Biochemical Changes in Aquatic Organisms in a Warming World

我们的世界正在变暖，但我们还没有完全理解这一影响深远、具有全球意义的事件的后果。植物和动物对水温升高的反应是通过被称为自粘适应的过程来调整细胞膜的化学成分。这种生化适应包括以牺牲多不饱和脂肪酸(PUFA)为代价增加饱和脂肪酸的数量。随着水的加热，多不饱和脂肪酸的损失，特别是omega-3长链多不饱和脂肪酸(LC-PUFA；即20和22碳长)的损失，对所有生命都有巨大的影响。浮游植物占全球初级总产量的1/2，是omega-3 LC-PUFA的主要生产者。然而，我们还不知道气候变暖将如何影响这些必需化合物的生产(来源)和分布(命运)。气候模型人员预测，地表气温和极端高温天数将会增加；伴随而来的是地表水温的上升(特别是在浅层生产系统中)。这些气温上升预计将在温带和北部地区感受最深刻。这些地区(水温较低，全民教育产量高)的大量(100万英镑)小型湿地(与陆地生态系统高度连通)表明，正是在生物圈的这些地区，淡水衍生的LC-PUFA的产量最大。大量证据表明，omega-3 LC-PUFA对动物的健康和福祉至关重要，支持提高生长速度和繁殖能力，并以积极的方式促进心血管、免疫和认知健康。动物可以简单地从短链(即18碳)脂肪酸前体合成LC-PUFA，然而，对各种鱼类和哺乳动物(包括人类)的研究表明，18碳omega-3 FA到LC-PUFA的转化是“不可靠和受限的”，因此脊椎动物大脑、神经组织、心血管系统、视网膜、精子和牛奶中发现的大部分必需脂肪酸(EFA)来自饮食。像EPA和DHA一样，omega-3 LC-PUFA不是由陆地植物或昆虫产生的。因此，陆生生物最终从水生生物(如昆虫出现；鱼类)获得大部分omega-3 LC-PUFA，而水生生物又从藻类获得它们。这种“预先形成”的全民教育来源是陆生(和水生)动物获得这些基本生物化学物质的最有效方式，其数量足以维持它们处于最佳状态。通过实验室和实地研究，我的学生(1名PDF、2名博士和1名硕士)和我以及我的同事将解决关键的知识差距，从而改变我们对气候变暖如何影响水生衍生omega-3 LC-PUFA的生产和再分配的理解。我们将：1)在受控的实验室条件下，量化与生物化学相关的脂质类别(如磷脂、二甘油)中多不饱和脂肪酸的产生，作为来自几个代表性分类群的几个世界性藻类物种中水的函数；以及2)测量天然池塘温和温暖的部分，并测量藻类中多不饱和脂肪酸的内部产生及其向浮游动物和昆虫幼虫阶段的营养转移，以及LC-PUFA从池塘到邻近陆地系统的重新分配(通过昆虫出现)。这种关于气候变化如何影响LC-PUFA生产的新概念将从根本上改变我们对我们的水生资源所发挥的关键作用的认识，以及在变暖的世界中这种作用可能会如何改变。

Our world is warming up yet we do not fully understand the ramifications of this far-reaching, globally-significant, event. Plants and animals respond to increasing water temperature by adjusting the chemical composition of their cell membranes through the process known as homeoviscous adaptation. This biochemical adaptation involves increasing the amount of saturated fatty acids at the expense of polyunsaturated fatty acids (PUFA). The loss of PUFA as water heats up, particularly the loss of omega-3 long-chain PUFA (LC-PUFA; i.e. 20- and 22-carbons long), has enormous implications for all life. Phytoplankton contribute >½ of the total primary production globally and are the main producers of omega-3 LC-PUFA. However, we do not yet understand how climate warming will affect the production (source) and distribution (fate) of these essential compounds. Climate modelers predict an increase in surface air temperatures and in the number of extreme heat days; with concomitant increases in surface water temperatures (especially in shallow productive systems). These temperature increases are expected to be most profoundly felt in temperate and northern regions. The large (> 1 million) number of small wetlands (with high connectivity to terrestrial ecosystems) in these regions (where water temperatures were colder and EFA production was high) suggest that it is in these regions of the biosphere that freshwater-derived LC-PUFA are being produced in the greatest quantities. A large body of evidence demonstrates that omega-3 LC-PUFA are critical for the health and well-being of animals, supporting enhanced growth rates and reproductive capacities and contributing in demonstratively-positive ways to cardiovascular, immunologic, and cognitive health. Animals could simply synthesize LC-PUFA from shorter-chain (i.e. 18-carbon) fatty acid precursors, however, studies with a variety of fishes and mammals (including humans) have demonstrated that the conversion of 18-carbon omega-3 FA to LC-PUFA such as EPA (20:5n-3) and DHA (22:6n-3) is “unreliable and restricted” and that therefore the majority of the essential fatty acids (EFA) found in the vertebrate brain, nervous tissue, cardiovascular system, retina, sperm and milk come from the diet. Omega-3 LC-PUFA like EPA and DHA are not produced by terrestrial plants or insects. Thus, terrestrial organisms ultimately obtain much of their omega-3 LC-PUFA from aquatic organisms (e.g. insect emergence; fish) which, in turn, obtain them from algae. This “pre-formed” source of EFA is the most efficient way for terrestrial (and aquatic) animals to obtain these essential biochemicals in amounts sufficient to maintain them in optimal condition. Using lab and field studies, my students (1 PDF, 2 PhD and 1 MSc) and I, and my colleagues, will address critical knowledge gaps thereby transforming our understanding of how climate warming affects the production and re-distribution of aquatic-derived omega-3 LC-PUFA. We will: 1) quantify, under controlled laboratory conditions, the production of PUFA in biochemically-relevant lipid classes (e.g. phospholipids, diacylglycerols) as a function of water in several cosmopolitan algal species from several representative taxa, and 2) gently warm portions of natural ponds and measure both the internal production of PUFA in algae and their trophic transfer to zooplankton and the larval stages of insects and also the flux of LC-PUFA (redistribution) from the ponds onto adjacent terrestrial systems (via insect emergence). This new conceptualization of how climate change affects LC-PUFA production will fundamentally transform our appreciation of the key role that our aquatic resources play and how this role may be altered in a warming world.