophila ovalis meadow, 14-16 m depth

Chesapeake Bay, USA. June

Cape Cod, USA. Summer

Canadian Arctic. Spring

McMurdo Sound, Antarctica.

Summer McMurdo Sound, Antarctica.

November Port Phillip Bay, Australia.

Werribee site, 10 m depth July (winter) March (early autumn)

Malta

Calif., USA Scotland

South Sulawesi, Indonesia

25 25 17 15 15 10

3-25 25

12.2

~320

158 83 108 149 208 133

0.18

-400

17 21 25 12

1349 258 1033 1139 811

323 367

Species or Temperature At At onset of compensation plant type Location, season °C saturation saturation Ek point Reference

Marine macroalgae

Chlorophyta

Enteromorpha intestinalis W. Baltic.

Marine macroalgae

Chlorophyta

Enteromorpha intestinalis W. Baltic.

Spring

10

450

-

6

691

Summer

20

-

-

15

Calif., USA

21

245

56

6

29

Cladophora glomerata

W. Baltic. Summer

20

700

-

8

691

Acrosiphonia centralis

Spring

10

200

-

7

Ulothrix speciosa

Spring

10

700

-

6

Monostroma grevillei

N. Baltic

5

120

-

-

1430

Ulva lactuca

Woods Hole, Mass. USA. Summer

23

250

160

1101

Ulva lobata

Calif., USA

16

245

76

11

29

Ulva rígida

21

412

50

9

Codium fragile

21

346

50

9

Chaetomorpha linum

21

418

82

10

Phaeophyta

Fucus serratas

W. Baltic. Winter

5

350

-

5

691

Autumn

15

200

-

12

Laminaria saccharina

W. Baltic. Winter

5

25

-

4

Autumn

15

700

-

18

Scytosiphon lomentaria

W. Baltic. Spring

10

700

-

8

Ectocarpus confen'oides

W. Baltic. Spring

10

200

-

5

Laminaria solidungula

Dictyosiphon foeniculaceus Pilayella littoralis

Macrocystis integrifolia

M. pyrifera

Nereocystis luetkeana

Fucoid-dominated macroalgal community

Rhodophyta

Dumontia incrassata

Phycodrys rubens

NE Greenland, Young Sound. New leaf blades. June, 10 m, under ice August, open water 2.5 m depth 10 m depth Alaskan High Arctic.

Summer N. Baltic N. Baltic

Vancouver I., BC,

Canada. September S. Calif., USA.

March-August Vancouver I., BC,

Canada. February September South Australia Summer Winter

W. Baltic. Winter

Spring W. Baltic. Spring

300 200 100 80

642 447

100 500 200

170 17

38-46

140-300

22 64

214 149

18 20

340 1430

1256

1463

Irradiance, |j,mol quanta (PAR) m 2 s 1

At

Species or

Temperature

At

At onset of

compensation

plant type

Location, season

°C

saturation

saturation Ek

point

Reference

Phycodiys rubens

Summer

20

200

-

14

Polysiphonia nigrescens

W. Baltic. Spring

10

400

-

7

Autumn

15

300

-

24

Ceramiiun tenuicome

N. Baltic

11

100

-

-

1430

Rhodomela confen'oides

N. Baltic

4, 10

40

-

-

Chondrus crispus

Woods Hole, Mass., USA. Summer

23

120

60

1101

Porphyra umbilicalis

Woods Hole, Mass., USA. Summer

23

250

90

-

Coral

Stylophora pistillata

Sinai, Egypt

378

High light form

28

600-2000

-

350

Low light form

28

200

-

40

Coral reef algal turf

Virgin Islands, Caribbean Sea. Jul, Oct, Nov, Dec

28

1400-1800

780-1060

60-105

209

different Ed values at intervals of only 10 to 15 s. ETR versus Ed curves obtained in this way are often referred to as rapid light curves (RLC). Although an RLC looks very like a classical P versus Ed curve, it is not the same since the cells have not been allowed to adapt to each actinic light intensity before the rate measurement is taken. Hawes et al. (2003), using PAM fluorometry to study photosynthesis in submerged meadows of the quillwort, Isoetes alpinus, in Lake Wanaka, New Zealand, found that for plants at 7 m depth the Ek value obtained from an RLC (117 mmol photons m-2 s-1) was markedly lower than that obtained (164 mmol photons m-2s-1) from leaves adapted to the varying ambient irradiance during the day. Ralph and Gademann (2005) offer suggestions as to how RLCs may usefully be interpreted.

A number of attempts have been made to find mathematical expressions that give a reasonable fit to the empirical curves relating P to Ed. Since it is a fact of observation that for any given phytoplankton population the curve will exhibit a fairly well-defined initial slope and a maximum, asymptotically approached, value of P, the values of a and Pm being characteristic of that population, then we may reasonably anticipate that the relationship we are seeking will express P as a function of a and Pm, as well as Ed. Furthermore the relationship will be such that P = f (a, Pm, Ed) reduces to P = aEd as Ed tends to zero, and approaches P = Pm as Ed tends to infinity. Jassby and Platt (1976) tested eight different expressions that have at various times been proposed, against 188 P versus Ed curves measured for marine phytoplankton in coastal Nova Scotia waters. The two that fitted best were

originally (in a somewhat different form) proposed by Smith (1936), and

proposed by Jassby and Platt (1976), the latter expression giving somewhat the better fit. These two expressions for P have been chosen simply on the basis of goodness of fit to the observations: they are not based on any assumptions about the mechanisms of photosynthesis.

There is a third, equally simple, equation

"Ed

originally proposed by Webb et al. (1974) to describe photosynthesis in the tree species, Alnus rubra, and which Peterson et al. (1987) have found to satisfactorily describe P versus Ed curves in a wide range of phyto-plankton systems that can be given a plausible rationale in terms of the mechanism of photosynthesis. By application of simple Poisson distribution statistics to the capture of photons by the photosynthetic unit per unit of time t, where t is the turnover time, and assuming that excess photons are not utilized, it can be shown that the rate of photosynthesis is proportional to (1 - e-m) where m is the mean number of incident photons captured by the photosynthetic unit in time t.1050 Since m must be proportional to the incident flux, Ed, it is clear that eqn 10.10, or its alternative version

(since Pm/a = Ek), is in accordance with this simple mechanistic model.

Equations 10.8 to 10.11 describe the variation of P with Ed only up to the establishment of saturation; they do not encompass the decline in P at higher values of Ed. Platt et al. (1980) have obtained an empirical equation that describes the photosynthetic rate of phytoplankton as a single continuous function of available light from the initial linear response up to and including photoinhibition.

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