Environmental characteristics and E. huxleyi coccoliths mass and morphology in the Mediterranean Sea during MedSeA and Meteor M84/3 cruises (May 2013, April 2011)

To understand the response of marine calcifying organisms under high CO2 scenarios, it is critical to study their calcification patterns in the natural environment. This paper focuses on a major calcifying phytoplankton group, the coccolithophores, through the interpretation of water samples collected along a W-E Mediterranean transect during two research cruises, in April 2011 (Meteor cruise M84/3) and May 2013 (MedSeA cruise 2013). The Mediterranean Sea is a marginal sea characterized by large biogeochemical gradients. Currently, it is undergoing both warming and ocean acidification, processes which are rapidly modifying species distribution and calcification. The species Emiliania huxleyi largely dominates the total coccolithophore production in the Mediterranean Sea. A series of morphometric measurements were performed on the coccoliths of this species to estimate their mass, length and calculate a calcification index (proxy for the size-normalized calcification degree). The most abundant morphotype of E. huxleyi in the Mediterranean Sea is Type A. Coccoliths of this morphotype were additionally analyzed based on scanning electron microscopy images: four calcification varieties were quantified, according to the relationship between slit length - tube width, and the state of the central area (open or closed). The average E. huxleyi coccolith mass along the Mediterranean oceanographic transect depended strongly on both the average coccolith length and calcification index. The variability in average coccolith length and calcification index across samples reflected oscillations in the relative abundance of the calcification varieties. We also demonstrated that the distribution of the calcification varieties followed the main environmental gradients (carbonate chemistry, salinity, temperature, nutrient concentrations). Hence, shifts in the distribution of the calcification varieties and of the average E. huxleyi coccolith mass are to be expected in the Mediterranean Sea under climate change. These physiological and ecological responses will modulate the net coccolithophore calcification and, ultimately, the regional carbonate export to the seafloor.为理解海洋钙化生物在高CO₂情景下的响应，研究其在自然环境中的钙化模式至关重要。本研究聚焦于一类主要的钙化浮游植物类群——颗石藻（coccolithophores），通过解读2011年4月（Meteor航次M84/3）与2013年5月（MedSeA 2013航次）两次科研巡航期间沿地中海东西向断面采集的水样数据展开研究。地中海作为边缘海，以显著的生物地球化学梯度为特征。当前，该海域正同时经历海洋变暖和酸化过程，这些过程正快速改变物种分布与钙化作用模式。赫氏颗石藻（Emiliania huxleyi）在地中海总颗石藻生产力中占据主导地位。研究团队针对该物种的颗石（coccolith）开展了一系列形态计量学测量，以估算其质量、长度，并计算钙化指数（calcification index，用于表征标准化尺寸后的钙化程度）。地中海海域中丰度最高的赫氏颗石藻形态型（morphotype）为A型。针对该形态型的颗石，研究团队还基于扫描电子显微镜（scanning electron microscopy, SEM）图像开展了额外分析：根据裂隙长度-管宽的关系以及中心区域状态（开放或闭合），共量化得到4种钙化变种。地中海海洋断面上的赫氏颗石平均颗石质量，与平均颗石长度及钙化指数均存在显著相关性。不同采样点间平均颗石长度与钙化指数的变异，反映了各钙化变种相对丰度的波动。本研究还证实，钙化变种的分布与主要环境梯度（碳酸盐化学、盐度、温度、营养盐浓度）密切相关。因此，在气候变化背景下的地中海海域，钙化变种的分布以及赫氏颗石平均颗石质量均将发生改变。这些生理与生态响应将调控颗石藻总净钙化量，并最终影响区域碳酸盐向海底的输出通量。