An important question in climate modeling is whether carbon cycling is fundamentally different in warm versus cold climate states. A key line of evidence regarding this question comes from the unusually large difference in carbon-isotope values (δ¹³C) between shallow-dwelling muricate foraminifera and foraminifera living deeper in the water column in the Paleogene. This has been interpreted as evidence that warmer temperatures elevated the metabolic rates of carbon-recycling bacteria, resulting in a steeper gradient in the δ¹³C of dissolved inorganic carbon (δ¹³C_DIC) and reduced carbon export. However, this interpretation depends on the assumption that vital effects—biological processes that bias foraminiferal δ¹³C—are constant throughout time. We test this assumption using a chemical model of the foraminiferal microenvironment and find that the hypothesized increase in metabolic rates should also increase vital effects, meaning that both Paleogene δ¹³C_DIC gradients and the temperature dependence of metabolism must have been significantly lower than previously estimated. We further propose that muricate foraminifera may have evolved a novel “mat” strategy for photosymbiosis wherein symbionts rested on the muricae in a thin layer surrounding the shell. This hypothesis can explain both the function of muricae and the observed isotopic data without the need for any change in metabolism. Our work thus challenges existing interpretations of δ¹³C and provides a path forward to empirically test the magnitude of temperature-dependent metabolic change in the deep past.

中文翻译：

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共生体的排列和代谢可以解释始新世浮游有孔虫中的高δ13 C

气候模型中的一个重要问题是，在温暖和寒冷的气候状态下，碳循环是否有根本不同。关于这个问题的证据的一个关键线来自于碳同位素值非常大的差异（δ ¹³浅栖muricate有孔虫和有孔虫在古近纪水柱生活更深之间C）。这已被解释为证据表明，较高的温度升高的碳循环的细菌的代谢率，导致更陡的梯度在δ ¹³溶解的无机碳（δ的Ç ¹³ Ç _DIC）和降低的碳出口。然而，这种解释依赖于假设的重要作用，生物过程偏见有孔虫δ ¹³C-在整个时间都是恒定的。我们使用有孔虫微环境的化学模型检验这一假设，并发现，在代谢率假设增加也应提高重要的作用，它意味着：古近系δ ^13个C ^ _DIC梯度和代谢与温度的关系一定是显著低于此前估计。我们进一步提出，有孔虫可能已经发展出一种新型的“光合”策略，用于光共生，其中，共生体以薄薄的壳壳围绕在粘菌上。这个假设可以解释鼠科动物的功能和观察到的同位素数据，而无需任何新陈代谢的变化。因此，我们的工作挑战δ现有的解释¹³C并提供了一条路径，可以根据经验来测试深层过去依赖温度的新陈代谢变化的幅度。