Functional adaptation of diatoms to environmental conditions in sea ice of the Southern Ocean

冬季，海冰覆盖的面积高达地球表面的7%，这显然是地球上最大的生物群之一。海冰可以被认为是覆盖在海洋表面的一层薄薄的毯子，它控制着，但也受到热量和水分通量的控制。从细菌、藻类、小动物、鲸鱼甚至人类在内的许多海洋生物的生命周期都受到大规模海冰形成周期的影响。因此，海冰被认为是地球系统的基本组成部分，这是我们预测未来气候条件的关键，过去十年来，由于全球迅速变暖，这一点变得越来越明显。由于全球气温升高，海冰逐渐消失的可能影响尚不完全清楚，但夏季北冰洋海冰缩小的图片已成为媒体的焦点和公众越来越多的关注。这项赠款申请的重点是生活在海冰多孔基质中的微藻。它们中的大多数属于硅藻类，通常硅藻在地球上扮演着关键角色，因为它们负责25%的主要二氧化碳固定，这相当于所有热带雨林的总和。为什么特别是硅藻主导着海冰藻类群落，目前还不完全清楚，但我们知道，它们实际上是冰盖水域中较高营养水平的固定碳的唯一来源。事实上，硅藻-磷虾-鲸鱼的食物链很短，依赖于冰藻，因为在冬季缺乏其他食物来源的情况下，冰藻为幼虾提供食物。这条食物链可能会受到全球变暖导致的海冰减少的严重影响。尽管极地冰藻意义重大，但人们对它们的基本生物学几乎一无所知。这就是为什么美国能源部(DOE)选择了最重要的海冰硅藻物种之一(Fragilariopsis Cylindrus)在申请者的指导下对其基因组进行测序。基因组序列现在已经可用，并为研究海水冰点以下的生命带来了令人兴奋的新见解。例如，基因组编码了一个以前未知的抗冻蛋白家族。它还编码了在这个基因组中没有预料到的基因，例如编码来自细菌的蛋白质的基因，用于收集光线来产生ATP。这是没有预料到的，因为圆柱藻的叶绿体和高等植物一样，在叶绿体中光合作用产生ATP。因此，这个基因的功能与基因组中的许多其他基因一样尚不清楚。阐明基因功能的第一步是研究它们在不同环境条件下的表达，这是这项提议的主要目标之一。我们建议进行实验，我们在海冰中存在的条件(例如冰冻温度、营养限制)下处理细胞，这些条件是极地地区全球变暖的预测结果(例如温度升高和二氧化碳浓度)。然后，我们将使用高通量测序技术对转录组进行测序，以确定在我们的实验条件下差异表达的基因。选定的基因将通过定量qPCR进行确认。这项研究将有助于确定圆柱藻对每年秋季形成的海冰的短期适应。将通过圆柱藻和近亲科尔盖耳藻的比较基因组分析来研究它们对海冰的适应情况，科尔盖耳藻也生活在寒冷的极地海水中，但在海冰中不能茁壮成长。这一比较不仅将揭示在海冰中茁壮成长所必需的具体适应，而且还将揭示全球变暖在极地海洋中的预测结果，因为无冰水域将越来越多地主导这些栖息地。高通量测序将用于对克氏狸殖吸虫的基因组进行测序，大小规模的基因组分析将揭示差异。

In winter sea ice covers an area of up to 7% of the earth surface, which clearly as such is one of the largest biomes on earth. Sea ice can be thought of as a thin blanket covering the ocean surface, which controls, but is also controlled by fluxes of heat and moisture. The life cycles of many marine organisms ranging from bacteria, algae, small animals, whales and even humans are influenced by large-scale cycles of sea ice formation. Thus, sea ice is recognized as a fundamental component of the system earth, which is key for our predictions of future climate conditions as has become increasingly apparent in the last decade due to rapid global warming. The possible implications of a gradual loss of sea ice due to higher global temperatures are not fully understood yet but pictures of shrinking Arctic sea ice in summer has become the focus of media and increasing attention from the general public. Focus of this grant application are microalgae that live inside the porous matrix of sea ice. Most of them belong to the group of diatoms, which in general play a key role on earth because they are responsible for 25% of primary fixation of carbon dioxide, which is as much as all tropical rain forests combined. Why especially diatoms dominate sea ice algal communities is not fully understood yet but we know that they are virtually the sole source of fixed carbon for higher trophical levels in ice-covered waters. In fact, the short food chain diatom-krill-whale is depend on ice algae because they provide food for young krill when other sources of food in winter are lacking. This food chain might be severely influenced by a reduction of sea ice due to global warming. Despite the significance of polar sea ice algae virtually nothing is know about their fundamental biology. This is why the US Department of Energy (DOE) has selected one of the most important sea ice diatom species (Fragilariopsis cylindrus) to sequence its genome under the guidance of the applicant. The genome sequence is now available and has brought exciting new insights into life at and below the freezing point of sea water. For instance, the genome encodes a previously unknown family of antifreeze proteins. It also encodes genes that were not anticipated to be present in this genome such as a gene that encodes a protein from bacteria for harvesting light to generate ATP. It was not anticipated because F. cylindrus has chloroplasts in which photosynthesis like in higher plants generates ATP. Thus, the function of this gene is not known yet as for many other genes in the genome as well. One first step to shed light on the function of genes is to study their expression under different environmental conditions, which is one main goal of this proposal. We propose to conduct experiments where we treat the cells under conditions that are present in sea ice (e.g. freezing temperatures, nutrient limitation) and which are predicted outcomes of global warming in polar regions (e.g. elevated temperatures and carbon dioxide concentrations). We will then sequence the transcriptome with high-throughput sequencing technology to identify genes that are differentially expressed under our experimental conditions. Selected genes will be confirmed by quantitative qPCR. This study will help to identify the short term acclimation of F. cylindrus to sea ice as it forms every autumn. Adaptation to sea ice will be studied by comparative genome analysis between F. cylindrus and the close relative F. kerguelensis, which lives also in cold polar sea water but doesn't thrive in sea ice. This comparison will not only shed light on specific adaptation necessary for thriving in sea ice but also the predicted outcomes of global warming in polar oceans because ice free waters will more and more dominate these habitats. High-throughput sequencing will be used to sequence the genome of F. kerguelensis and large and small scale genome analysis will reveal the differences.