Biogeosciences, 10, 6759–6767, 2013 Open Access www.biogeosciences.net/10/6759/2013/ doi:10.5194/bg-10-6759-2013 Biogeosciences © Author(s) 2013. CC Attribution 3.0 License. A new model for biomineralization and trace-element signatures of Foraminifera tests G. Nehrke1, N. Keul2, G. Langer3, L. J. de Nooijer4, J. Bijma1, and A. Meibom5 1Alfred Wegener Institute, Bremerhaven, Germany 2Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA 3Department of Earth Sciences, Cambridge University, Cambridge, UK 4Department of Marine Geology, Royal Netherlands Institute of Sea Research, Horntje, the Netherlands 5Laboratory for Biological Geochemistry, School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland Correspondence to: G. Nehrke ([email protected]) and A. Meibom (anders.meibom@epfl.ch) Received: 27 May 2013 – Published in Biogeosciences Discuss.: 18 June 2013 Revised: 24 September 2013 – Accepted: 24 September 2013 – Published: 29 October 2013 Abstract. The Mg / Ca ratio of Foraminifera calcium car- environmental conditions, their relative taxonomic abun- bonate tests is used as proxy for seawater temperature dances as well as the elemental (e.g., Mg / Ca) and isotopic and widely applied to reconstruct global paleo-climatic (e.g., δ18O) composition of their tests are frequently used to changes. However, the mechanisms involved in the carbon- reconstruct global climate change associated with, for ex- ate biomineralization process are poorly understood. The ample, glacial–interglacial cycles (Elderfield and Ganssen, current paradigm holds that calcium ions for the test are 2000) and mass extinctions (Kiessling et al., 2008). How- supplied primarily by endocytosis of seawater. Here, we ever, the use of Foraminifera tests as paleo-environmental combine confocal-laser scanning-microscopy observations archives is complicated by biological processes, which cause of a membrane-impermeable fluorescent marker in the extant their chemical and isotopic compositions to be significantly benthic species Ammonia aomoriensis with dynamic 44Ca- different from Ca carbonates precipitated inorganically under labeling and NanoSIMS isotopic imaging of its test. We in- the same environmental conditions. These differences are re- fer that Ca for the test in A. aomoriensis is supplied primar- ferred to as the “vital effect” (Urey et al., 1951). A better ily via trans-membrane transport, but that a small compo- understanding of the Foraminifera biomineralization process nent of passively transported (e.g., by endocytosis) seawater is essential to the identification of the mechanism(s) respon- to the site of calcification plays a key role in defining the sible for such chemical and isotopic fractionations. One fun- trace-element composition of the test. Our model accounts damental question in this regard is how the constituents of for the full range of Mg / Ca and Sr / Ca observed for benthic the tests, especially the dominant cation Ca2+ and the mi- Foraminifera tests and predicts the effect of changing seawa- nor/trace elements Mg and Sr, are transported from the am- ter Mg / Ca ratio. This places foram-based paleoclimatology bient seawater to the site of calcification. into a strong conceptual framework. A widely accepted model holds that endocytosis of sea- water is the principal mechanism involved in test forma- tion: special vesicles transport seawater to the site of cal- cification (Erez, 2003; Bentov et al., 2009). This model 1 Introduction is based on the key observation that labeling of seawater with the membrane-impermeable fluorescent marker FITC- Calcium carbonate tests (shells) formed by unicellular D (fluorescein isothiocyanate-dextran) results in staining of Foraminifera are present in marine sedimentary records since newly formed chambers in previously decalcified benthic the Ordovician (∼ 290 million years ago) (Martin, 1995; Foraminifera Amphistegina lobifera. This observation led Schallreuter, 1983). With different species adapted to specific Published by Copernicus Publications on behalf of the European Geosciences Union. 6760 G. Nehrke et al.: A new model for biomineralization and trace-element signatures to the conclusion that endocytosed seawater is the primary source of ions for calcification. However, the endocytosis model suffers from two problems. First, a mechanism for the modification of the elemental composition of the vacuolized seawater, in particular Mg re- moval, during transport to the site of calcification has to be postulated in order to explain the chemical composition of the tests (Erez, 2003). Secondly, the endocytosis model is challenged by a cal- culation of the volume of seawater needed to supply the re- quired Ca to the site of calcification. For the ubiquitous ben- thic species Ammonia tepida, this is about 75 times the vol- ume of the Foraminifera itself (de Nooijer et al., 2009), as- suming initial seawater composition in the vesicles. De Nooi- jer and co-workers (de Nooijer et al., 2009) did observe sea- water endocytosis in A. tepida using the FITC-D marker, but the low vesicle activity observed during normal calcification renders endocytosis insufficient as the major transport mech- anism for Ca from seawater to the site of calcification. To overcome this problem, an internal Ca pool241 was hypothesized for A. tepida (de Nooijer et al., 2009), but never experimen- tally demonstrated. (It should be noted that in some studies Fig. 1. Laser scanning confocal micrographs of three A. aomorien- 242 Figure 1: Laser scanning confocal micrographs of three A. aomoriensis specimens after we cite (de Nooijer et al., 2009; Dissard et al., 2010), the sis specimens after incubation in FITC-D-labeled seawater. (A) authors have misidentified the species investigated243 incubation as Am- in FITCSpecimen-labeled 1seawater. was incubated (A) Specimen in FITC-D 1 was labeled incubated seawater in FITC for 3.5 labeled h seawater for monia tepida, which actually was Ammonia244 aomoriensis 3 1/2 hoursT6 priorprior to chamber to chamber formation. formation. Immediately Immediately after after the onsetonset of of chamber chamber formation the (the species also used in the present study) (Schweizer et al., formation, the FITC-D was removed (see supporting information) 245 FITC was removed (see supporting information) and the image taken. Vesicles (ca. 20) were 2011; Hayward et al., 2004).) and the image taken. Vesicles (ca. 20) were observed inside the cell (arrow). Due to remaining FITC-D-labeled seawater, which could Nonetheless, it is important to emphasize246 that endocytosis,observed inside the cell (arrow). Due to remaining FITC-labeled seawater, which could not be not be washed out, fluorescence is seen inside the newly forming or vacuolization of seawater in Foraminifera,247 is anwashed observed out, fluorescencechamber (arrow is seen head). in Thisside fluorescencethe newly diffusesforming out chamber of the cham- (arrow head). This fact, which allows FITC-D to be transported to the site of 248 fluorescence diffusesber within out of 15 the min. chambe(B) Imager within of specimen 15 minutes 1 two. (B) hours Image after of the specimen first 1 two hours calcification and, as will become apparent below, does play image (A) was taken. Chamber formation is almost completed. A an important role for the trace-element composition249 after ofthe thefirst imagefew (A) small was vesicles taken. areChamber still present formation (arrow). is almost(C) Image completed. of specimen A few small vesicles test. 250 are still present2 (arrow). immediately (C) afterImage chamber of specimen formation. 2 Specimen immediately 2 was after incubated chamber formation. Here we use the fluorescent marker FITC-D together with in FITC-D-labeled seawater during chamber formation. No vesicles Specimen 2 was incubated in FITC-labeled seawater during chamber formation. No vesicles are confocal-laser scanning microscopy, and 25144Ca pulse-chase are present. The new chamber is fluorescent. Note the difference experiments in combination with NanoSIMS252 isotopic present. imag- The newbetween chamber the is fluorescence fluorescent. of N theote new the chamberdifference in (A)betweencaused the by fluorescence re- of the maining FITC-D-labeled seawater that diffused out within 15 min ing to investigate the hypothesis of an intracellular253 new calcium chamber in (A) caused by remaining FITC-labeled seawater that diffused out within 15 pool in A. aomoriensis. (For details on sample collection, cul- and (C) due to FITC-D incorporated into the newly formed cham- ber. (D) Image of specimen 3 four hours after FITC-D incubation turing, and incubation conditions, see Appendix)254 minu Incubationtes and (C) due to FITC incorporated into the newly formed chamber. (D) Image of showing the highest number of vesicles (arrow) observed in this specimen 3 four hours after FITC incubation showing the highest number of vesicles (arrow) experiments with FITC-D were performed255 prior to, during, study. Before incubation the specimen was not fed for 48 h. Scale and after chamber formation to visualize and256 relate observed seawater in this barsstudy. are Before 40 µm. incubation the specimen was not fed for 48 h. Scale bars are 40 vacuolization to calcification (i.e., formation of new cham- bers). Formation of a new chamber in A. aomoriensis257 µm. starts with an organic sheet, onto which calcitic CaCO258 3 nucleates. 12 In our incubated specimens, chamber formation takes 3–5 h, by de Nooijer