Institute of Geosciences, University of Mainz, Mainz, Germany.
Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands.
PLoS One. 2021 Feb 26;16(2):e0247968. doi: 10.1371/journal.pone.0247968. eCollection 2021.
Bivalve shells are increasingly used as archives for high-resolution paleoclimate analyses. However, there is still an urgent need for quantitative temperature proxies that work without knowledge of the water chemistry-as is required for δ18O-based paleothermometry-and can better withstand diagenetic overprint. Recently, microstructural properties have been identified as a potential candidate fulfilling these requirements. So far, only few different microstructure categories (nacreous, prismatic and crossed-lamellar) of some short-lived species have been studied in detail, and in all such studies, the size and/or shape of individual biomineral units was found to increase with water temperature. Here, we explore whether the same applies to properties of the crossed-acicular microstructure in the hinge plate of Arctica islandica, the microstructurally most uniform shell portion in this species. In order to focus solely on the effect of temperature on microstructural properties, this study uses bivalves that grew their shells under controlled temperature conditions (1, 3, 6, 9, 12 and 15°C) in the laboratory. With increasing temperature, the size of the largest individual biomineral units and the relative proportion of shell occupied by the crystalline phase increased. The size of the largest pores, a specific microstructural feature of A. islandica, whose potential role in biomineralization is discussed here, increased exponentially with culturing temperature. This study employs scanning electron microscopy in combination with automated image processing software, including an innovative machine learning-based image segmentation method. The new method greatly facilitates the recognition of microstructural entities and enables a faster and more reliable microstructural analysis than previously used techniques. Results of this study establish the new microstructural temperature proxy in the crossed-acicular microstructures of A. islandica and point to an overarching control mechanism of temperature on the micrometer-scale architecture of bivalve shells across species boundaries.
双壳贝类越来越多地被用作高分辨率古气候分析的档案。然而，仍然迫切需要不需要了解水化学的定量温度代用指标——这是基于δ18O 的古温度计所需要的——并且能够更好地抵抗成岩作用的影响。最近，微观结构特性已被确定为满足这些要求的潜在候选者。到目前为止，只有少数几种短寿命物种的不同微观结构类别（珍珠层、棱柱层和交错层）被详细研究过，在所有这些研究中，单个生物矿化单元的大小和/或形状都被发现随着水温的升高而增加。在这里，我们探讨了这种情况是否也适用于冰岛北极蛤铰链板交错针状微观结构的特性，在该物种中，这种微观结构是最均匀的贝壳部分。为了专注于温度对微观结构特性的影响，本研究使用在实验室中控制温度条件（1、3、6、9、12 和 15°C）下生长贝壳的双壳类动物。随着温度的升高，最大单个生物矿化单元的大小和结晶相占据贝壳的相对比例增加。最大孔的大小增加，这是冰岛北极蛤的一个特定微观结构特征，本文讨论了其在生物矿化中的潜在作用。最大孔的大小呈指数增长与培养温度。本研究采用扫描电子显微镜结合自动化图像处理软件，包括一种创新的基于机器学习的图像分割方法。新方法极大地方便了微观结构实体的识别，并使微观结构分析比以前使用的技术更快、更可靠。本研究的结果确立了冰岛北极蛤交错针状微观结构中新的微观结构温度代用指标，并指出了温度对物种边界跨越的双壳类贝壳微观结构的整体控制机制。
