Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Proc Natl Acad Sci U S A. 2009 Dec 22;106(51):21500-4. doi: 10.1073/pnas.0906636106. Epub 2009 Dec 10.
Foraminifera are unicellular organisms that inhabit the oceans in various ecosystems. The majority of the foraminifera precipitate calcitic shells and are among the major CaCO(3) 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 Ca(2+) 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 CO(3)(2-) 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.
有孔虫是一种生活在各种海洋生态系统中的单细胞生物。大多数有孔虫沉淀碳酸钙外壳，是海洋中主要的 CaCO3 生产者之一。它们是全球碳循环的重要组成部分，同时也基于其外壳中稳定同位素和微量元素（示踪剂）的相对丰度提供有价值的古海洋学信息。了解有孔虫的生物矿化过程对于预测它们对海洋酸化的钙化反应以及对古海洋学示踪剂的可靠解释非常重要。大多数生物矿化模型都涉及膜离子转运体（通道和泵）在将 Ca2+和其他离子输送到钙化部位的作用。相比之下，我们在这里表明，在滨岸有孔虫 Amphistegina lobifera（一种浅海物种）中，通过液相内吞作用运输海水可能是供应给钙化部位的大部分离子的来源。在它们的细胞内运输过程中，海水液泡经历碱化，从而提高了 CO32-浓度，并进一步增强了它们的钙化潜力。这种生物矿化机制可以解释为什么许多钙质有孔虫可以很好地记录古海洋学条件。它也可以解释在几种浮游生物和底栖生物中观察到的对海洋酸化的敏感性。
