Ice nucleation activity of boreal plants with focus on birch trees

Abstract

Zwei Drittel der Erde sind mit Wolken bedeckt. Die Erde ist daher eher ein weißer Planet als ein Blauer. Wolken haben großen Einfluss auf die Albedo der Erde. Allerdings verhalten sich Wolken sehr unterschiedlich. Der Aggregatszustand des enthaltenen Wassers beeinflusst das Abregnen und die Wechselwirkung mit der ein- und ausfallenden Strahlung. Reines Wasser gefriert nicht bei

Two thirds of the Earth are covered by clouds, rendering our planet a white planet rather than a blue one. Hence, clouds have a huge impact on the albedo of the Earth; however, not every cloud acts in the same manner. The microphysical state of the water present in clouds affects precipitation and lifespan of a cloud, as well as their radiative properties. Pure water does not freeze at 0C (the thermodynamic freezing point of water), since freezing is a kinetically hindered process, but rather at far lower temperatures. Pure water droplets in the micrometre size range, such as cloud droplets, freeze only at temperatures around -36 C. Impurities can influence the freezing process by shifting the freezing event to higher temperatures. These are called ice nucleating particles and the process is referred to as heterogeneous nucleation. So far, ice nucleating particles have been found in almost all kingdoms of life, playing a part in the life of many different organisms for a wide range of functions. This work focusses on the ice nucleation activity of plants native to the boreal region. Boreal regions exhibit night frosts even during the warm seasons and winter temperatures that can be as cold as -40 C. However, even under these conditions, life has prevailed and found various mechanisms to cope with the cold. Several plants have developed mechanisms, allowing them to control freezing within their tissue. They exhibit many different molecules regulating not just temperature but also crystal size and cell stability. These properties make plants native to the boreal regions very interesting candidates for ice nucleation research. So far, very little is known about the ice nucleation activity of plants. In 2001, Diehl and colleagues showed that single birch pollen can act as efficient ice nucleating particles. More than ten years later, Pummer and colleagues (2012) published data showing that the size of the ice nucleating component is in the submicron size and that it can be easily washed off the pollen-surface. Due to its small size and its stability towards reactive oxygen species, it has the potential for a long atmospheric lifespan. The chemical nature of the ice nuclei extractable from birch pollen is not fully understood. One goal of this work is to further elucidate their composition. Different biochemical separation techniques (e.g. solid phase extraction cartridges) as well as vibrational spectroscopy were applied. Further, chaotropic reagents and enzymes are used to analyse the role of proteins in the process of heterogeneous ice nucleation. The examined plant material was collected from several parts of ten different birch trees. In this work it was shown that all analysed parts of birch trees (primary wood, secondary wood, leaves, bark, and drill cores of the stem) were ice nucleation active. The contained ice nucleating particles show strong similarities to those found in pollen. Different sample preparations and extractions allowed us to estimate not just the total contained concentration in the plant material, but also the amount of ice nuclei that is easily accessible for the surrounding environment of the tree. Apart from birch trees, berries from several perennial plants (black currant, blueberry, cranberry, chokeberry, lingonberry, sambucus, sea buckthorn, raspberry, and rowanberry) were investigated. Further leaves of blueberry, juniper, raspberry, and sea buckthorn were analysed. All samples were ice nucleation active, showing how far spread this ability is in the Boreal plant kingdom.