The existence of certain bacteria can be the deciding factor between a plant surviving a cold snap or suffering from frost damage. While many people associate bacteria with disease, some species play a surprising role in the environment by influencing where and when ice forms. These are known as ice-nucleating bacteria. In contrast, “ice-minus” bacteria are variants that lack this ice-forming capability. This distinction makes them a subject of scientific and agricultural interest, as their presence can help protect plants from the very frost their relatives might otherwise encourage.
How Bacteria Can Promote Ice Formation
Certain bacteria on plant leaves, most notably Pseudomonas syringae, can catalyze ice formation. While pure water can supercool well below freezing, these bacteria produce ice-nucleating proteins (INPs) on their outer membranes. These proteins act as templates, organizing water molecules into an ice-like lattice structure.
This process allows ice to form at much warmer temperatures, between -2°C and -5°C, than it would otherwise. The resulting ice crystals can rupture plant cells, causing significant damage even during a light frost. The bacteria then exploit this damage to access nutrients within the plant tissue.
The INP’s structure is directly related to its function, containing repeating amino acid sequences that create a stable surface. This surface mimics the crystalline structure of ice, providing a scaffold for water molecules to begin the freezing process. Without these bacterial surfaces, much colder temperatures would be required for ice to form on plants.
Creating Ice Minus Variants
Ice-minus bacteria are strains of ice-nucleating bacteria, like Pseudomonas syringae, that have lost their ability to promote ice formation. This change can occur through natural mutations or be induced in a laboratory using genetic engineering. By removing or disabling the specific gene that codes for the INP, scientists can create a stable ice-minus variant.
The defining characteristic is the absence of a functional ice-nucleating protein. Without this protein, the bacteria cannot organize water into an ice lattice at higher sub-zero temperatures. Water on a leaf colonized by these bacteria can remain supercooled to lower temperatures, which lowers the point at which frost forms on the plant.
The practical application for these bacteria is based on competitive displacement. The idea is that introducing a large population of the ice-minus version onto crops displaces the naturally occurring “ice-plus” bacteria, which led to some of the earliest field tests of a genetically modified organism.
Practical Uses of Ice Minus Bacteria
The primary application for ice-minus bacteria is as an agricultural protectant against frost. Sprayed onto crops like strawberries and potatoes before a frost, they colonize leaf and flower surfaces. By establishing a dominant population, they prevent ice-nucleating bacteria from settling on the plant, which reduces the sites where ice can form and lowers the plant’s freezing temperature.
This technology was commercialized under brand names such as Frostban. Application must occur well before a frost to allow the bacteria time to multiply and establish themselves. To be effective throughout a frost season, repeated applications may be necessary as bacterial populations change.
Beyond crop protection, these bacteria are used in artificial snowmaking. While their ice-nucleating relatives help create snow at higher temperatures, ice-minus variants are used where ice formation is undesirable, such as preventing ice buildup on surfaces. However, their most studied application remains preventing frost damage in agriculture.
Ecological Considerations and Public Perception
The proposed release of genetically engineered ice-minus bacteria in the 1980s sparked significant public debate and regulatory scrutiny. As one of the first genetically modified organisms (GMOs) approved for release into the environment, it raised concerns about unforeseen ecological consequences. Activist groups initiated legal challenges that delayed the first field trials, citing worries that the bacteria could disrupt natural processes like cloud formation.
Before field trials could proceed, the ice-minus bacteria underwent extensive safety assessments. The U.S. Environmental Protection Agency (EPA) classified the organism as a pesticide because it was intended to mitigate the “pest” of naturally occurring ice-plus bacteria. This subjected it to a rigorous approval process, which ultimately concluded that the risks were minimal and paved the way for controlled field tests.
Despite the scientific consensus on their safety and EPA approval, the initial controversy left a lasting impact on public perception of agricultural biotechnology. The first outdoor test on a small plot of strawberries in California in 1987 was met with protests. While the bacteria were shown to be effective and safe in trials, the combination of stringent regulations, high costs, and public stigma discouraged widespread commercialization.