Photoprotection in Diverse, Desiccation-Tolerant, Desert Green Algae and Their Close Aquatic Relatives

本研究的目的是确定沙漠绿藻在干燥和复水过程中保护光合作用机构的机制(S)背后的过程。地球上生命进化的关键一步是从生活在水中转移到在陆地上生存。实现这一飞跃的有机体面临着高光、极端温度和干燥。开花植物、针叶树、苔藓和蕨类植物都是成功入侵陆地的单一绿藻祖先的后代。它们共有的特征可能是在陆地上生存的关键，也可能仅仅是从共同祖先那里继承下来的特征。为了区分这两种可能性，必须研究其他绿色的、具有光合作用的陆地生物，这些生物不是来自那个共同的绿藻祖先的后代。在生活在美国西南部沙漠微生物结壳中的各种自由生活的微小绿藻中都发现了这种微生物。沙漠绿藻比它们的水生藻类更能在干燥中生存，沙漠藻类在复水后很快(在几分钟内)就能从干燥中恢复过来。因此，它们为研究脱水耐性的基本机制提供了一个理想的系统。随着农业向边际地区扩张，同时气候也发生变化，陆地植物为在干旱、高温和寒冷期间保护自己免受强烈阳光照射而发展起来的特征尤为重要。荒漠绿藻具有许多特点，不仅具有较大的农业和生态重要性的绿色植物，而且越来越多地被用于生产生物燃料。研究与教育和培训的结合将包括制定6-8年级的教案，供教师在科德角国家海滨带领短途旅行时使用，重点是“沙丘上令人惊讶的微生物”。本科生的研究机会将集中在绿色植物和微生物结壳中的干燥胁迫和光生物学的研究项目上。总体研究目标是确定快速可逆机制(S)在干燥和复水过程中保护光合作用装置的潜在过程(和系统发育分布)，以及四个藻类中的成对沙漠和水生绿藻。光保护或耐脱水机制在多个绿藻纲沙漠成员中的广泛系统发育分布将表明(通过简约)这些机制可能是较大的绿色植物支系的祖先特征。对沙漠和近缘水生动物的特征进行比较，将确定特定的谱系特征。研究将确定干燥耐受性和干燥诱导的光保护能力是否具有持久的遗传成分(即使藻类生长在水合状态下)，以及光和干燥速度对光保护效率的影响是否类似于它们对地衣和苔藓的影响。最近提出的干燥诱导光保护的机制(PSII到PSI溢出，P700+氧化，以及叶绿素-叶绿素电荷转移状态)将在硫磺地衣和耐干燥绿藻中进行测试，使用荧光衰减动力学和模型，EPR，以及线性和圆二色谱。整个转录组鸟枪测序将被用来测试，就像在苔藓植物中一样，典型的沙漠绿藻圆生栅藻的转录组是否具有防止干燥的结构性细胞保护作用。

The objective of this research is to determine the process(es) underlying the mechanism(s) protecting the photosynthetic apparatus of desert green algae during desiccation and rehydration. A key step in the evolution of life on Earth was the move from living in water to surviving on land. Organisms making that leap faced high light levels, extreme temperatures, and desiccation. Flowering plants, conifers, mosses, and ferns are all descended from a single green algal ancestor that successfully invaded land. Traits they share may be essential for surviving on land, or they might simply be traits inherited from the common ancestor. To tease apart these two possibilities, other green, photosynthetic, terrestrial organisms must be studied that are not descended from that common green algal ancestor. Such organisms are found among diverse, free-living, microscopic green algae inhabiting microbiotic crusts of the desert Southwestern U.S. Desert green algae survive desiccation much better than their aquatic algal relatives, and desert algae recover from desiccation very rapidly (within minutes) when rehydrated. Thus, they make an ideal system to study basic mechanisms of desiccation tolerance. The traits land plants developed to protect themselves from intense sunlight during drought, high heat, and cold are particularly important as agriculture expands into marginal lands while climate also shifts. Desert green algae share many characteristics with larger agriculturally and ecologically important green plants, and also with the green algae increasingly used for biofuel production. Research integration with education and training will include development of 6-8th grade Lesson Plans, available to teachers leading excursions at Cape Cod National Seashore focusing on "Surprising Microbes of the Dunes". Research opportunities for undergraduates will be focused on projects studying desiccation stress and photobiology in green plants and microbiotic crusts.The overall research objective is to determine the process(es) underlying (and phylogenetic distribution of) rapidly-reversible mechanism(s) protecting the photosynthetic apparatus during desiccation and rehydration, in paired desert and aquatic green algae within four algal classes. Broad phylogenetic distribution of photoprotective or desiccation-tolerance mechanisms across desert members of multiple green algal classes will suggest (through parsimony) that these mechanisms are likely ancestral traits in the larger green plant clade. Comparison of traits in desert and closely-related aquatic relatives will identify lineage-specific characteristics. Research will establish whether the capacity for desiccation tolerance and desiccation-induced photoprotection has a persistent genetic component (even when algae are grown hydrated), and whether influences of light and speed of desiccation on the efficiency of photoprotection are similar to their effects in lichens and mosses. Mechanisms that have been recently proposed to underlie desiccation-induced photoprotection (PSII to PSI spillover, P700+ oxidation, and chlorophyll-chlorophyll charge transfer states) will be tested in the lichen Parmelia sulcata and desiccation-tolerant green algae using fluorescence decay kinetics and modeling, EPR, and linear and circular dichroism spectroscopy. Whole transcriptome shotgun sequencing will be used to test whether, as in bryophytes, the transcriptome in the representative desert-derived green alga Scenedesmus rotundus possesses constititive cellular protection against desiccation.