EAGER: Did Oxygenic Photosynthesis Cause Marine Group 1 Crenarchaeota to Take a Dive?

表层海水中过氧化氢的存在与太阳辐射有关，其浓度随季节和日变化而变化。环境中的H2O2浓度相对于许多细菌所能耐受的浓度是低的，但相对于细菌，海洋1组绿原藻（MG1C）似乎缺乏对它的防御。因此，如果他们没有开发出减轻H2O2毒性的替代策略，例如通过消除、替换或保护细胞中不易受H2O2中毒影响的敏感靶点，那么H2O2的光区浓度可能足以抑制MG1C代谢。由于目前仅有2个MG1C基因组被测序，而且这些序列还没有得到完整的注释，更不用说对这些注释进行实验验证了，因此很难从基因组证据中推断出MG1C是否具有其他适应环境中H2O2的能力。因此，本项目将通过结合中浮游生物组合的毒性生物测定和微自放射-荧光原位杂交（MAR-FISH）来确定目标群体，通过测试MG1C对H2O2的敏感性来直接解决这个问题。如果提出的实验验证了MG1C对H2O2敏感，那么MG1C的这一特性可以回答许多问题。1) MG1C的全球和季节分布-为什么它们不在表层生活；高纬度地区的季节分布；它们相对于Euryarchaeota的分布，Euryarchaeota含有更多的过氧化氢酶和过氧化物酶，在地表水和近岸环境中更为常见。2)底物利用和代谢模式-如果它们对它敏感，MG1C只能利用底物或通过不产生H2O2的途径代谢。3)缺乏古细菌病原体——活性氧（ROS）的爆发是动物宿主抵御病原体的第一道防线。4)为什么MG1C很难培养-许多标准实验室操作会产生ROS；这可能也适用于难以培养的细菌，因为不是所有的细菌都有过氧化氢酶或过氧化物酶基因。5)细菌和古细菌对过氧化物敏感性的差异可能是由于“冷冻代谢事故”造成的，这是古细菌生物化学的一个基本特性，大约在氧气光合作用进化的时候。6)细菌和古生菌之间的一些生化和组成差异——一些细菌适应高ROS生活方式的一种方式是消除或保护其细胞机制中的ROS目标。这是一个范围和预算有限的试点项目。尽管如此，它将直接促进研究生和本科教育。研究结果将在会议和课堂上通过出版物和报告进行传播，数据将根据美国国家科学基金会的数据发布政策向公众提供。尽管这个问题很重要，而且支持这一假设的论据和证据具有挑衅性和说服力，但它们是间接的、推测性的，无法通过同行评议的检验。这个假设的含义是变革性的，然而，这个项目将允许对这个假设进行有限的测试。

The presence of hydrogen peroxide in surface seawater is tied to solar radiation and its concentration varies seasonally as well as diurnally. H2O2 concentrations in the environment are low relative to those tolerated by many Bacteria, but relative to Bacteria, marine group 1 Crenarchaeota (MG1C) appear to lack defenses against it. Thus, photic zone concentrations of H2O2 may be sufficient to inhibit MG1C metabolism if they have not developed alternative strategies for mitigating H2O2 toxicity, for example by eliminating, replacing or protecting sensitive targets in the cell with structures that are less susceptible to H2O2 poisoning. Given that only 2 MG1C genomes have been sequenced and that these sequences have not been fully annotated, let alone had the annotations verified experimentally, it is difficult to infer from genomic evidence whether MG1C have other adaptations for dealing with H2O2 in the environment. Thus, this project will to address this question directly by testing MG1C sensitivity to H2O2 by combining toxicity bioassays of mesopelagic plankton assemblages with MicroAutoRadiography-Fluorescent In Situ Hybridization (MAR-FISH) to identify target groups. If the proposed experiments verify that MG1C are sensitive to H2O2, this property of MG1C could answer a number of questions. 1) The global and seasonal distributions of MG1C - why they do not live in the surface layer; their seasonal distribution at high latitudes; and their distributions relative to Euryarchaeota, which have more catalases and peroxidases and are more common in surface waters and nearshore environments. 2) Substrate utilization and metabolism patterns - if they are sensitive to it, MG1C can only use substrates or metabolize via pathways that do not generate H2O2. 3) Lack of Archaeal pathogens - a burst of reactive oxygen species (ROS) is an animal host's first line of defense against pathogens. 4) Why MG1C are hard to culture - many standard lab practices generate ROS; this may apply to Bacteria that have proven hard to culture as well, as not all Bacteria have catalase or peroxidase genes. 5) The divergence between Bacteria and Archaea - peroxide sensitivity may result from a "frozen metabolic accident" that was a fundamental property of Archaeal biochemistry at about the time that oxygenic photosynthesis evolved. 6) Some of the biochemical and compositional differences between Bacteria and Archaea - one way some Bacteria have adapted to high-ROS lifestyles has been to eliminate or protect ROS targets in their cellular machinery.This is a pilot project of limited scope and budget. Nonetheless it will contribute directly to graduate and undergraduate education. Results will be disseminated via publications and presentations at meetings and in the classroom, and data will be made publicly available in accordance with NSF's data release policy. Although the problem is important and that the arguments and evidence in favor of the hypothesis are provocative and convincing, they are circumstantial, speculative and would not pass the test of peer-review. The implications of the hypothesis are transformative, however, and this project would allow a limited test of this hypothesis.