Phytoplankton growth and light absorption as regulated by light, temperature, and nutrients DALE A. KIEFER and JOHN J . CULLEN * Kiefer, D. A . & Cullen, J . J . 1991: Phytoplankton growth and light absorption as regulated by light, temperature. and nutrients. Pp. 16x172 in Sakshaug. E.. Hopkins. C. C. E. & Britsland, N . A . (eds.): Proceedings of the Pro Mare Symposium on Polar Marine Ecology. Trondheim, 12-16 May 1990. Polar Research l O ( 1 ) . Numerous studies of the growth of phytoplankton in the laboratory have demonstrated the dependence of cellular pigment concentration and growth rate upon light intensity, photoperiod, temperature, and nutrient supply. These same environmental parameters vary with season in the polar seas and presumably affect the growth rate and cellular pigment concentration of the phytoplankton crop. Unfortunately, there has not been a complete mathematical description of the interaction of all four environmental parameters. This study presents an approach to describing these interactions. It can reasonably be assumed that the gross specific growth rate. g. is a function of the specific rate of light absorption: g = n r (l-exp(-a, $,,, E d n 6 ) ) . The dependent variables in this equation are g, the gross specific growth rate, n , the maximum carbon- specific photosynthetic rate, and, 6 . the ratio of carbon to chlorophyll. The value of all three dependent variables is constrained. The independent variables are E,). the light intensity (assumed constant during the photoperiod), and r. the photoperiod (as a fraction of 24 hours) that the cells are illuminated. n is the instantaneous capacity of the dark reactions to assimilate electrons, while the product a p $mrr E,JB is the instantaneous capacity of the light reactions to supply electrons. If the capacity for photochemistry exceeds the capacity for assimilation. dissipative processes occur, and the quantum yield is low. We have applied this equation to the analysis of the growth and light absorption by Skelefonema costafum cultured under light. temperature, and nutrient limitation. Decreases in nutrient supply and tcmpcrature cause decreases in n and increases in 6; thus both the capacity for electron supply and utilisation decrease. However. decreases in temperature decrease the capacity for electron assimilation more rapidly than the capacity for supply; quantum yield drops. Decreases in nutrient supply cause the capacity for supply and assimilation to drop in parallel; quantum yeield is maintained. Decreases in day length cause dccrcases in 0 and increases in n. The capacity to assimilate electrons and the capacity to supply electrons increase in parallel; quantum yield is maintained. Decreases in light intensity cause decreases in both Band the capacity to supply electrons. Although the changes in n with light intensity are difficult to assess. the capacity to assimilate electrons appears to be little changed by light limitation. Quantum yields increase with decreasing light levels. Dale A. Kiefer. Department of Biological Sciences, University of Southern California, University Park, Los Angeles. California WO89-0371, USA; John J . Cullen. Bigelow Laboralory for Ocean Science, McKown Point. West Boothbay Harbor, Maine 04575, USA. Introduction A number of factors may limit the growth rate of phytoplankton in the sea. These include light intensity, day length, temperature, and nutrient supply. Given t h e large seasonal variability that exists at high latitudes, it is likely that the extent to which each of these four variables determines the growth rate may fluctuate. For example in the Barents Sea, which is between 70--80"N, the water temperature ranges over the year between 0 and 6"C, the day length ranges between 0 and 24 hours, midday incident photosynthetically avail- able radiation ranges between 20 and 1000 p E , and the concentration of nitrate ranges m-2 s - I between 0.1 and 1 2 p m o l kg-'. Although the growth of phytoplankton is clearly influenced by this variability, there have been few, if any, pub- lications describing the effects of all four vari- ables - light intensity, day length, temperature, and nutrient concentation - upon growth rate. In this study a tentative description of the regu- lation of phytoplankton growth by more than o n e or two environmental parameters is presented. Through examination of data on growth rate and 164 D. A . Kiefer & J . J . Cicllen chemical composition obtained from the con- tinuous cultures of phytoplankton where all four factors mentioned above have been varied. i t is suggested that there are strong patterns in the responses of phytoplankton to variations in tem- perature. light, and nutrient limitation. This infor- mation has been incorporated into a model that provides a reasonably satisfactory description of the relationship between the light absorption, chemical composition. and growth rates of phyto- plankton. Model Many models of the growth of phytoplankton a r e energy budgets (Ryther & Yentsch 1957; W e b b et al. 1974; Jassby & Platt 1976; Geider 1990) that describe growth as a function of incident irradiance; photosynthetic efficiency and the amount of carbon that must be synthesized to make another cell. While such models are very useful, they d o little to explain how environmental factors regulate growth and chemical composition of phytoplankton (Cullen 1990). Additional insight comes from descriptions of the effects of irradiance, day length, nutrients, and tempera- ture on light absorption. photosynthetic efficiency. and chemical composition. Although these effects have been described many times. (Smith 1980; Laws & Bannister 1980: Fasham & Platt 1983; Kiefer & Mitchell 1983; Osborne & Geider 1986; Geider 1987: Falkowski e t al. 1985). a comprehensive description is still lacking. By examining experimental d a t a to specify how each environmental factor affects parameters of a simple model. this study aims at describing phytoplankton growth as regulated by light, tem- perature. and nutrients. T h e conceptual model used to relate the rate of light absorption by phytoplankton t o t h e growth rate is based upon both a phenomenological description and a mech- anistic description. T h e phenomenological description. which will be used t o analyze the data presented in the following section, simply states that the specific growth rate of phytoplankton is the product of the rate of light absorption by the cell and the quantum yield of cellular carbon fixation. More specifically, the following can be stated: g. which is the specific gross rate of photosynthesis and a dependent variable (units of d a y - ' ) , is a function of independent variables: light intensity, E,]: photoperiod. r; temperature, T; and nutrient concentration, N . g is equal t o t h e sum of the specific growth rate. 1-1, and the specific respiration rate (Bannister 1979). In this study the light regime is rectified; the lights are on (units of mol m-' day-I) for a fraction, r, of each 24-hour cycle; T is in units of "C; N is in units of mg-at m - I . a p is the chlorophyll a-specific absorption coefficient (units of m' mg-') and is assumed here to be a constant. 6 is the ratio of cellular carbon to cellular chlorophyll a (units of mg-at C mg Chl-I) and is a dependent variable. 4 is the quan- tum yield (units of mg-at C mol-I) and is a depen- dent variable. Although the two dependent variables may be functions of all four independent variables, we will see that this is true only for 8; Table of Symbols 8. n . r. E,, aP. I # J ~ . ~ ~ . 9. H . fc Fc . T.d 01 . '!IT. JCC. a dependent \ariable as indicated by equation 7 . i s the gross specific growth rate of the cells a dependent Varidbk. is the maximum. instantaneous. carbon specific rate of carbon fixation an independent Lariahlc. i s the duration of the photoperiod as a fraction of d ?+hour day a n independent bariable. i s the light intensity during the photoperiod a constant. i5 the chlorophyll (1 specific absorption coefficient of the cell suspension a constant and t h u s independent of light intensit!. tcmpcrature. day length. and nutrient concentration. is the a dependent variahlc as indicated by cqudtions ( 5 ) and ( 8 ) . is the quantum yield of carbon fixation a dependcnt variable. i s the ratio of cellular carbon to chlorophyll o a dependcnt \ a n a b l e as indicated b) cqudtion ( 2 ) . 15 the maximum. light-limited. instantancoub. carbon specific a dependent \ariahle a r indicated b! equation ( 3 ) . i s thc maximum. light-limited daily. carbon specific rate of dependent Lariahle. is the minimum steady state turnober time of the photosynthetic electron transport system a dependent \ariahie. 15 the mean. cffccti\c absorption crors-section of photosvrtem I I a dependent bariahle. is the ratio of the ccllular concentration of photosystcm I I to carbon a con\tant which depends upon the chemical composition of the cell. i s the ratio ot the number of carhon atnms maximum q u a n t u m bield of carbon fixation rate of carbon tixatinn carbon fixation fixed t o the numhcr of clcctronr p a s c d through t h e elcctron transport s p t c m Phytoplankton growth and light absorbance 165 14 H A J p C h l * , 1 ‘ r A t ji, 7 O2 t 28 H 7 H20 t 3 H t 14 H 2 0 (3xixc7( P reaajons reactions Chl AOX 7 C 0 2 t N H 3 7 O2 t 28 H Fig. 1 . A conceptual model of phytoplanktonic growth and rcgulation. Variations in light intensity. photopcriod, temperature, and nutrient supply will cause changes in both the rates of energy transformations and the cellular concentrations of the components shown in the figure. These components are the photosynthetic unit. the electron transport chain. and the cnzymes of the dark reactions. The cellular concentrations of these three components are regulated so that the growth rate is maintained without excessive dissipation of energy @ varies with temperature and light intensity but is little affected by variations in day length and nutrient Concentration. We represent the maximum, light-limited instantaneous carbon specific rate of carbon fix- ation as fc (units of day-’): The maximum, light-limited, daily carbon specific rate of carbon fixation is Fc: (3) The mechanistic description is represented in Fig. 1. Photosynthetic growth is described in terms of the cycling of the pool of electron trans- port compounds. symbolized by Aox/Arc, the oxi- dized and reduced forms of the carrier pool. Electrons are supplied to this pool by the activity of photosystem 11, symbolized by ChlII/Chl d , the ground and excited states of chlorophyll in the photosystem. Although not shown in the figure, each photosystem consists of a n antenna and a reaction centre. Photosystem I is not shown but is assumed to be under metabolic control (Foyer et al. 1990) and does not restrict electron flow. While rates of electron supply will depend directly upon the size and cellular concentration of the photosystems and upon light intensity, rates will also be regulated by day length, temperature, and nutrient concentration. Although evidence is limited, it appears that the cellular concentration of the electron transport pool is proportional to the concentration of photosystem I1 (Sukenik et al. 1987). Electrons are lost from the electron transport pool by the assimilation of inorganic compounds such as carbon dioxide and nutrients. While the rates of electron assimilation will dep- end directly upon the cellular concentration of the enzymes of the dark reactions and temperature, rates will also be regulated by day length, nutrient concentration, and light intensity. Transformations of the system include the dissipation of free energy in photosystem I1 by fluorescence and heat production, the dis- sipation of heat from the electron transport sys- tem by short circuiting of electrochemical gradients, and the dissipation of heat and excreted compounds by the “photorespiratory” pathways of the dark reactions. Of course, such dissipation causes reductions in the quantum yield of carbon assimilation. W e assume that the process of meta- bolic regulation involves adjustments t o maintain growth rates and t o minimize the costs that are associated with dissipation. (For example see Kie- fer & E n n s 1976 and Shuter 1979.) Such regu- lation can generally be achieved by adjusting the rate of supply of electrons from the light reactions with rates of utilization by the dark reactions. Specifically, this is achieved by varying the cellular concentrations of all three components shown in Fig. 1, photosystem 11, the electron transport pool, and the enzymes of the dark reactions. A t present we d o not have a complete mathematical formulation describing the steady state, optimized condition. A mathematical description of the relationship between light absorption and growth is most simply represented in terms of the cellular con- 166 D . A . Kiefer & J. J . Cullen centration of photosystem 11. its effective absorp- tion cross-section, and its minimum, steady state, turnover time. A similar derivation has been pre- sented by Bannister (1979). T h e minimum, steady state, turnover time of photosystem 11, t, is the minimum time required for the reaction center to process an electron under steady state conditions (Myers & Graham 1971). T h e time required for charge separation by the unit is many times shorter. t, which is a dependent variable (unit of days), can be defined as the quotient of the cellu- lar concentration of photosystem I1 and the maxi- mum carbon specific rate of photosynthesis: r = qll jce/II. (4) q I I , a dependent variable (in units of m gm-at C), is the concentration of photosystem I1 normalized to cellular carbon (units of moles mg-at C - ] ) . n, a dependent variable, is the maximum carbon specific rate of photosynthesis (units of d a y - ] ) . jce, which is a constant, is the stoichiometric coefficient of the number of carbon atoms fixed per electron that is passed through the transport chain (units of gm-at C mole e - l ) . Increases in the concentration of reaction centers tend to increase the turnover time while increases in t h e maximum rate of carbon assimilation tend t o decrease the turnover time. If the time between the capture of photons by photosystem I1 is less than the minimum turnover time of the photosystem, the photosynthetic quantum efficiency will be reduced. This relationship is expressed by the Poisson distribution function (Dubinsky e t al. 1986; Peterson e t al. 1987; Cullen 1990): (5) @,,,, a constant, is the maximum photosynthetic quantum yield; its value can be measured when the time between each capture of a photon by the photosystem is much longer than the minimum turnover time. uI1 is the effective cross-section of photosystem I1 (units of m2/mole) (Ley & Mauzerall 1982). The mean specific absorption coefficient. ap, is assumed to be contributed solely by the photo- synthetic pigments of photosystems I and 11. If the effective absorption by the two photosystems is equal, o n e writes: @ = @ma, ( ~ - ~ x P ( - u I I TE"))/UII rEw By appropriate substitution within equations 1, 1. 5 , 6. o n e eliminates the two dependent variables. n l l and ull and obtains an equation for gross specific growth rate in terms of two constants, @,,, and ap, two independent variables, Eo and r, and two dependent variables, I3 and 8: g = nT (1 -exp( -ap &,axEo/n - 8)). (7) The quantum yield is G = no (l-exp(-a, $ma, Eo/n * 8 ) / a p Eo. (8) Equation (7) is the exponential form of photo- synthetic response curve. 8II/a,@,,, is Ik, g 8 is P i . If it is assumed that a p and $I,,, a r e constants, then variations in quantum yield that a r e effected by temperature, day length, a n d nutrient con- centration must result from changes in the product no. A s mentioned earlier, it appears that this product is sensitive to changes in light intensity and temperature but not to changes in day length and nutrient concentration. Growth and light absorption in cultures The laboratory studies used to examine t h e model presented above provide information on changes in two (g and 8 ) of the three dependent variables which are effected by variations in light intensity, day length, temperature, and nutrient supply. Two studies which describe the steady state growth of Skeletonema costatum will now be examined. O n e study by Yoder (1979) examined the cellular concentration of chlorophyll a and growth rate of cells limited by light intensity and temperature. T h e other study by Sakshaug et al. (1989) examined cellular concentration of chloro- phyll a and growth rate of cells limited by light intensity, day length, a n d rates of nutrient supply. Cultures in these studies were grown a t 15°C. It will be seen that g, n, a n d 8 vary with tempera- ture, nutrient concentration, light intensity, and day length. It will also be seen that although the product n8 varies little with day length and nutrient concentration, it does vary with tem- perature. Thus, only light intensity and tem- perature effect the steady state quantum yield of carbon assimilation. Yoder (1979) grew Skeletonema costaturn in turbidostat a t five temperatures (0, 5 , 10, 16, and 22°C) and at five light intensities. Although cultures were also grown under a number of dif- ferent photoperiods, values for the ratio of cellu- lar carbon to chlorophyll u were reported only for Phytoplankton growth and light absorbance 167 - 2.00 .- 1.60 -- 1.20 F -- 0.80 -- 0.40 * Clchl 1- growth rate 10.00 8.00 6.00 0 4.00 2.00 Fig. 2. Variations in specific growth rate, p (d-I). and the cellular ratio of carbon to chlorophyll, L9 (gm-at C gm Chl-', of Skeletonema costatum caused by variations in temperature. The five cultures were grown under light intensities of about 130 pmol m-* SKI and a photoperiod of 12 hours. - ._ - - .- -- 6.00 5.00 4.00 e 3.00 - .- .- -- 0.00 L 0.00 -- t 0 .- 0.90 - - 0.60 P 0.30 0.00 a 12-hour light and 12-hour dark cycle. Sakshaug et al. (1989) grew Skeletonema costatum con- tinuously by daily dilution with a medium with an elemental composition that ensured nitrogen limitation. The cultures were maintained at 20°C, at six light intensities, ranging from 12 to 1200 pE * m-2 - s-l, 3 photoperiods, 6 h light, 14 h light, and 24h light, and from 4-6 rates of dilution. The matrix of light intensities, dilution rates, and photoperiods was not complete. Both studies include measurements of the temperature, light intensity, day length, specific growth rate, p, and the ratio of cellular carbon to chlorophyll a , 8. The data from these two studies have been analyzed by introducing values for the measured independent and dependent variables into equation (7). ap has been assigned a value of 0.016 m2 mg Chl-' and a value of 0.10 g-at mol-'. Since neither g nor the specific rate of 5 10 1 6 2 2 Temperature respiration in the dark was measured, we assumed that g was equal to p and that dark respiration was only a small fraction of p. This assumption is questionable for cultures grown at very low light intensities or very short photoperiods. Growth rates and cellular concentrations of chlorophyll a Figs. 2, 3, 4, and 5 summarize the changes in growth rate and cellular chlorophyll a con- centration that occurred with limitation by the four independent variables, light intensity, day length, temperature, and nutrient supply. Tem- perature limitations (Fig. 2) causes decreases in growth rate that are accompanied by increases in the ratio of cellular carbon to chlorophyll a . Such decreases in growth rate have been interpreted to result from decreases in the maximum specific I .A C/chl -.- growth rate T 1.20 168 D . A . Kiefer & J . J . Ciilleri 1 .1 C i c h l growth fate 1 - I 5 .. 1 2 -- 0.9 Fig. 4. Variations in specific growth rate. p ( d - I ) . and the cellular ratio of carbon to chlorophyll. tJ (gm-at C gm caused by variations in photoperiod, r. The three cultures werc grown at 0 6 o . 3 Chl-I). of Skeleronernu costalum 0 o 1S"C and under a light intensity of 0.25 0 . 5 8 r activity of enzymes of the dark reactions. Limi- tations by light intensity (Fig. 3) cause decreases in growth rate that are accompanied by decreases in the ratio of cellular carbon t o chlorophyll a . Growth rate declines with Eo because the cellular concentration of chlorophyll a does not increase sufficiently with decreasing intensity. Limitations by day length (Fig. 4) cause decreases in growth rate that are accompanied by little change in the ratio of cellular carbon to chlorophyll a . Since the instantaneous rate of cellular light absorption remains constant for the three photoperiods, the decreases in growth a r e simply caused by decreases in the daily rate of light absorption. Limitations by rates of nutrient supply (Fig. 5) cause decreases in growth rate that are 1 .oo 100pmol m-' S K I and with a rapid wpply of nutrients. accompanied by increases in the ratio of cellular carbon t o chlorophyll a . Growth rates and maximal daily rates of photochemistry The data from the two studies can b e further analyzed by examining the relationship between the specific growth rate of the cultures, p, and the cell's maximal daily capacity to supply electrons, Fc of equation (3). In t h e case of temperature and light limitation t o growth the relationship is complicated by a n absence of linearity. As shown in Fig. 6, growth rate is a curvilinear function of Fc. Furthermore. this relationship is clearly a function of temperature; temperature decreases 5 0 T \ ~ ',, e 30 t Fig. 5. Variations in the cellular ratio of carbon t o chlorophyll. H (gm-at C grn C h l - l ) , of Skeleronernu cosfurum caused by variations in >pccific rates of nutrient supply. T h c culture3 were grown a t 15°C. at the light intcnsitics 2 0 + 40 i "'L c* c-- .-.----_ I 0 2 0 4 0 6 0 8 1 1 2 I 4 shown (pmol m - : 5 - l ) . and under a u photoperiod of 24 hours. 0-' Phytoplankton growth and light absorbance 169 rate. variations Fig. 6. p. Variations of in Skeletonemu cellular. in specific daily costaturn capacity growth with of ( d a y - 1 p 0.50 1.00 1 electron supply, Fc of equation 3. The grown at the temperatures shown in variations between cultures that were 0.00- *.O0 T 1 _ _ .- .@.@ 0 T=O X- T=5 * T=10 + T=16 * T=22 [3x ~ _ _ . ... ~ ~~~~ ~ ~ .. . ,-.~- ~~~ ~ . -., .. - cause decreases in rates of electron supply that saturate growth. It is apparent from this figure that a t 5°C a daily rate of electron supply of 0.5 d - ' is sufficient t o meet a rate of assimilation of about 0.6 d-I. A t 22°C a daily rate of electron supply of 2.0 d-' is needed t o meet maximal daily rates of assimilation of about 1.8 d-'. In the case of limitations t o growth by light intensity, day length, and nutrient supply, the relationship appears t o b e linear. A s shown in Fig 7, when Skeletonema's growth rate at a given light intensity is plotted as a function of its daily capacity for photochemistry, the relationship is linear despite changes in day length and rate of nutrient supply. T h e slope of this line, which in Fig. 7. Variations in spccific growth rdtc, p. of Skeletonemu costaturn with variations in cellular. daily capacity for electron supply. Fc of equation (3). The variations bctwecn those cultures that were grown under the light levels, E,, (pmol m-? s-I). shown in the legend were caused hy variations in both rates of nutrient supply and photoperiod. The temperature was 15°C. 0 '.I 1 fact is the proportional to quantum yield, increases with increasing light intensity. Quantum yields and growth rates T h e variations in quantum yield implied in Figs. 6 and 7 are explicitly presented in Figs. 8 and 9. When the quantum yields for growth at a given light intensity are plotted as a function of growth rate, there is evidenced a clear distinction between the adaptation to limitation by tem- perature and the adaptation t o limitation by either day length or rate of nutrient supply. As shown in Fig. 8, decreases in the temperature-dependent growth rate a t a given light intensity are 0 S 170 D . A . Kiefer & J . J . Cullen 0 0 4 . 0.02 0 00 0 50 1 0 0 1 5 0 u ( d a y - 1 ) accompanied by decreases in quantum yield. Each of the five categories of light intensity display a trend of decreasing yield with decreasing growth rate. One also notes by comparing the five cat- egories that quantum yields tend t o decrease with increasing light levels. Decreases in growth rate at a given light level effected by decreases in nutrient supply or day length d o not appear to be accompanied by decreases i n quantum yield (Fig. 9). An exam- ination of the six categories of light levelsindicates that in four of the six categories there is little or no covariation between quantum yield and growth rate. The two exceptions are the categories of the 4 2.00 lowest light levels, 12 and 41 ymol m-* d - ' , where decreases in quantum yield appear to parallel decreases in growth rate. Again, it can be noted that quantum yields decrease with increases in light levels. Capacity for instantaneous electron supply and assimilation The final analysis of the two studies of Ske- letonema consisted of introducing values for Eo, r, and 8, into equation (7). The equation is then solved for n , the maximum, instantaneous, specific rate of carbon fixation, by introducing 004 t I 0 0 2 4 0 4 - 0 Fig. 8. Variations in the photosynthetic quantum yield, I#J (gm- at C E-I). of Skeletonema costaturn w i t h variations in specific growth rate. p. The variations between those cultures that were grown under the light levels. 6, (pmol m - * s-I). shown in the legend were caused by variations in temperature. The photoperiod was 12 hours. A * o Fig. Y. Variations in the photosynthetic quantum yield. I#J (gm- w i t h variations in specific growth rate. p. The variations between those *I at C E - ' ) . of Skeletonema costaturn * o ; 0 0 0 0 cultures that were grown under the " light levcls. E,, (pmol m - ' s - ' ) . shown in the legend were caused by variations in nutrient supply and 0 2 0 . 4 0 6 0.8 1 1.2 1.4 photoperiod. The temperature was ( d a y - 1 ) 15°C. Phytoplankton growth and light absorbance 171 significantly greater than the rate of change in photochemistry. While at 15°C and above, the specific capacity for electron supply is comparable to the specific capacity of electron assimilation; at 0°C the capacity for supply is about three times greater than the capacity for assimilation. The large decreases in quantum yield with decreases in temperature are explained by the differences between the two slopes. Decreases in nutrient supply (Fig. 11) cause decreases in both the capa- city to supply and assimilation electrons. However, in the case of nutrient limitation, decreases in capacity for supply are paralleled by decreases in assimilation. Thus, unlike tem- perature adaptation, adjustments of the light and dark systems to nutrient limitation help to main- tain the constancy of the quantum yield. Decreases in light intensity cause decreases in the cellular instantaneous capacity to supply elec- trons but have little effect on the capacity to assimilate electrons (Fig. 12). Thus, quantum yields increase with decreasing light levels. Unfor- tunately, values of ll calculated from equation 7 for light limitation are too variable to allow us to confidently conclude that there is no trend. On the other hand, there is abundant evidence that quantum yields increase with decreasing light levels, and thus cellular capacity for electron supply must drop more rapidly with light level than cellular capacity for electron assimilation. Finally, unlike decreases in light intensity, decreases in day length cause increases in both 4.00 - 3.00 ~ ( d a y l ) 2.00 - 1.00 - 4 ' 0 0 2.00 I- I c 0.00 I 0 5 I 0 I5 2 0 2 5 Temperature Fig. 10. Variations in the cellular, instantaneous capacity for electron supply by the light reactions, fc of equation (2):and the instantaneous capacity for electron assimilation by the dark reactions, n. with variations in temperature. The five cultures were grown under light intensities of about 130 pmol m-* s-' and a photoperiod of 12 hours. values of 1 for g. Changes in ll with temperature, light level, day length, and nutrient supply are then compared with changes in maximum instan- taneous rates of photochemistry, fc, defined in equation (2). Such a comparison provides insight into the differences in adaptation to the four environmental variables. Decreases in tempera- ture at a given light level and photoperiod (Fig. 10) cause decreases in both the maximal rates of photochemistry and maximal rates of assimilation. Of most importance is the rate of change in assimilation with temperature which is 5.00 T 0.00 J I 0 0.2 0.4 0.6 0.8 1 I .2 P ( d a y l ) Fig. 11. Variations in the cellular, instantaneous capacity for electron supply by the light reactions, fc of equation 2, and the instantaneous capacity for electron assimilation by the dark reactions. n. with variations in specific rates of nutrient supply. p. (p = dilution rate). The six cultures werc grown under a light intensity of about 100 pmol m - 2 s - ' and a photoperiod of 14 hours. T 16 .' r0.5 0.00 I 0 4 0 8 0 120 160 200 E o ( u E m . 2 ~ - I ) Fig. 12. Variations in the cellular, instantaneous capacity for electron supply by the light reactions, fc of equation (2). and the instantaneous capacity for electron assimilation by the dark reactions, n. with variations in light intensity, E,). The five cultures were grown at 16°C and under a photoperiod of 12 hours. 172 D. A . Kiefer & J. J. Cullen i 4 03s Eo 1W o o a 8 0 DO 0 20 0 4 0 0 6 0 0 8 0 I00 r Fig. 13. Variations in the cellular. instantaneous capacity for electron supply hy the light reaction\. fc of equation ( 2 ) . and the inStantancous capacity for electron assimilation hy the darh reactions. n. u i t h \ariations in photo-pcriod. r. T h e three cultures were grown at IYC under light intensities of ahout 1001~mol m ' 5 ' and at a dilution rate of 0 . 3 5 d ~ ' . the cellular instantaneous capacity to supply and assimilate electrons (Fig. 13). Since changes i n both capacities parallel each other, photosyn- thetic quantum yields a r e little affected by changes i n day length. These results indicate that adaptation to day length is to a large extent inde- pendent of adaptation to light intensity. A r k n o w l e d g e m e m . - This work was sponsorcd by both the Oceanic Processes Branch of the National Aeronautics and Space Administration (NAGW-317 and NAGW-2072) and the Biological Oceanograph) Program of the Office of N a v a l Research (N~XX)14-RY-J-30XI. 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