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

过冷水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 et al., 2012; Vergara-Temprado et al., 2018）。已知与某些植物花粉粒相关的冰活性多糖（IAPs）可以有效地成核冰，并且可能大量存在于大气中（Dreischmeier等人，2017，Pummer等人，2012，O'Sullivan等人，2015，Diehl等人，2002，von Blohn等人，2005）。桦树花粉iap引起的冰成核在本质上具有显著的特异性，在-16℃左右急剧发生（图1(a)），表明iap已经进化到与冰相互作用。目前尚不清楚桦树花粉产生iap的原因，也不知道绝大多数花粉成核冰的能力如何，因为只有不到10种桦树被研究过（von Blohn et al., 2005）。花粉进化iap有两个明显的原因。1)控制降水，帮助被风抬升到高空后返回地面。这是“生物降水”的一个例子，生命与云相互作用以传播自己。2)通过控制云层或地面结冰来提高对低温的耐受性。已知不同种类的花粉在不同程度上具有干燥耐受性，这取决于它们采用的特定传播方法。这可能与它们形成冰核的能力有关。该学生将从皇家植物园（RGB） Kew收集的植物物种中收集花粉，并测量它们的冰核能力和耐寒性，以生成一个全面的多物种数据集。有了这个数据集，学生将对花粉冰成核的系统发育背景和生态作用进行分析，从而评估花粉冰成核的原因。这一信息将具有生态学意义，并增进对生物圈/大气相互作用的了解。方法：将从RGB Kew收集的各种植物中获得花粉粒，并使用液滴冷冻技术评估其冰核能力（Whale et al., 2015）。冰成核的多糖将通过冰色谱分离和表征。然后将开发花粉iap存在的测试和选择性地使其变性的方法。冰核与花粉干燥耐受性之间的关系将通过文献和经验测定获得（Nebot et al., 2018）。在Williams（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冰冻中存活。结果将与现有的系统发育和生态数据集进行比较，以评估哪些植物科形成冰核，并追溯植物冰核的遗传起源（例如Colville和Pritchard， 2019）。

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)