Gross and net production during the spring bloom along the Western Antarctic Peninsula Johanna ...

35 Pages·2015·0.57 MB·English

1 Gross and net production during the spring bloom along the
2 Western Antarctic Peninsula
3
4 Johanna A.L. Goldman1, Sven A. Kranz1,4, Jodi N. Young1, Philippe
5 D. Tortell2, Rachel H.R. Stanley3, Michael L. Bender1, Francois
6 M.M. Morel1.
7 Corresponding author: Johanna A.L. Goldman, email:
8 [email protected], Phone: [phone]
9 1 Department of Geosciences, Princeton University, Princeton, NJ,
10 08544, USA
11 2 Department of Earth, Ocean and Atmospheric Sciences, Dept. of
12 Botany, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada
13 3 Department of Marine Chemistry and Geochemistry, Woods Hole
14 Oceanographic Institution, Woods Hole, MA, USA
15 4 Present address: Department of Earth, Ocean and Atmospheric
16 Sciences, Florida State University, Tallahassee, FL, 32306, USA
17 Word count:
18 Total 6303
19 Introduction 730
20 Material and methods 1738
21 Results 2568
22 Discussion 1122
23 Acknowledgements 145
24 4 Figures (Figures 1 and 2 in colors)
25
26 Supporting information:
27 Word count: 1550
28 6 Figures, 3 Tables, 4 Notes
1
29 Summary
30 • This study explores some of the physiological mechanisms
31 responsible for high productivity near the shelf in the Western
32 Antarctic Peninsula despite a short growing season and cold
33 temperature.
34 • We measured gross and net primary production at Palmer
35 Station during the summer 2012/2013 via three different
36 techniques: 1) incubation with H 18O; 2) incubation with 14CO ;
2 2
37 and 3) in situ measurements of O /Ar and triple oxygen isotope.
2
38 Additional laboratory experiments were performed with the
39 psychrophilic diatom Fragilariopsis cylindrus.
40 • During the spring bloom, which accounted for more than half of
41 the seasonal gross production at Palmer Station, the ratio of net
42 to gross production reached a maximum greater than ~60%,
43 among the highest ever reported. The use of multiple-
44 techniques showed that these high ratios resulted from low
45 heterotrophic respiration and very low daylight autotrophic
46 respiration. Laboratory experiments revealed a similar ratio of
47 net to gross O production in F.cylindrus and provided the first
2
48 experimental evidence for an important level of cyclic electron
49 flow (CEF) in this organism.
50 • The low ratio of community respiration to gross primary
51 production observed during the bloom at Palmer Station may
52 be characteristic of high latitude coastal ecosystems and
53 partially supported by a very active CEF in psychrophilic
54 phytoplankton.
55
56 Key words: cold adaptation, cyclic electron flow, Fragilariopsis cylindrus,
57 gross production, net community production, respiration, Western
58 Antarctic Peninsula.
2
59 Introduction
60 The Western Antarctic Peninsula (WAP) is among the most
61 productive regions in the Southern Ocean (Arrigo et al., 2008a; Arrigo et
62 al., 2008b), where spring phytoplankton blooms result in a large Net
63 Community Production (NCP= photosynthesis minus respiration)
64 supporting an abundant and diverse ecosystem (Ducklow et al., 2007).
65 Over the past several decades, the WAP has also experienced some of
66 the most extreme atmospheric warming on the planet (IPCC, Parry
67 (2007)) with a concomitant rise of more than 1°C in sea surface
68 temperature since the 1950s (Meredith & King, 2005). Significant
69 research to date has focused on determining the effects of rising
70 temperature on the magnitude of the spring bloom (e.g. Montes-Hugo
71 et al. (2009)) but there is limited understanding of the mechanisms that
72 control the rates of carbon fixation and consumption in these waters.
73 Photochemical reactions are nearly temperature independent
74 and phytoplankton have developed several adaptive mechanisms to
75 mitigate the effect of low temperature on the photosynthetic electron
76 transport chain (see reviews by Mock and Hoch (2005), Morgan-Kiss et
77 al. (2006) and Dolhi et al. (2013)). As a result, the light reaction of
78 photosynthesis is little affected by low temperature under nutrient
79 replete conditions. For the dark reaction, the reduced catalytic rates of
80 various enzymes in the Calvin cycle at cold temperature can be partially
81 compensated by the increased solubility of CO over O in water, an
2 2
82 efficient carbon concentrating mechanism (Kranz et al., accepted), and
83 an increase in the cellular concentration of key enzymes, including
84 Ribulose-1,5-bisphosphate carboxylase oxygenase, Rubisco (Young et
85 al., accepted). In contrast, respiratory processes have been reported to
86 be highly sensitive to the change in temperature. Large decreases in
87 respiration rate with decreasing temperature have been shown in
88 previous studies of individual phytoplankton and macro-algae (Staehr &
89 Birkeland, 2006; Padilla-Gamino & Carpenter, 2007) as well as in
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90 numerous ecosystems (Valentini et al., 2000; Regaudie-de-Gioux &
91 Duarte, 2012; Yvon-Durocher et al., 2012). It is unclear why
92 phytoplankton growing at a given growth rate would need less energy
93 from respiration at low than at higher temperature. One possibility is
94 that psychrophilic phytoplankton may take advantage of the
95 maintenance of the high activity of their photosystems to generate ATP
96 via the photosynthetic apparatus.
97 In this study we used three different techniques to study primary
98 production and respiration in the WAP (LTER-station B, Palmer station,
99 Latitude: -64.7795; Longitude: -64.0725, sampling depth: 10m, total
100 depth ~70m; Fig. S1) during the austral summer of 2012/2013.
101 Comparing the results of three techniques provides insights into some
102 of the mechanisms that control the primary productivity in the WAP.
103 Two of these techniques are bottle incubations that measure
104 production from the light reaction of photosynthesis (H 18O incubation)
2
105 or the dark reaction of photosynthesis (14CO incubation). In addition,
2
106 measurements of biologically-derived O supersaturation as well as of
2
107 the isotopic composition of the O pool, from in situ sample collection,
2
108 allowed us to derive time-integrated ratios of Net/Gross production (the
109 “triple oxygen isotope technique”; Luz and Barkan (2000); Prokopenko
110 et al. (2011); Juranek and Quay (2013)). To further explore the
111 underlying physiological processes, laboratory experiments were
112 conducted with the model psychrophilic diatom Fragilariopsis cylindrus,
113 including measurements of net and gross photosynthesis, respiration
114 and cyclic electron flow.
115
116 Materials and methods
117 Field sampling and incubation experiments.
118 Sampling took place twice weekly in the morning at LTER-station B
119 (Latitude: -64.7795; Longitude: -64.0725; total depth ~70m). Water
120 from 10m depth was collected (monsoon pump; Waterra WSP-SS-80-
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121 NC) and dispensed into acid washed (10% HCl) containers, rinsed
122 thoroughly with MilliQ and on site water, for transport back to the
123 shore-based laboratories. Back at shore, all containers were kept in a
124 cold room (2°C) until processing, which typically occurred within 1 -2
125 hours of sample collection. Incubation experiments (described below)
126 were conducted on sub-samples placed in a flow-through seawater tank
127 shaded with neutral density screening to reduce irradiance by 50% (LEE
128 filters 209 0.3ND). A continuous flow of seawater pumped from the
129 shore (10m depth) was used for temperature control.
130 Pigment measurements and identification.
131 A known volume of sample (0.2-1L) was collected in duplicate and
132 filtered onto a glass fiber filter (Whatman GF/F, nominal pore size=
133 0.7µm). After overnight extraction in 90% acetone at -20°C, chlorophyll
134 a (Chl a) concentration for each duplicate was determined with a
135 fluorometer (Turner Designs 10-AU), measuring the un-acidified and
136 acidified sample to correct for phaeopigments (Welschmeyer, 1994).
137 For pigment identification (chlorophyll and carotenoids), one liter of
138 seawater was filtered under low light onto a GF/F, wrapped in
139 aluminum foil, flash frozen in liquid nitrogen and subsequently stored at
140 -80°C. Phytoplankton species composition for the three major taxa
141 (diatoms, Phaeocystis, and cryptophytes) was determined from the
142 abundance of the pigments fucoxanthin, 19’ hexanoyloxyfucoxanthin
143 and alloxanthin (Everitt et al., 1990; Arrigo et al., 2000). Quantification
144 of pigments was conducted using HPLC analysis at the Estuarine Ecology
145 Lab (University of South Carolina) following the protocol described in
146 Pinckney et al. (1998).
147 Seawater Hydrography and meteorological measurements.
148 The Palmer station Long Term Ecological Research (PAL-LTER) program
149 conducted regular depth profiles of temperature and salinity at LTER-
150 station B using a Seabird SBE 19plus Seacat Profiler. These depth profile
151 (Fig. S2) measurements were made within 1 – 2 hours of our regular
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152 sampling times. Seawater density (Fig. S3) was computed from salinity
153 and temperature using the TEOS-10 seawater equation of state
154 (http://www.teos-10.org/software.htm). Surface PAR levels (i.e.
155 irradiance between 400 and 700 nm wavelength) were measured by the
156 Palmer Station Terra Laboratory using a LI-COR LI-190SA quantum
157 sensor. Wind-speed data were obtained from the meteorological
158 sensors on top of the Palmer Station Terra Laboratory.
159 O measurements.
2
160 Dissolved [O ] was measured by Winkler titration. Seawater was
2
161 collected with a Go-flo bottle and transferred directly to Winkler
162 bottles, taking care to avoid bubbles and to seal samples and reagents
163 with no headspace. Samples for each sampling day were taken in
164 duplicate. Titrations were performed using an amperometric oxygen
165 titrator designed by Dr. Chris Langdon (Langdon, 1984; Culberson &
166 Huang, 1987).The instrumental precision of the measurements is ± 2
167 µmol L-1 but the average precision of the duplicate was ± 4 µmol L-1.
168
169 H18O incubations.
2
170 Gas tight bottles (145 ml, Pyrex) were rinsed and filled with collected
171 seawater from the carboys. One bottle was set aside for immediate
172 transfer (initial sample) while two bottles were spiked with 125µL H 18O
2
173 (Medical Isotopes, 97.6%) for a final enrichment of 412.4‰. The two
174 experimental bottles were incubated for 4 to 8 hours in the aquarium
175 tank outside the station. For our laboratory experiments we followed
176 the same procedure but duplicate bottles were taken for immediate
177 transfer (initial samples) and duplicate bottles were incubated in an
178 illuminated incubator (150 µE m-2 s-1) at 0.5°C. The samples were
179 subsequently transferred into custom-made 500mL flasks equipped
180 with Louwers-Hapert valves (LH flask), that had been previously spiked
181 with 100µL of saturated HgCl , dried, and evacuated (~1.5 mTorr=
2
182 ~0.2Pa). The LH flasks were then analyzed back in the laboratory at
183 Princeton within 6 months. Prior to analysis, the liquid phase was first
6
184 equilibrated, and then carefully drained. LH flasks were then put in a
185 freezing bath and the gas in each flask transferred into a stainless steel
186 tube kept in a liquid helium tank. Each tube was allowed to warm up for
187 one hour before analysis in a Delta Plus XP mass spectrometer for
188 dO /Ar, d18O and d15N (see Emerson et al. (1995)). The increase of
2
189 [18O16O] provides a measurement of Gross photosynthesis (GP) in the
190 bottle, while the change in [O ] during the incubation provides the net
2
191 production in the light in the bottle (NBP). The difference of the two
L
192 gives respiration in the light in the bottle. The errors were calculated as
193 standard deviation of the duplicates. The values were converted from
194 oxygen evolution to carbon fixation using a photosynthetic quotient
195 (PQ) based on C/N ratios from Young et al. (accepted) and derived from
196 electrons balance:
197 PQ= = = . This equation assumes all nitrogen
1 −
𝑚𝑚𝑚 𝑂2 4𝑚𝑚𝑚 𝑒 𝑁
198 sourc𝑚e 𝑚is𝑚 𝐶NO3- 𝑚an𝑚𝑚d 𝐶 an a1ve+ra2ge∗ p𝐶hotosynthetic product of CH2+3N/C ONN/C.
199 All the measurements and values used can be found in Table S1.
200 14C incubations.
201 We used a 14C-based incubation approach to measure C fixation rates by
202 phytoplankton assemblages. Samples were incubated in polycarbonate
203 bottles (125 ml), with varying amounts for NaH14CO in the flow-through
3
204 seawater tank. Triplicate samples for Gross Primary Production (GPP)
205 measurements were spiked with 10 µCi H14CO - and incubated in the
3
206 tank for 2h. Both incubations were initiated at midday. Triplicate
207 samples for Net Primary Production (NPP) were spiked with 5 µCi
208 H14CO - and incubated for 24h. In addition to the six NPP and GPP
3
209 bottles, two bottles were used as controls and spiked with 5 µCi H14CO -:
3
210 one bottle was filtered immediately after addition (blank), while the
211 other was incubated in the dark for 24h in the seawater tank (negative
212 control).To quantify total 14C activity for specific activity calculations, a
213 subsample (125 µl) was collected from each bottle after the incubation
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214 period. Subsequently, samples were filtered onto a 0.7µm glass fiber
215 filter, which was placed into 20mL scintillation vials and acidified with
216 6N HCl for at least 24h. Radioactivity in the samples was measured by
217 scintillation counting on a Beckman-Coulter Liquid Scintillation counter
218 (LSC 6500), using a standard quench curve correction to derive
219 disintegrations per minute. The errors were calculated as standard
220 deviation of the triplicate. An estimation of the correction for
221 respiration of unlabeled carbon during the 24h experiment was also
222 calculated (see Notes S1).
223 In situ measurement of ΔO /Ar and triple oxygen isotopes.
2
224 Samples were collected with a Go-flo bottle and transferred
225 immediately into a 500mL custom-made pre-evacuated and pre-
226 poisoned bottles with Louwers-Hapert (LH) valves (Emerson et al.,
227 1995). Great care was taken during the sample collection process to
228 avoid entrainment of atmospheric oxygen. The samples were stored at
229 ambient temperature and analyzed at Woods Hole Oceanographic
230 Institution within 6 months. Analysis followed the method of Barkan
231 and Luz (2003) with the modification that the GC column was 5.3 m and
232 held at -3 °C, and that each sample was collected on a cryogenic trap at
233 <10°K which was then warmed to room temperature and directly
234 released into an isotope ratio mass spectrometer. The ratio O /Ar
2
235 reflects the mass balance between NCP and gas exchange (Craig &
236 Hayward, 1987; Emerson et al., 1991), while the triple oxygen isotope
237 measurement reflects the mass balance between GPP and gas exchange
238 (using the equation described in Prokopenko et al. (2011)). The details
239 of this approach have been described previously (Luz & Barkan, 2000;
240 Reuer et al., 2007) and are explained in Supplementary notes S2. Values
241 for NCP and GPP were converted from µmol O m-2 d-1 to µmol O L-1 d-1
2 2
242 using the computed mixed layer depth (derived from a density
243 difference criterion of 0.125 kg m-3) or our sampling depth of 10m,
244 whichever was greater.
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245 Bacterial productivity and respiration.
246 30mL bottles were filled with seawater sub-samples. Triplicate control
247 bottles were immediately spiked with 200µL formalin (37%
248 formaldehyde) to stop biological activity. All bottles were then spiked
249 with 50µL 3H-thymidine. The biologically active bottles (in triplicate)
250 were then incubated in the outdoor seawater tank for 2h. At the end of
251 the incubation, biological activity was stopped by adding 200 µL
252 formalin. A subsample was taken from all bottles for specific activity
253 determination, while the remaining volume was filtered on a 0.2µM
254 cellulose nitrate filter. The filters were dried overnight, dissolved in
255 ethyl acetate, and counted on a Beckman-Coulter Liquid Scintillation
256 counter (LSC 6500), with quench correction. Following (Kirchman et al.,
257 1982), the uptake rate of labelled compound v(t) is assumed
258 proportional to the bacterial growth rate: v(t)= , where N is the
1 𝑑𝑁(𝑡)
259 number of cells and the conversion factor C= 1. 2𝐶 x∗ 10𝑑6𝑡 cells pmol-1
260 (Delille & Cahet, 1997). Bacterial production and respiration were
261 calculated using an average value of 10 fgC cell-1 and a growth efficiency
262 factor of 0.15 (Ducklow et al., 2012). Errors were calculated as standard
263 deviation of the triplicates.
264 Laboratory culture experiments.
265 Fragilariopsis cylindrus (CCMP 1102) was grown in semi continuous
266 batch culture using 0.2 µm filtered coastal seawater supplemented with
267 Aquil nutrients (Sunda et al., 2005) under continuous light (c. 150 µE m-2
268 s-1) at 0.5°C. Cells densities were counted with a Coulter Counter Z2
269 (Beckman Coulter Inc, Fullerton, CA). Cells were harvested during
270 exponential phase after ~2 weeks of growth. Incubations with H 18O in
2
271 the light were performed (as described above) to yield estimates of net
272 and gross photosynthesis as well as respiration in the light. Additional
273 incubations were performed in the dark to derive estimates of
274 respiration in the dark from the change in O concentration.
2
9
275 Cyclic electron flow.
276 For the determination of relative cyclic electron transport rates,
277 Fragilariopsis cylindrus cells in exponential growth were filtered gently
278 and resuspended into Aquil medium with 20% w/w Ficoll. Following
279 continuous illumination during which the redox state of P in PSI
700
280 reaches steady state, the reduction rate of P was measured in the
700
281 dark by fast spectrophotometry as the increase in absorbance at 700nm
282 (JTS-10 spectrophotometer, BioLogic, France). The total number of PSI
283 as well as the rate of their dark reduction, due to either cyclic or linear
284 electron flow, was obtained by the technique reviewed in Alric (2010).
285 To discriminate between the cyclic and linear electron flow, the
286 measurements were done under two different conditions: without any
287 inhibitor and in the presence of DCMU (3-(3,4-dichlorophenyl)-1,1-
288 dimethylurea, 20µM) a specific inhibitor of PSII that blocks the linear
289 flow of electrons going to PSI. To obtain the initial rate of P reduction,
700
290 the spectroscopic data were fitted with an exponential function and
291 corrected for an instrumental artefact as explained in supplemental
292 information (Notes S3).
293 RESULTS
294 Community composition and phytoplankton biomass
295 The near shore waters adjacent to Palmer station are typically ice-
296 covered in the winter. During the austral spring 2012, the ice retreated
297 at the end of October (~ October 25th). A large bloom of phytoplankton
298 was observed in late November-early December of 2012 (up to 45µg Chl
299 a L-1; Fig. 1a). Based on the analyses of three marker pigments
300 (fucoxanthin, 19’-hexanoyloxyfucoxanthin , and alloxanthin, see Arrigo
301 et al., (2000)), the phytoplankton community at the beginning of the
302 season was equally dominated by diatoms and Phaeocystis (Fig. 1a). As
303 the diatom bloom developed, the Phaeocystis share of the community
304 decreased and at the peak of the bloom the population was essentially
305 all diatoms. The bloom was interrupted by a mixing event which
10