How to freeze water biologically
Ice formation is a fundamental process for life. Certain organisms such as ice active bacteria can catalyse the formation of ice through specific ice-nucleating proteins (INPs). It’s unknown why ice-nucleating organisms have this skill, though one idea is that they could use it to attack plants through frost. Some organisms also promote local ice nucleation to avoid freezing in other places. This remarkable ability is well-recognised for its importance in global precipitation levels, and is of great interest to climate researchers. It has also been co-opted for a range of applications including frost prevention in agriculture, or to create artificial ice. Despite these use cases, the molecular mechanisms underpinning protein-driven ice formation remain a mystery, mostly due to a lack of proper instrumentation and methods. “To follow the process of ice nucleation, you need to look at a few layers of water molecules interacting with a single layer of proteins,” explains Tobias Weidner, associate professor in the Department of Chemistry at Aarhus University, in Denmark and ProIce project coordinator. While some techniques can do that, the equipment and theory have only become available recently, he says. To fill in this fundamental knowledge gap, the EU-funded ProIce project researched INPs at a molecular level, taking advantage of recent advances in ultrafast vibrational spectroscopy – a technique that uses laser pulses to vibrate molecules and gather information from them. “The most important breakthroughs were the discovery of pH control of ice activity, and the development of methods to track protein-water interactions,” says Weidner.
Probing ice nucleation
The first step of the project was to investigate INPs at the interface with pure water, to see how these proteins fold in contact with water; proteins fold in order to carry out specific biological functions. “It turns out the proteins fold into beta helices, a structure previously predicted in simulations,” adds Weidner. Through further experiments, ProIce researchers discovered that INPs reorient themselves at lower temperatures, to optimise their contact with water. The researchers also found that changes in pH – the measure of how acidic water is – can act as a trigger for ice nucleation activity. The project, which was undertaken with the support of the Marie Skłodowska-Curie Actions programme, ended with a study of how water interacts with proteins, using vibrational spectroscopies to follow the interaction of water molecules with protein backbones (the part that holds proteins together) and side chains (the parts that bond with each other).
Forming new research connections
The successful research led to new collaborations with atmospheric scientists also studying ice nucleation in clouds. The ProIce team recently received funding for a new centre that will investigate this topic: the Center for Chemistry of Clouds (C3), which is located at Aarhus University. The ProIce project also opened up new questions that Weidner and his team will pursue further. “I am fascinated by the new possibility of following water-protein interactions,” Weidner adds. “We have observed that energy is transferred quickly between water molecules near the surface of ice bacteria. Where is the energy going?” he asks. Weidner now aims to follow this energy transfer from water to proteins at molecular timescales. Further potential research avenues include looking at other ice-nucleating organisms, such as fungi and anthropogenic materials.
Keywords
Prolce, ice, formation, nucleation, fungi, energy, pH, water, organisms, bacteria