Bio-precipitation or self-cryopreservation: Why does pollen nucleate ice?

来自过冷水的结晶冰的成核在混合相云的冰川作用中起着关键作用，从而影响天气和气候（Murray等人，2012年，Vergara-Temprado等人，2018年）。已知与一些植物物种的花粉粒相关的冰活性多糖（IAP）有效地使冰成核，并且以潜在的显著量存在于大气中（Dreischmeier等人，2017年，Pummer等人，2012年，O 'Sullivan等人，2015年，Diehl等人，2002年，von Blohn等人，2005年）。由桦树花粉IAPs引起的冰成核在自然界中是非常特异的，在-16 ℃左右急剧发生（图1（a）），表明IAPs已经进化到与冰相互作用。还不知道桦树花粉为什么产生IAP，或者绝大多数花粉如何使冰成核，因为已经研究了不到10个物种（von Blohn等人，2005年）。有两个明显的原因，花粉进化IAP。1)为了控制降水，帮助被风吹到高空的飞机返回地面。这是一个“生物沉淀”的例子，生命与云相互作用，传播自己。2)通过控制冰的形成来提高对寒冷温度的耐受性，无论是在云中还是在地面。已知不同种类的花粉具有不同程度的耐干燥性，这取决于它们采用的特定传播方法。这可能与它们的成核能力有关。学生将从皇家植物园（RGB）基尤的植物标本中收集花粉，并测量它们的成核能力和抗冻性，以生成一个全面的多物种数据集。有了这个数据集，学生将进行花粉冰成核的系统发育背景和生态作用的分析，允许为什么花粉成核冰的评估。方法：从RGB基尤植物园收集的各种植物的花粉粒将被获得，并使用液滴冷冻技术评估它们的冰成核能力（Whale等人，2015年）。负责冰成核的多糖将通过冰色谱法分离并表征。然后将开发花粉IAP存在的测试和选择性变性它们的方法。冰成核和花粉干燥耐受性之间的关系将从文献和经验测定中获得（Nebot等人，2018年）。在威廉姆斯（2013）之后，将花粉粒冷冻在液态水中，并评估解冻后的活力。花粉活力测试将在物种水平上进行优化。这将显示IAP是否有助于花粉粒在冷冻中存活。结果将与现有的系统发育和生态数据集进行比较，以评估哪些植物家族使冰成核，并追溯植物冰成核的遗传起源（例如，科尔维尔和普里查德，2019年）

Nucleation of crystalline ice from supercooled water plays a critical role in the glaciation of mixed-phase clouds and thereby impacts both weather and climate (Murray et al., 2012, Vergara-Temprado et al., 2018). Ice-active polysaccharides (IAPs) associated with pollen grains of some plant species are known to nucleate ice effectively and are present in the atmosphere in potentially significant quantities (Dreischmeier et al., 2017, Pummer et al., 2012, O'Sullivan et al., 2015, Diehl et al., 2002, von Blohn et al., 2005). Ice nucleation caused by birch pollen IAPs is remarkably specific in nature, occurring sharply at around -16C (Fig. 1(a)), indicating that IAPs have evolved to interact with ice. It is not known why birch pollen produces IAPs or how well the vast majority of pollens nucleate ice as less than 10 species have been studied (von Blohn et al., 2005). There are two obvious reasons for pollens to evolve IAPs. 1) To control precipitation and aid return to the ground having been lofted to altitude by wind. This is an example of 'bio-precipitation' where life interacts with clouds to propagate itself. 2) To improve tolerance of cold temperatures by controlling ice formation, be it in clouds or at ground level. Different species pollens are known to be desiccation tolerant to varying extents, depending on the specific dispersal methods they employ. This may be related to their ability to nucleate ice.The student will collect pollen from plant species held in the collections at Royal Botanic Gardens (RGB) Kew and measure their ice nucleation ability and freeze tolerance to generate a comprehensive multi-species dataset. Armed with this dataset the student will conduct an analysis of the phylogenetic background and ecological role of pollen ice nucleation, allowing an assessment of why pollen nucleates ice. This information will be of ecological interest and improve understanding of biosphere/atmosphere interaction.Methodology: Pollen grains from a wide range of plant species will be obtained from the RGB Kew collections and their ice nucleation ability assessed using droplet freezing techniques (Whale et al., 2015). The polysaccharides responsible for ice nucleation will be isolated by ice chromatography and characterized. A test for the presence of pollen IAPs and a method for selectively denaturing them will then be developed. The relationship between ice nucleation and pollen desiccation tolerance will be obtained from the literature and empirical determination (Nebot et al., 2018). Pollen grains will be frozen in liquid water after Williams (2013), and post-thaw viability assessed. Pollen viability tests will be optimized at the species level. This will show whether the IAPs help pollen grains to survive freezing. Results will be compared with existing phylogenetic and ecological datasets to assess which plant families nucleate ice and to trace back the genetic origin of plant ice nucleation (e.g. Colville and Pritchard, 2019)