Carbonic anhydrase, coral calcification and a new model of stable isotope vital effects
Carbonic anhydrase, coral calcification and a new model of stable isotope vital effects
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DOI：
10.1016/j.gca.2018.02.032
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发表时间：
2018-09-01
影响因子：
5
通讯作者：
Adkins, Jess F.
中科院分区：
文献类型：
--
作者：
Chen, Sang;Gagnon, Alexander C.;Adkins, Jess F.
生物成因碳酸盐的稳定同位素组成已被用于古海洋学和古气候重建，并产生了一些在该领域最具代表性的记录。然而，我们仍然缺乏对稳定同位素代用物的完整机制理解，特别是对被称为“生命效应”的环境信号的生物套印。在海洋钙化生物中稳定同位素生命效应的一个普遍特征是δ O-18和δ C-13在从无机方解石/文石中耗尽的一系列值之间具有很强的相关性。人们提出了两种机制来解释这种相关性，一种是基于CO2(aq)- hco3 -相互转化过程中的动力学同位素效应，另一种是基于溶解无机碳（DIC）池的pH依赖形态形成过程中的平衡同位素交换。这两种机制都不能解释在生物成因碳酸盐岩中观察到的所有稳定同位素特征。在这里，我们提出了一个生物矿化及其同位素效应的全动力学模型，以深海珊瑚为试验生物。我们模型的一个关键组成部分是考虑了碳酸酐酶在催化细胞外钙化液（ECF）中CO2(aq)- hco3 -相互转化反应中的作用。我们发现碳酸酐酶的量不仅调节钙化流体的碳酸盐化学性质，而且有助于解释δ o -18- δ C-13相关性的斜率。生物矿化过程中CA活性的差异可能解释了不同钙化生物中δ o -18- δ C-13斜率的变化范围。利用数值模式对稳定同位素生命效应的机理认识有助于开发更好的古海洋示踪剂。(C) 2018 Elsevier Ltd.版权所有。
The stable isotope compositions of biogenic carbonates have been used for paleoceanographic and paleoclimatic reconstructions for decades, and produced some of the most iconic records in the field. However, we still lack a fully mechanistic understanding of the stable isotope proxies, especially the biological overprint on the environmental signals termed "vital effects". A ubiquitous feature of stable isotope vital effects in marine calcifying organisms is a strong correlation between delta O-18 and delta C-13 in a range of values that are depleted from inorganic calcite/aragonite. Two mechanisms have been proposed to explain this correlation, one based on kinetic isotope effects during CO2(aq)-HCO3- inter-conversion, the other based on equilibrium isotope exchange during pH dependent speciation of the dissolved inorganic carbon (DIC) pool. Neither mechanism explains all the stable isotope features observed in biogenic carbonates. Here we present a fully kinetic model of biomineralization and its isotope effects using deep-sea corals as a test organism. A key component of our model is the consideration of the enzyme carbonic anhydrase in catalyzing the CO2(aq)-HCO3- inter-conversion reactions in the extracellular calcifying fluid (ECF). We find that the amount of carbonic anhydrase not only modulates the carbonate chemistry of the calcifying fluid, but also helps explain the slope of the delta O-18-delta C-13 correlation. Differences in CA activity in the biomineralization process can possibly explain the observed range of delta O-18-delta C-13 slopes in different calcifying organisms. A mechanistic understanding of stable isotope vital effects with numerical models can help us develop better paleoceanographic tracers. (C) 2018 Elsevier Ltd. All rights reserved.