The ecological and biogeochemical role of acanthariain the Southern Ocean

Joachim Henjes1, Stephanie Jacquet2, Philipp Assmy1, Damien Cardinal3, Frank Dehairs2, Nicolas Savoye4, Marina Montresor5 & Victor Smetacek1

1Alfred-Wegener-Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany2Department of Analytical and Environmental Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium3Department of Geology, royal Museum for Central Africa, B-3080 Tervuren, Belgium4OASU, UMR EPOC, Université Bordaux 1, CNRS Station Marine d´Arcachon, 33120 Arcachon, France5Stazione Zoologica 'A. Dohrn', Villa Comunale, 80121 Napoli, Italy

IntroductionThe 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 acanthariaIn 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 Baxs signal, despite a rather low relation between Baxs DWAv and total acantharia (Fig. 2N.). Acanthariaalso strongly affect the distribution of particulate biogenic Sr indicated by a particularly good relation between Srp and >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.) Epifluorescencemicrograph displaying endosymbiotic zooxanthellae inside the ectoplasma of the acantharia(Stauracon spp.). (F.) Acantharia engulfed by an athecate heterotrophic dinofalgellate. (G.) Acantharian skeleton (arrow) inside a metazoan faecal pellet during EIFEX.

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Fig. 2: Particulate biogenic Baxs profiles (pM; 0-500 m) inside (A.-C.) and outside (D.) the fertilized patch during EIFEX. Particulate Srp profiles (nM; 0-500 m) inside (E.-G.) and outside (H.) the fertilized patch during EIFEX. <50 µm (full diamonds) and >50 µm (open triangles) acantharianabundance 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 DWAv vs. total acantharia (N.) and Ba/Sr molar ratios of individual acantharia (O.) between 0-150 mduringEIFEX.

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I. Ecological importance of acanthariaBy 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.).

AcknowledgementsWe 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 SixthFrame-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 Baand Sr