The role of seawater endocytosis in the biomineralization process in calcareous foraminifera

Foraminifera are unicellular organisms that inhabit the oceans in various ecosystems. The majority of the foraminifera precipitate calcitic shells and are among the major CaCO3 producers in the oceans. They comprise an important component of the global carbon cycle and also provide valuable paleoceanographic information based on the relative abundance of stable isotopes and trace elements (proxies) in their shells. Understanding the biomineralization processes in foraminifera is important for predicting their calcification response to ocean acidification and for reliable interpretation of the paleoceanographic proxies. Most models of biomineralization invoke the involvement of membrane ion transporters (channels and pumps) in the delivery of Ca2+ and other ions to the calcification site. Here we show, in contrast, that in the benthic foraminiferan Amphistegina lobifera, (a shallow water species), transport of seawater via fluid phase endocytosis may account for most of the ions supplied to the calcification site. During their intracellular passage the seawater vacuoles undergo alkalization that elevates the CO32− concentration and further enhances their calcifying potential. This mechanism of biomineralization may explain why many calcareous foraminifera can be good recorders of paleoceanographic conditions. It may also explain the sensitivity to ocean acidification that was observed in several planktonic and benthic species.

Bentov, S., Brownlee, C. & Erez, J., 2009. The role of seawater endocytosis in the biomineralization process in calcareous foraminifera. Proceedings of the National Academy of Sciences of the United States of America 106(51):21500-21504. Article (subscription required).

3 Responses to “The role of seawater endocytosis in the biomineralization process in calcareous foraminifera”

  1. 1 Michael Crawford 18 December 2010 at 16:27

    It is claimed in a report from the Royal Society that mineralisation or building of calcium carbinate shells actually adds CO2 to the sea. Is that true? That would mean that oyster farming for example was increasing ocean acdification?They call it counter intuitive!

    • 2 Jean-Pierre Gattuso 19 December 2010 at 05:48

      That is correct. Consider the calcification reaction:

      Ca2+ + 2 HCO3- -> CaCO3 + CO2 + H2O

      Frankignoulle, M., Canon, C. and Gattuso, J.-P. (1994). Marine calcification as a source of carbon dioxide- Positive feedback of increasing atmospheric CO2. Limnology and Oceanography, 39, 458-462.

      Ware, J.R., Smith, S.V. and Reaka-Kudla, M.L. (1992). Coral reefs: sources or sinks of atmospheric CO2? Coral Reefs, 11, 127-130.

      • 3 Michael Crawford 19 December 2010 at 15:52

        But the Carbon the oyster uses comes not from the HCO3 but from the fixation of CO2 and N2 by phytoplankton and micro-algae I would have thought.

        Even supposing the oyster only used carbon from the bicarbonate you have effectively 2*C02 (from atmosphere) -> CaCO3 + CO2

        So one CO2 of the two only is left in the water. The other is on my dinner table???


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