Relative distribution of heterococcolithophores and holococcolithophores in the Mediterranean Sea during the MedSeA and Meteor M84/3 cruises (May 2013, April 2011)

Coccolithophores are unicellular pelagic algae, capable of calcification. In the Mediterranean Sea, several species have a well-known haplo-diploid life cycle, alternating the production of different types of calcite plates, the holo- and hetero-coccoliths. The environmental triggers of haplo-diploid transformations in coccolithophores are still uncertain and studies in the natural environment are fundamental to reveal their ecology. We analyzed the distribution of both phases along a W-E Mediterranean transect during April 2011 and May 2013 (spring season), following strong environmental gradients. The proportion of holococcolithophores:heterococcolithophores of selected species varies not only vertically along the water column, but also longitudinally, following the overall biogeochemical gradients. Based on the environmental affinities of the coccolithophore life phases, we conclude that a dimorphic life cycle might provide additional adaptability to the south-eastern (SE) Mediterranean environment (particularly rich in carbonate ions, warm, stratified and nutrient limited) and support the survival of species whose diploid phases are in contrast adapted to Atlantic or south-western (SW) Mediterranean conditions. These mechanisms can alter the total coccolithophore response to ongoing climate changes.

颗石藻（Coccolithophores）是一类能够进行钙化作用的单细胞浮游藻类。在地中海海域，多个已知物种具备典型的单倍-二倍体生命周期，会交替产生两种不同类型的方解石鳞片：全颗石（holococcolith）与异颗石（heterococcolith）。目前学界对于颗石藻单倍-二倍体转化的环境触发机制仍不明确，而开展自然环境下的相关研究对揭示其生态特征至关重要。
本研究于2011年4月与2013年5月的春季，沿西-东向地中海断面开展采样，覆盖了显著的环境梯度，分析了两类生命阶段的分布特征。所选物种的全颗石藻（holococcolithophore）与异颗石藻（heterococcolithophore）占比，不仅随水柱垂直剖面发生变化，同时也顺着整体生物地球化学梯度呈现纵向差异。
基于颗石藻不同生命阶段的环境亲和性，本研究认为二型生命周期或许能为颗石藻适应东南地中海（SE）海域环境——该区域碳酸根离子富集、水温较高、水体层化且营养盐受限——提供额外适应性，并助力那些二倍体阶段更适配大西洋或西南地中海（SW）环境的物种存活。这类生存机制或能改变颗石藻群落对当前气候变化的整体响应模式。