# Bioprecipitation

Bioprecipitation is the concept of rain-making bacteria and was proposed by David Sands from Montana State University in the 1970s. This is precipitation that is beneficial for microbial and plant growth, it is a feedback cycle beginning with land plants generating small air-borne particles called aerosols that contain microorganisms that influence the formation of clouds by their ice nucleation properties. The formation of ice in clouds is required for snow and most rainfall. Dust and soot particles can serve as ice nuclei, but biological ice nuclei are capable of catalyzing freezing at much warmer temperatures. The ice-nucleating bacteria currently known are mostly plant pathogens. Recent research suggests that bacteria may be present in clouds as part of an evolved process of dispersal.
Ice-nucleating proteins derived from ice-nucleating bacteria are used for snowmaking. A symbiotic relationship between sulphate reducing, lead reducing, sulphur oxidizing, and denitrifying bacteria was found to be responsible for biotransformation and bioprecipitation.

## Plant pathogens

Most known ice-nucleating bacteria are plant pathogens. These pathogens can cause freezing injury in plants. In the United States alone, it has been estimated that frost accounts for approximately $1 billion in crop damage each year. The ice-minus variant of P. syringae is a mutant, lacking the gene responsible for ice-nucleating surface protein production. This lack of surface protein provides a less favorable environment for ice formation. Both strains of P. syringae occur naturally, but recombinant DNA technology has allowed for the synthetic removal or alteration of specific genes, enabling the creation of the ice-minus strain. The introduction of an ice-minus strain of P. syringae to the surface of plants would incur competition between the strains. Should the ice-minus strain win out, the ice nucleate provided by P. syringae would no longer be present, lowering the level of frost development on plant surfaces at normal water freezing temperature (0°C).

## Dispersal of bacteria through rainfall

Bacteria present in clouds may have evolved to use rainfall as a means of dispersing themselves into the environment. The bacteria are found in snow, soils and seedlings in locations, such as, Antarctica, the Yukon Territory of Canada and the French Alps, according to Brent Christner, a microbiologist at Louisiana State University. It has been suggested that the bacteria are part of a constant feedback loop between terrestrial ecosystems and clouds. According to Christine, this bacteria may rely on the rainfall to spread to new habitats, in much the same way as plants rely on windblown pollen grains, which could possibly a key element of the bacterial life cycle.

## Snowmaking

Certain species of bacteria and fungi are known to act as efficient biological ice nuclei at temperatures between −10 and 0 °C. Without ice nuclear agents, to freeze water the temperature has to be at least -40 °C. But ice nucleating bacteria can freeze at -1 °C instead of -40 °C. Even after the death of the bacteria, the glycoproteins continue ice crystallization. It does so by mimicking ice at the site of ice nucleating sites, which it acts as a template for the formation of ice lattice. Many ski resorts use a commercially available freeze-dried preparation of ice-nucleating proteins derived from the bacterium species Pseudomonas syringae to make snow in a snowgun. Pseudomonas syringae is a well studied plant pathogen that can infect plants, which results in loss. By studying this pathogen it can help us understand the plant immune system.