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

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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.