Response of Emiliania huxleyi to a high CO2 world: assessing the extent of genetic diversity in the pattern of gene expression

赫胥黎Emiliania huxleyi是一种快速生长的“球石藻”浮游植物，在其细胞外部形成碳酸钙（CaCO3）板。在现代海洋中，赫胥黎是最丰富的“水华形成”浮游植物物种之一，因此在从地表水中去除（输出）碳和碱度方面起着重要作用。大量的实验室研究之前已经研究了环境因素，如光、温度和营养物质，如何相互作用，影响赫胥黎氏杆菌的生长和钙化。然而，对于赫胥黎蛤来说，在生长和钙化之间保持平衡的关键因素是海水的pH值。考虑到这一点，全球的注意力都集中在赫胥黎e.h uxleyi将如何应对由于大气二氧化碳浓度升高而预测的海洋pH值下降（海洋酸化）。最近的研究表明，大气中二氧化碳的增加直接减少了E. huxleyi的钙化；反过来，这种生物从海洋表面输出物质的效率可能会降低。尽管取得了这样的进展，政府间气候变化专门委员会的上一份报告强调，“海洋酸化对海洋生物群的影响，特别是对实现生物钙化的生物的影响，仍然是一个关键的不确定性”。最令人担忧的是，赫胥黎线虫是由“数不清的”遗传变异组成的，而独立实验（包括二氧化碳扰动）并不总是能检测到同一种变异的环境驱动特征。我们实验室的结果支持这一说法：两个变种对生长的光和二氧化碳条件的扰动表现出非常不同的适应模式。基因表达的变化是这些生物对环境变化作出反应的基础，这一事实导致了基因组学和转录组学应用于增加我们对海洋生物地球化学知识的建议。然而，分子遗传学和生物地球化学之间仍然存在着巨大的概念差距：地球化学家需要能够在很长一段时间内应用于整个海洋的概括；生物学家关注的是是什么让生物体独一无二。弥合目前分子生物学和生物地球化学之间的差距的关键是检查基因表达的变异性在多大程度上是由于分离株之间的遗传差异而不是对环境强迫的一般反应。这项研究建立在先前NERC资助的基础上，通过解决赫胥黎e.h遗传变异的基因表达如何响应海洋pH值的变化。我们提出了一个涉及埃塞克斯大学和英国海洋生物协会的合作研究计划，该计划在SOFI的优先主题领域海洋生物地球化学循环中被称为“恒化培养和微阵列分析中的球石团基因表达谱”（WP 2.8, 2.9）。我们实验室开发的“pH-stat”技术将用于在两种pH条件下（目前与2100年以后的预测）培养四种赫胥黎氏杆菌基因变异。将比较基于微阵列的分子信号对不同pH条件的响应，并与生理信号（光合作用和钙化）一起分析。这里提出的工作将在埃塞克斯大学和英国海洋生物协会（海洋2025中心）这两个研究中心之间建立核心联系，这两个研究中心在研究赫胥黎生物方面有着出色的记录。

Emiliania huxleyi is a fast growing 'coccolithophorid' phytoplankton species that forms calcium carbonate (CaCO3) plates on the outside of its cells. In the modern ocean, E. huxleyi is one of the most abundant 'bloom forming' phytoplankton species and consequently plays a major role in removal (export) of both carbon and alkalinity from surface waters. Substantial laboratory research has previously examined how environmental factors, such as light, temperature and nutrients, interact to affect the growth and calcification of E. huxleyi. However, the major factor that is critical to the balance between growth and calcification for E. huxleyi is the pH of seawater. With this in mind, global attention has focused upon how E. huxleyi will respond to the decrease in ocean pH (ocean acidification) that has been predicted as a result of elevated atmospheric CO2 concentrations. Recent research has demonstrated that an increase in atmospheric CO2 directly reduces calcification by E. huxleyi; in turn, the efficiency with which this organism can export material from the surface ocean will likely decrease. Despite such progress, the last report of the Intergovernmental Panel on Climate Change highlighted that 'the impact of ocean acidification on marine biota especially for organisms achieving bio-calcification remains a key uncertainty'. Of major concern is that the species of E. huxleyi is comprised of an 'untold number' of genetic variants and independent experiments (including CO2 perturbations) do not always examine environmentally-driven characteristics for the same variant. Results from our laboratory support this statement: two variants exhibited very different modes of acclimation to perturbations of light and CO2 conditions for growth. Changes in gene expression are the bases by which these organisms appear to respond to environmental change, a fact that has led to suggestions that genomics and transcriptomics should be applied to increase our knowledge of ocean biogeochemistry. However, a huge conceptual gap still exists between molecular genetics and biogeochemistry: geochemists need generalisations that can be applied to the entire ocean over long time periods; biologists focus on what makes an organism unique. Key to bridging the current gap between molecular biology and biogeochemistry is to examine the extent with which variability in gene expression is due to genetic differences amongst isolates versus general responses to environmental forcing. This study builds immediately upon previous NERC grants held by the investigators by addressing how gene expression responds to changes of ocean pH for genetic variants of E. huxleyi. We propose a programme of collaborative research involving the University of Essex and Marine Biological Association of the UK under the SOFI call 'Coccolithophore gene expression profiles in chemostat culture and microarray analysis' (WP 2.8, 2.9) within priority topic area marine biogeochemical cycles. 'pH-stat' technology developed in our laboratory will be used to grow four E. huxleyi genetic variants at two pH conditions (present day versus that predicted beyond the year 2100). Microarray-based molecular signals in response to the different pH conditions within and between variants will be compared but also analysed alongside physiological signals (photosynthesis and calcificiation). Work proposed here will establish a core link between two research centers with an excellent track record investigating E. huxleyi biology, the University of Essex and the UK's Marine Biological Association, which is an Ocean 2025 Centre.