in McMurdo Sound
Photoadaptive strategies in a natural population of Phaeocystis pouchetii in McMurdo Sound A.C. PALMISANO and
second. Photosynthetic rates normalized to chlorophyll a were fitted to the empirically derived equations of Platt, Gallegos, and Harrison (1980).
J.B. SooHoo
I a I
Allan Hancock Foundation University of Southern California Los Angeles, California 90089 Si.
'
-
SooHoo
Scientific Data Center Cedars Sinai Medical Center Beverly Hills, California 90048
L.L. CRAFT and C.W. SULLIVAN Department of Biological Sciences University of Southern California Los Angeles, California 90089
The microalga Phaeocystis pouchetii (Hariot) Lagerheim enjoys a virtually worldwide distribution including antarctic marine ecosystems. Classified as a prymnesiophyte, its life history consists of both a colonial form in a mucilaginous envelope (figure 1) and a motile or "swarmer" stage. Colonies of Phaeocystis have been reported in McMurdo Sound both in the water column and in association with sea ice (Bunt 1964; Dayton and Oliver 1977; Palmisano and Sullivan 1985). McMurdo Sound provides a natural situation for examining photoadaptation in this common microalga. Prior to the Phaeocystis bloom, primary production is virtually restricted to seaice microalgae, with only low levels of chlorophyll a (less than 0.4 microgram per liter) found in the under-ice water column. With the onset of the Phaeocystis bloom in late December, P/iaocystis accounted for more than 99 percent of the phyoplankton in surface waters of east McMurdo Sound. Surface rrents on the east sound move southward from ice-free Waers to beneath fast ice (Dayton and Oliver 1977). Thus, Phaocystis would be carried from a relatively high-light, ice-free nvironment to the low-light conditions (typically less than 1 percent of surface downwelling irradiance) beneath annual sea ice. Surface seawater samples were collected along a north-south transect in east McMurdo Sound at five stations beginning at the edge of the fast ice (figure 2). The onset of the bloom in the east sound was followed between 18 and 28 December 1984. Lowsalinity seawater carrying Phaeocystis was advected from the ice edge to beneath 1.5 meters of fast ice. Levels of dissolved oxygen and chlorophyll a and the number of Phaeocystis cells increased with time as the bloom progressed (Palmisano et al. in preparation). To examine photoadaptive strategies in Phaeocystis, photosynthesis-irradiance [(i)] relationships were determined using small-volume, short-term (1-hour) incubations at — 1.8°C, which is the mean ambient surface water temperature in McMurdo Sound (Littlepage 1965). Photosynthetic rate was estimated by the uptake of 14 C-sodium bicarbonate over a range of irradiances from 0 to 874 microEinsteins per square meter per 1985 REVIEW
-, 411
f ,: A
Figure 1. Scanning electron micrograph of a colony of Phaeocystis pouchetii. Individual cells can be seen within a mucilaginous envelope.
166E
167°E
Scale 30' 5km S
pe RoydsN
ROSS SEA
ROSS ISLAND 1 .,
Cape Evans • Turks Head
2•
t
77 45 0,
/
ice
3. Annual Sea Ice
77 45. S
4.
Ross Ice Shelf 166E
pMcMurdoStatioi 5.V... \.
167°E
Figure 2. Map of east McMurdo Sound. Samples were collected along a transect from station 1 at the fast-ice edge to stations 2 through 5 beneath 1.5 meters of fast ice. ("km" denotes "kilometer:')
We found that Phaeocystis demonstrated a unique photoadaptive strategy in response to reduced irradiance beneath annual ice. A series of P(I) curves from samples collected on 24 De133
cember 1984 (figure 3) revealed that the photosynthetic efficiency or oc (the initial slope of the P(i) curve where light is limiting) increased by fourfold as the Phaeocystis adapted to the reduced irradiance. The maximum photosynthetic rate increased gradually from 3.5 to 7.3 milligrams carbon per milligram chlorophyll a per hour. The low-light adapted cells not only used lower irradiance more efficiently but also reached their maximum photosynthetic rate at 50 percent lower irradiance. The increased photosynthetic efficicency was not the result of an increase in cellular chlorophyll a. Instead, it may reflect changes in accessory pigments, the enzymes of carbon fixation, or energy flow between photosystems (Prezelin 1981). Adaptation to reduced irradiance was also evaluated by measuring in vivo absorption and fluorescence excitation spectra. Patterns observed in the spectra are consistent with the response of other microalgae to reduced growth irradiance (SooHoo et al. in press). One long-range goal of our research is the development of a model for carbon cycling in McMurdo Sound. We estimated that in situ production by Phaeocystis beneath annual sea ice in the east sound contributed 16 grams of carbon per square meter to the annual carbon budget. This value is comparable to primary production by ice algae in McMurdo Sound of 18 grams of carbon per square meter estimated by Grossi (1985). The study of photoadaptation in natural populations of Phaeocystis in McMurdo Sound will help us to estimate more accurately primary production in the changing light regimes that characterize pack-ice regions. The authors thank U.S. Navy squadron VXE-6 for helicopter support and Steven T. Kottmeier for field assistance. Research was supported by National Science Foundation grant DPP 8304985.
ikT.
6.0
December 24, 1984
4.8
i!VTT
Station 1
Station 2
Station 3
7.2•
Station 4
7.2 cc °•
S
::;:
0.0
Station 5
References 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. Dayton, P.K., and J.S. Oliver. 1977. Antarctic soft bottom benthos in oligotrophic and eutrophic environments. Science, 197, 55 - 58. Grossi, S.M. 1985. Response of a sea ice microalgal community to a gradient in under-ice irradiance. (Doctoral dissertation, University of Southern California, Los Angeles, California.) Littlepage, J.L. 1965. Oceanographic investigations in McMurdo Sound, Antarctica. Antarctic Research Series, 5, 1 - 37. Palmisano, A.C., J.B. SooHoo, S. SooHoo, S.T. Kottmeier, L.L. Craft, and C.W. Sullivan. In preparation. Photoadaptation in Phaeocystis pouchetii advected beneath annual sea ice in McMurdo Sound, Antarctica. Palmisano, A. C., and C.W. Sullivan. 1985. Pathways of photosynthetic carbon assimilation in sea ice in microalgae from McMurdo Sound, Antarctica. Limnology and Oceanography, 30, 674-678. Platt, T., C.L. Gallegos, and W.G. Harrison. 1980. Photoinhibition of photosynthesis in natural assemblages of marine phytoplankton. Journal of Marine Research, 38, 687 - 701. Prezelin, B.B. 1981. Light reactions in photosynthesis. In T. Platt (Ed.), Physiological bases of plankton ecology. Canadian Bulletin of Fisheries and Aquatic Sciences, 210, 1 - 46.
134
2.4jr 0 200 400 600 800 1000
IRRADIANCE j.aEIn/m2/s Figure 3. Photosynthesis/irradiance relationships normalized per chlorophyll a in samples from five stations on 24 December 1984. ("mgC/mg Chi alhr" denotes "milligrams of carbon per milligram of chlorophyll a per hour." " R EinIm 2/s" denotes "microEinsteins per square meter per second:')
SooHoo, J.B., D.A. Kiefer, D.J. Collins, and I.S. McDermid. In press. In vivo fluorescence excitation absorption spectra of marine phytoplankton: I. Taxonomic characteristics and responses to photoadaptation. Journal of Plankton Research.
ANTARCTIC JOURNAL
second. Photosynthetic rates normalized to chlorophyll a were fitted to the empirically derived equations of Platt, Gallegos, and Harrison (1980).
J.B. SooHoo
I a I
Allan Hancock Foundation University of Southern California Los Angeles, California 90089 Si.
'
-
SooHoo
Scientific Data Center Cedars Sinai Medical Center Beverly Hills, California 90048
L.L. CRAFT and C.W. SULLIVAN Department of Biological Sciences University of Southern California Los Angeles, California 90089
The microalga Phaeocystis pouchetii (Hariot) Lagerheim enjoys a virtually worldwide distribution including antarctic marine ecosystems. Classified as a prymnesiophyte, its life history consists of both a colonial form in a mucilaginous envelope (figure 1) and a motile or "swarmer" stage. Colonies of Phaeocystis have been reported in McMurdo Sound both in the water column and in association with sea ice (Bunt 1964; Dayton and Oliver 1977; Palmisano and Sullivan 1985). McMurdo Sound provides a natural situation for examining photoadaptation in this common microalga. Prior to the Phaeocystis bloom, primary production is virtually restricted to seaice microalgae, with only low levels of chlorophyll a (less than 0.4 microgram per liter) found in the under-ice water column. With the onset of the Phaeocystis bloom in late December, P/iaocystis accounted for more than 99 percent of the phyoplankton in surface waters of east McMurdo Sound. Surface rrents on the east sound move southward from ice-free Waers to beneath fast ice (Dayton and Oliver 1977). Thus, Phaocystis would be carried from a relatively high-light, ice-free nvironment to the low-light conditions (typically less than 1 percent of surface downwelling irradiance) beneath annual sea ice. Surface seawater samples were collected along a north-south transect in east McMurdo Sound at five stations beginning at the edge of the fast ice (figure 2). The onset of the bloom in the east sound was followed between 18 and 28 December 1984. Lowsalinity seawater carrying Phaeocystis was advected from the ice edge to beneath 1.5 meters of fast ice. Levels of dissolved oxygen and chlorophyll a and the number of Phaeocystis cells increased with time as the bloom progressed (Palmisano et al. in preparation). To examine photoadaptive strategies in Phaeocystis, photosynthesis-irradiance [(i)] relationships were determined using small-volume, short-term (1-hour) incubations at — 1.8°C, which is the mean ambient surface water temperature in McMurdo Sound (Littlepage 1965). Photosynthetic rate was estimated by the uptake of 14 C-sodium bicarbonate over a range of irradiances from 0 to 874 microEinsteins per square meter per 1985 REVIEW
-, 411
f ,: A
Figure 1. Scanning electron micrograph of a colony of Phaeocystis pouchetii. Individual cells can be seen within a mucilaginous envelope.
166E
167°E
Scale 30' 5km S
pe RoydsN
ROSS SEA
ROSS ISLAND 1 .,
Cape Evans • Turks Head
2•
t
77 45 0,
/
ice
3. Annual Sea Ice
77 45. S
4.
Ross Ice Shelf 166E
pMcMurdoStatioi 5.V... \.
167°E
Figure 2. Map of east McMurdo Sound. Samples were collected along a transect from station 1 at the fast-ice edge to stations 2 through 5 beneath 1.5 meters of fast ice. ("km" denotes "kilometer:')
We found that Phaeocystis demonstrated a unique photoadaptive strategy in response to reduced irradiance beneath annual ice. A series of P(I) curves from samples collected on 24 De133
cember 1984 (figure 3) revealed that the photosynthetic efficiency or oc (the initial slope of the P(i) curve where light is limiting) increased by fourfold as the Phaeocystis adapted to the reduced irradiance. The maximum photosynthetic rate increased gradually from 3.5 to 7.3 milligrams carbon per milligram chlorophyll a per hour. The low-light adapted cells not only used lower irradiance more efficiently but also reached their maximum photosynthetic rate at 50 percent lower irradiance. The increased photosynthetic efficicency was not the result of an increase in cellular chlorophyll a. Instead, it may reflect changes in accessory pigments, the enzymes of carbon fixation, or energy flow between photosystems (Prezelin 1981). Adaptation to reduced irradiance was also evaluated by measuring in vivo absorption and fluorescence excitation spectra. Patterns observed in the spectra are consistent with the response of other microalgae to reduced growth irradiance (SooHoo et al. in press). One long-range goal of our research is the development of a model for carbon cycling in McMurdo Sound. We estimated that in situ production by Phaeocystis beneath annual sea ice in the east sound contributed 16 grams of carbon per square meter to the annual carbon budget. This value is comparable to primary production by ice algae in McMurdo Sound of 18 grams of carbon per square meter estimated by Grossi (1985). The study of photoadaptation in natural populations of Phaeocystis in McMurdo Sound will help us to estimate more accurately primary production in the changing light regimes that characterize pack-ice regions. The authors thank U.S. Navy squadron VXE-6 for helicopter support and Steven T. Kottmeier for field assistance. Research was supported by National Science Foundation grant DPP 8304985.
ikT.
6.0
December 24, 1984
4.8
i!VTT
Station 1
Station 2
Station 3
7.2•
Station 4
7.2 cc °•
S
::;:
0.0
Station 5
References 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. Dayton, P.K., and J.S. Oliver. 1977. Antarctic soft bottom benthos in oligotrophic and eutrophic environments. Science, 197, 55 - 58. Grossi, S.M. 1985. Response of a sea ice microalgal community to a gradient in under-ice irradiance. (Doctoral dissertation, University of Southern California, Los Angeles, California.) Littlepage, J.L. 1965. Oceanographic investigations in McMurdo Sound, Antarctica. Antarctic Research Series, 5, 1 - 37. Palmisano, A.C., J.B. SooHoo, S. SooHoo, S.T. Kottmeier, L.L. Craft, and C.W. Sullivan. In preparation. Photoadaptation in Phaeocystis pouchetii advected beneath annual sea ice in McMurdo Sound, Antarctica. Palmisano, A. C., and C.W. Sullivan. 1985. Pathways of photosynthetic carbon assimilation in sea ice in microalgae from McMurdo Sound, Antarctica. Limnology and Oceanography, 30, 674-678. Platt, T., C.L. Gallegos, and W.G. Harrison. 1980. Photoinhibition of photosynthesis in natural assemblages of marine phytoplankton. Journal of Marine Research, 38, 687 - 701. Prezelin, B.B. 1981. Light reactions in photosynthesis. In T. Platt (Ed.), Physiological bases of plankton ecology. Canadian Bulletin of Fisheries and Aquatic Sciences, 210, 1 - 46.
134
2.4jr 0 200 400 600 800 1000
IRRADIANCE j.aEIn/m2/s Figure 3. Photosynthesis/irradiance relationships normalized per chlorophyll a in samples from five stations on 24 December 1984. ("mgC/mg Chi alhr" denotes "milligrams of carbon per milligram of chlorophyll a per hour." " R EinIm 2/s" denotes "microEinsteins per square meter per second:')
SooHoo, J.B., D.A. Kiefer, D.J. Collins, and I.S. McDermid. In press. In vivo fluorescence excitation absorption spectra of marine phytoplankton: I. Taxonomic characteristics and responses to photoadaptation. Journal of Plankton Research.
ANTARCTIC JOURNAL
