The ecological and biogeochemical role of acantharia in the Southern Ocean
Joachim Henjes 1 , Stephanie Jacquet 2 , Philipp Assmy 1 , Damien Cardinal 3 , Frank Dehairs 2 , Nicolas Savoye 4 , Marina Montresor 5 & Victor Smetacek 1
1Alfred-Wegener-Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
2Department of Analytical and Environmental Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
3Department of Geology, royal Museum for Central Africa, B-3080 Tervuren, Belgium
4OASU, UMR EPOC, Université Bordaux 1, CNRS Station Marine d´Arcachon, 33120 Arcachon, France
5Stazione Zoologica 'A. Dohrn', Villa Comunale, 80121 Napoli, Italy Introduction
The study of the diversity and the function of larger protozooplankton (especially foraminifera, radiolaria and acantharia) in pelagic food webs, despite the extensive use of their mineral skeletons as proxies for palaeoceanographic reconstructions, has started only fairly recently. Process studies like iron fertilization experiments provide an ideal context to determine the role of larger protozoa as a trophic link between smaller protozooplankton and larger metazooplankton and to investigate a size fraction and group of taxa which are important for biogeochemical cycles of certain elements (Si, Ca, Ba, Sr) and palaeoceanographic studies. An important group, which comes to the fore as a biological proxy, are the acantharia. These delicate, free living, microphagic organisms form barium-enriched celestite (Ba/Sr SO4) skeletons. Thus our approach was to study the response of acantharia, the water column distribution of particulate biogenic Ba, Sr and the individual acantharian skeleton Ba/Sr molar ratios during an iron-induced phytoplankton bloom (EIFEX) to determine the ecological and biogeochemical role of acantharia in the Southern Ocean.
II. Biogeochemical importance of acantharia
In the wind mixed layer (0-150m), results indicate that the particulate biogenic Baxs (Figs. 2A.-D.) and Sr (Figs. 2E.-H.) signals follow mainly phytoplankton carbon (data not shown) and acantharian stocks (Figs. 2I.- L.). Taking the high individual Ba/Sr acantharian skeleton ratios (Fig. 2O.) into account, calculations suggest that the acantharian-derived Ba could account for 96% of the upper 150 m Baxssignal, despite a rather low relation between BaxsDWAv and total acantharia (Fig. 2N.). Acantharia also strongly affect the distribution of particulate biogenic Sr indicated by a particularly good relation between Srpand >50 µm acantharia in surface water.
Fig. 1: Temporal development of (A.) acantharia <50 µm and (B.) acantharia >50 µm. Percentage of carbon standing stock (mg C m-2) inside the fertilized patch (C.) and outside the fertilized patch (D.) during EIFEX. All values integrated over 150 m mixed layer depth. (E.) Epifluorescence micrograph displaying endosymbiotic zooxanthellae inside the ectoplasma of the acantharia (Stauraconspp.). (F.) Acantharia engulfed by an athecate heterotrophic dinofalgellate. (G.) Acantharian skeleton (arrow) inside a metazoan faecal pellet during EIFEX.
Days since first Fe-release 0 4 8 12 1620 2428 3236 40
Acantharia <50 µm (105
i-2nd. m)
0 20 40 60 80 100 120
IN-PATCH OUT-PATCH
3-station running average (IN-PATCH) A.
D.
0 10 20 30 40 50 60 70 80 90 100
D2 D12 D18 D27 D35
Days since first Fe-release C.
0 10 20 30 40 50 60 70 80 90 100
D0 D11 D13 D22 D24 D29 D32 D34 D37 Days since first Fe-release
% of biomass (mg C m-2)
Acantharia Foraminifera Radiolaria
Days since first Fe-release 0 4 8 12 16 20 24 28 32 36 40
Acantharia >50 µm (105 ind. m-2)
0 20 40 60 80 100 120 B.
20 µm
E. F. G.
Fig. 2:Particulate biogenic Baxsprofiles (pM; 0-500 m) inside (A.-C.) and outside (D.) the fertilized patch during EIFEX. Particulate Srpprofiles (nM; 0-500 m) inside (E.-G.) and outside (H.) the fertilized patch during EIFEX. <50 µm (full diamonds) and >50 µm (open triangles) acantharian abundance profiles (indiv./m3; 0-500 m) inside (I.-K.) and outside (L.) the fertilized patch during EIFEX. Srp DWAv(Depth Weighted Average values) vs. >50 µm acantharia (M.) and Baxs DWAvvs.
total acantharia (N.) and Ba/Sr molar ratios of individual acantharia (O.) between 0-150 mduring EIFEX.
0 100 200 300 400 500
0 500 1000 1500 2000
Baxs[pM]
Z [m]
424/17
0 100 200 300 400 500
0 500 1000 1500 2000
Baxs[pM]
587/10 D.
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5 9 3/1 2 C.
B.
A.
t = 37
t = 35
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0 50 0 10 0 0 150 0 2 0 0 0 Baxs [p M]
5 53 /10 t = 29 t = 0
IN-PATCH IN-PATCH IN-PATCH OUT-PATCH
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0 5 10 15 20 2 5
Srp [nM]
Z [m]
424/17t = 0 IN-PATCH
0
100
200
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0 5 10 15 2 0 2 5
Srp[nM]
593/12 t = 37
IN-PATCH OUT-PATCH 0
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Srp[nM]
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0 5 10 15 2 0 2 5
Srp [nM]
553/10 t = 29 IN-PATCH
E. F. G.
0 100 200 300 400 500
0 50000 100000 150000 Acantharians [indiv/m3]
Z [m]
<50µm
>50µm t = 0 IN-PATCH
0 100 200 300 400 500
0 500 0 0100 0 0 01500 0 0 Acantharians [indiv/m3]
<50µm
>50µm IN-PATCH t = 29
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0 50000100000 150000 Acantharians [indiv/m3]
<50µm
>50µm IN-PATCH
t = 37
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0 50000 100000 150000 Acantharians [indiv/m3]
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I. J. K.
H.
L.
N. O.
M.
0.00 0.01 0.02 0.03 0.04 0.05 0.06
0 10 20 30
Sample number
Ba/Sr (molar ratio)
R2 = 0.83
0 5 10 15 20 25 30
10000 30000 50000 70000
Acant >50µm [indiv/m3]
Srp [DWAv; nM]
R2 = 0.32
0 200 400 600 800 1000
10000 50000 90000 130000
Total Acant [indiv/m3] Baxs [DWAv; pM]
Acantharia
Foraminifera
Radiolaria I. Ecological importance of acantharia
By the peak of the EIFEX experiments, phytoplankton carbon stocks had increased 3fold in the deeply mixed surface layer (down to 150 m) but decreased again significantly in the final week of the experiment (see poster Assmy et al.). Acantharia, but not other protozooplankton, also showed a marked population increase in the first four weeks of the experiment inside the patch (Figs. 1A., B.), and in general their temporal response resembled the development of the phytoplankton bloom. Hence acantharia clearly have the capability to respond to enhanced biological productivity with population growth and indicating that they were actively feeding on the bloom. Compared to foraminifera and radiolaria, acantharia clearly dominated carbon stocks inside and outside the fertilized patch (Figs. 1C., D.) and estimates of the carbon fixation ability of acantharian endosymbionts (Michaels AF, 1988, Marine Biology, 97: 559-569) show that these symbionts (Fig. 1E.) could significantly contribute, at least locally, to total primary productivity. Moreover the EIFEX experiment showed that acantharia are an important food source for other microprotozoan and metazoan grazers (Figs. 1F., G.).
Acknowledgements
We gratefully acknowledge the enthusiastic support of the captain and crew of R.V.
Polarstern. P.A. and J.H. have been funded by Carbo-Ocean (contract no. GOCE-511176-2) within the Community´s Sixth Frame-work Program.
Conclusions Acantharia...
• are the dominant large protozoa in the Southern Ocean
• respond to enhanced primary productivity with population growth
• are food source for other grazers
• Have a significant impact on the biogeo- chemical cycles of Ba and Sr
