Diel periodicities of photosynthesis in antarctic phytoplankton
Diel periodicities of photosynthesis in antarctic phytoplankton: Species-specific responses M. PUTT Department of Biological Sciences University of California at Santa Barbara Santa Barbara, California 93106
R.B. RIvKIN and M. MOLINE Horn Point Environmental Laboratories Center for Environmental and Estuarine Studies University of Maryland Cambridge, Maryland 21613
Light is believed to be a major factor regulating the productivity of phytoplankton in the Antarctic (Bunt 1964; Holm-Hansen et al. 1977). The photic regime experienced by these microalgae is one of the most extreme in the world. For example, in McMurdo Sound the transition from the continuous darkness of winter to the continuous light of summer takes about 2 months. By summer, the 1.5 to 2 meters of annual sea ice and the developing epontic community reduce light available to phytoplankton to irradiances below those often considered necessary for growth by temperate microalgae (Garrison, Sullivan, and Ackley 1986). Changes in the photosynthetic capacity over the course of the 24-hour day are common in temperate regions and appear to have evolved in response to predictable light-dark cycles (Sournia 1974). Diel patterns of photosynthesis may impart some selective advantage to these phytoplankters by enhancing their ability to fix carbon. Photosynthetic periodicities may also influence estimates of daily water-column production in temperate regions (Harding et al. 1982). During the 1985-1986 field season, we conducted prelimin ary studies to determine whether phytoplankton from McMurdo Sound exhibit diel patterns in photosynthetic capacity. An unmodified water sample contains many algal species, thus temporal changes in photosynthesis may appear small if the relative maxima and minima of the different species within the sample occur at the same time. Hence to maximize the resolution with which periodicities of photosynthesis of these polar phytoplankton were examined, we used the species-specific radioisotope techniques of Rivkin and Seliger (1981). Samples were collected in McMurdo Sound at our seasonal field station located about 25 kilometers north of Cape Armitage and 9 kilometers west of Tent Island. Plankton were sampled through a hole in the sea ice using a 0.5 meter-diameter, 20micrometer aperture plankton net and were incubated under simulated in situ conditions of temperature and irradiance. Incident insolation was continuously measured. Photosynthesis was determined at irradiances previously determined to satu1986 REVIEW
rate photosynthesis. To facilitate comparisons among species, relative rates of photosynthesis are presented here: Relative photosynthesis = - x 100 PtIlax where P is the observed rate of photosynthesis at any time and Pma, is the maximum rate of photosynthesis observed over the 24-hour period. Two centric diatoms were among the dominant net plankton at our sample site in McMurdo Sound during the austral spring of 1985. On 12 September daylength was about 9 hours and maximum daily incident irradiance was measured at about 156 microEinsteins per square meter per second (figure 1A). 'max for Coscinodiscus sp. was about 6 times greater than the observed minimum rate of photosynthesis Maximum rates occurred midday with minimum rates at midnight. For Porosira pseudodenticulata the ratio of Pn,a. to P i,, was about 2. Like Coscinodiscus sp. minimum rates for P. pseudodenticulata occurred during the dark period but P max occurred at dawn. On 30 September daylength was 14 hours and maximum irradiances were about 600 microEinsteins per square meter per second (figure 1B). The diel pattern for Cosinodiscus sp. was similar to that observed on 12 September. The ratio of Pma, to P,i, was about 5 and occurred midday. In P. pseudodenticulata, P,ax to Pmin was about 2, and early morning and late afternoon peaks in photosynthesis were observed. On 10 November, the sun was continuously above the horizon; maximum and minimum incident irradiances were about 850 microEinsteins per square meter per second and 200 microEinsteins per square meter per second, respectively. All four species examined showed diel changes in photosynthetic capacity (figure 2). For three of the species, Pm i. to Pmin was about 4, which was similar to values observed earlier in the season. A) 12 September 1988 B) 30 September 1985 e q4 U U
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it
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o; •. ____________ 0800 1600 2400 0000 0800 1600 2400 0800 Local Standard Time Figure 1. Diel changes in photosynthesis for Coscinodiscus sp. and measured on A. 12 September 1985 and B. 30 September 1985. Incident irradiance on each date is indicated. Photosynthesis was measured at 240 microEinsteins per square meter per second on 12 September and at 140 microEinsteins per square meter per second on 30 September.
(•) Porosira pseudodenticulata (A)
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Species-specific Pmax for these species occurred at times ranging from early morning to midnight. Our study shows that diel patterns of photosynthesis are common in antarctic phytop!ankton throughout the season and that both timing of maximum rates of photosynthesis and the ratio of P,,,, , to Pmifl vary among species. Future studies will examine the environmental regulation of these diel changes and their impact on the estimation of primary productivity in polar regions. This research was supported by National Science Foundation grant DPP 83-14607 to Richard B. Rivkin and by a scholarship awarded to Mary Putt by the Natural Sciences and Engineering Research Council of Canada. Other members of the 1985-1986 field party were M. Voytek and E. Lessard.
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Figure 2. Diel changes in photosynthesis rate for Thalassiosira Scotia (), Amphiprora kufferathii (A), Fragilariopsis kerguelensis sp. (A) and Thalassiosira sp. (•), measured on 10 November 1985. Incident irradiance is indicated. Photosynthesis was measured at 80 microEinsteins per square meter per second.
186
Bunt, J.S. 1964. Primary production under sea ice in Antarctic waters. 2. Influence of light and other factors on photosynthetic activities of Antarctic marine microalgae. Antarctic Research Series, 1, 27-31. Garrison, D.C., C.W. Sullivan, and S. Ackley. 1986. Sea ice microbial communities in Antarctica. BioScience, 36, 243-250. Harding, L.W., Jr., B.B. Prezelin, B.M. Sweeney, and J.L. Cox. 1982. Diel oscillations of the photosynthesis-irradiance relationship in natural assemblages of phytoplankton. Marine Biology, 67, 167-178. Holm-Hansen, 0., S.Z. El-Sayed, G.N. Franceshini, and R.L. Cuhel. 1977. Primary production and the factors controlling phytoplankton growth in the Southern Ocean. In. G. Llano (Ed.), Ada pt:ons within antarctic ecosystems. (Proceedings of the Third Scientific Committee on Antarctic Research Symposium on Antarctic Biology, Washington, D. C. 1976.) Rivkin, R. B., and H. H. Seliger. 1981. Liquid scintilation counting for 14C uptake of single algal cells isolated from natural populations. Limnology and Oceanography, 26, 780-785. Sournia, A. 1974. Circadian periodicities in natural populations of marine phytoplankton. Advances in Marine Biology, 12, 325-389.
ANTARCTIC JOURNAL
R.B. RIvKIN and M. MOLINE Horn Point Environmental Laboratories Center for Environmental and Estuarine Studies University of Maryland Cambridge, Maryland 21613
Light is believed to be a major factor regulating the productivity of phytoplankton in the Antarctic (Bunt 1964; Holm-Hansen et al. 1977). The photic regime experienced by these microalgae is one of the most extreme in the world. For example, in McMurdo Sound the transition from the continuous darkness of winter to the continuous light of summer takes about 2 months. By summer, the 1.5 to 2 meters of annual sea ice and the developing epontic community reduce light available to phytoplankton to irradiances below those often considered necessary for growth by temperate microalgae (Garrison, Sullivan, and Ackley 1986). Changes in the photosynthetic capacity over the course of the 24-hour day are common in temperate regions and appear to have evolved in response to predictable light-dark cycles (Sournia 1974). Diel patterns of photosynthesis may impart some selective advantage to these phytoplankters by enhancing their ability to fix carbon. Photosynthetic periodicities may also influence estimates of daily water-column production in temperate regions (Harding et al. 1982). During the 1985-1986 field season, we conducted prelimin ary studies to determine whether phytoplankton from McMurdo Sound exhibit diel patterns in photosynthetic capacity. An unmodified water sample contains many algal species, thus temporal changes in photosynthesis may appear small if the relative maxima and minima of the different species within the sample occur at the same time. Hence to maximize the resolution with which periodicities of photosynthesis of these polar phytoplankton were examined, we used the species-specific radioisotope techniques of Rivkin and Seliger (1981). Samples were collected in McMurdo Sound at our seasonal field station located about 25 kilometers north of Cape Armitage and 9 kilometers west of Tent Island. Plankton were sampled through a hole in the sea ice using a 0.5 meter-diameter, 20micrometer aperture plankton net and were incubated under simulated in situ conditions of temperature and irradiance. Incident insolation was continuously measured. Photosynthesis was determined at irradiances previously determined to satu1986 REVIEW
rate photosynthesis. To facilitate comparisons among species, relative rates of photosynthesis are presented here: Relative photosynthesis = - x 100 PtIlax where P is the observed rate of photosynthesis at any time and Pma, is the maximum rate of photosynthesis observed over the 24-hour period. Two centric diatoms were among the dominant net plankton at our sample site in McMurdo Sound during the austral spring of 1985. On 12 September daylength was about 9 hours and maximum daily incident irradiance was measured at about 156 microEinsteins per square meter per second (figure 1A). 'max for Coscinodiscus sp. was about 6 times greater than the observed minimum rate of photosynthesis Maximum rates occurred midday with minimum rates at midnight. For Porosira pseudodenticulata the ratio of Pn,a. to P i,, was about 2. Like Coscinodiscus sp. minimum rates for P. pseudodenticulata occurred during the dark period but P max occurred at dawn. On 30 September daylength was 14 hours and maximum irradiances were about 600 microEinsteins per square meter per second (figure 1B). The diel pattern for Cosinodiscus sp. was similar to that observed on 12 September. The ratio of Pma, to P,i, was about 5 and occurred midday. In P. pseudodenticulata, P,ax to Pmin was about 2, and early morning and late afternoon peaks in photosynthesis were observed. On 10 November, the sun was continuously above the horizon; maximum and minimum incident irradiances were about 850 microEinsteins per square meter per second and 200 microEinsteins per square meter per second, respectively. All four species examined showed diel changes in photosynthetic capacity (figure 2). For three of the species, Pm i. to Pmin was about 4, which was similar to values observed earlier in the season. A) 12 September 1988 B) 30 September 1985 e q4 U U
100
41 C.-.
/4'
0
4J X
50 .0 U
it
- 1200 U CY • UJ
800 400
o; •. ____________ 0800 1600 2400 0000 0800 1600 2400 0800 Local Standard Time Figure 1. Diel changes in photosynthesis for Coscinodiscus sp. and measured on A. 12 September 1985 and B. 30 September 1985. Incident irradiance on each date is indicated. Photosynthesis was measured at 240 microEinsteins per square meter per second on 12 September and at 140 microEinsteins per square meter per second on 30 September.
(•) Porosira pseudodenticulata (A)
185
in tJnvmhrn" 1gR5 U) .1-1
U)
0) 4J
c '>
0
4X 00 0-a-
Species-specific Pmax for these species occurred at times ranging from early morning to midnight. Our study shows that diel patterns of photosynthesis are common in antarctic phytop!ankton throughout the season and that both timing of maximum rates of photosynthesis and the ratio of P,,,, , to Pmifl vary among species. Future studies will examine the environmental regulation of these diel changes and their impact on the estimation of primary productivity in polar regions. This research was supported by National Science Foundation grant DPP 83-14607 to Richard B. Rivkin and by a scholarship awarded to Mary Putt by the Natural Sciences and Engineering Research Council of Canada. Other members of the 1985-1986 field party were M. Voytek and E. Lessard.
> •1•I
4J I0 0
a:
0 References
1200
'I
800
lu rm
400
o 0
I-I
OBQO t60.0 2400 0800 Local Standard Time
Figure 2. Diel changes in photosynthesis rate for Thalassiosira Scotia (), Amphiprora kufferathii (A), Fragilariopsis kerguelensis sp. (A) and Thalassiosira sp. (•), measured on 10 November 1985. Incident irradiance is indicated. Photosynthesis was measured at 80 microEinsteins per square meter per second.
186
Bunt, J.S. 1964. Primary production under sea ice in Antarctic waters. 2. Influence of light and other factors on photosynthetic activities of Antarctic marine microalgae. Antarctic Research Series, 1, 27-31. Garrison, D.C., C.W. Sullivan, and S. Ackley. 1986. Sea ice microbial communities in Antarctica. BioScience, 36, 243-250. Harding, L.W., Jr., B.B. Prezelin, B.M. Sweeney, and J.L. Cox. 1982. Diel oscillations of the photosynthesis-irradiance relationship in natural assemblages of phytoplankton. Marine Biology, 67, 167-178. Holm-Hansen, 0., S.Z. El-Sayed, G.N. Franceshini, and R.L. Cuhel. 1977. Primary production and the factors controlling phytoplankton growth in the Southern Ocean. In. G. Llano (Ed.), Ada pt:ons within antarctic ecosystems. (Proceedings of the Third Scientific Committee on Antarctic Research Symposium on Antarctic Biology, Washington, D. C. 1976.) Rivkin, R. B., and H. H. Seliger. 1981. Liquid scintilation counting for 14C uptake of single algal cells isolated from natural populations. Limnology and Oceanography, 26, 780-785. Sournia, A. 1974. Circadian periodicities in natural populations of marine phytoplankton. Advances in Marine Biology, 12, 325-389.
ANTARCTIC JOURNAL
