A brief insight into the ice-making and weather-changing capabilities of Pseudomonas syringae

Emma Knox

It’s that time of year again. The days are getting shorter, the air is getting crisper, and the rain takes on a new level of vengefulness. Indeed, it shouldn’t be too much longer before the dreaded – or the much anticipated, for those better equipped to resist falling than I – snow and ice warnings start hitting our screens.

At this point you may be wondering what on earth micro-organisms have to do with any of these natural phenomena. Well, it turns out that the atmosphere is laden with microbes. Just like bacteria in the soil are involved in nitrogen fixation, and bacteria that invade your gut after you’ve eaten some dodgy food are responsible for making you sick, microbes in the atmosphere are involved in their own array of processes.

A chilling process

One very interesting microbial process is ice-nucleation, carried out most efficiently by a Gram-negative bacterium, Pseudomonas syringae, using special ice-nucleation active proteins (INAPs) embedded in its outer membrane (Wolber et al., 1986). Channelling the powers of possibly one of the most underrated Marvel characters – Iceman – these bacteria can promote ice crystal formation at temperatures of up to –2°C (i.e. temperatures higher than those typically required for the freezing of environmental water).

So, how exactly does this nucleation process work?

Up until recently, it was known that INAPs – such as InaZ in P. syringae – were directly involved in the nucleation process; however, their specific interactions with water molecules remained a mystery. Recent spectroscopic research has shown that INAPs alternatively attract and repel water molecules, aligning the molecules to promote ice formation (this unique ordering mechanism was shown to be enhanced by decreasing temperature). The freezing process also involves dispersal of heat energy away from the nucleation site, such that the water in the immediate vicinity of the nucleation site becomes particularly cold and more likely to freeze (Pandey et al., 2016).

INAPs are so effective in the process of ice-nucleation that freeze-dried protein preparations derived from P. syringae are used to produce snow for ski slopes when the level of snowfall is insufficient, or when there is no snowfall at all.

slopes

Enjoying the slopes

What is the motive?

One question we must ask is why microbes would evolve this peculiar ice-nucleating ability. I know what my own personal motivation would be – i.e. the ability to make endless supplies of ice-cream on demand (well, that’s pretty much it). However, the current hypothesis as to why P. syringae developed this skill is that it constitutes part of its life cycle and enables dispersion of the bacteria over long distances.

P. syringae begins by infecting leafy plants, crops and trees, particularly those already damaged by frost. The bacterium uses its INAPs to stimulate frost formation at relatively high subzero temperatures and then penetrates into the plant through frost-damaged tissue where it has access to nutrients. The bacteria can then spread locally by air from one plant to another and the infection process continues.

frost

Leaves frosted over, potentially due to Pseudomonas syringae (Image Source)

In addition to local dispersion, a recent study indicates that P. syringae cells can be swept high up into the atmosphere by wind currents, allowing them to travel long distances to remote ecological niches. This study involved isolating strains of P. syringae from a variety of non-agricultural niches such as streams and lakes, and examining their relationship with strains isolated from agricultural niches. The fact that strains from the same clonal lineage – i.e. a closely-related set of variants descended from a single bacterial cell – were isolated from snow, irrigation water and infected crops provides evidence for long-distance dispersion, and supports a previously suggested hypothesis that the life cycle of the bacterium may be linked to the hydrological cycle (Morris et al., 2008). In this model, large amounts of P. syringae can be swept up from one area, such as the Amazon, and ride on air currents to locations as distant as Antarctica. These bacteria, present in the atmosphere and in clouds, then catalyze the formation of ice crystals leading to bioprecipitation – e.g. rain, hail and snow. P. syringae present in the rain and melting snow can then attack plants in far-distant regions, while also being deposited in lakes, streams, rivers, and so on.

Human relevance

So, at this stage we know that micro-organisms are present throughout the troposphere and stratosphere, we know how the ice-nucleation process works in a physical sense, and we know that ice-nucleating bacteria have been isolated from snow and rainwater – but what exactly does this tell us? Are micro-organisms responsible for the more extreme weather patterns we have been experiencing worldwide? Could they be the key to tackling climate change head on?

These are exciting questions. However, for now it seems we can only correlate the effects of these weather-changing microbes with changes in local and regional weather patterns, not global climate change. One paper published in the International Research Journal of Biological Sciences speculates that human practices that alter the level of P. syringae in the atmosphere, can indirectly affect precipitation (Prasanth et al., 2015). Deforestation and the extensive use of pesticides – in areas such as the Amazon, for example – dramatically reduce the number of rain-making bacteria being swept up into the atmosphere and in turn may reduce precipitation.

deforestation

Amazonian deforestation may have an impact on bioprecipitation (Image Source)

Conclusion

In any case, whether greenhouse gases, land-use change, ice-nucleating bacteria, or a combination of all three are driving climate change, the most important thing we can do is treat our environment with respect and appreciation (and for the time being, invest in some decent rain gear).

References:

  1. Wolber, P.K., Deininger, C.A., Southworth, M.W., Vandekerckhove, J., van Montagu, M. and Warren, G.J. (1986) Identification and purification of a bacterial ice-nucleation protein. Proceedings of the National Academy of Sciences. 83(19), 7256–7260.
  2. Pandey, R., Usui, K., Livingstone, R.A., Fischer, S.A., Pfaendtner, J., Backus, E.H., Nagata, Y., Fröhlich-Nowoisky, J., Schmüser, L., Mauri, S., Scheel, J.F., Knopf, D.A., Poschl, U., Bonn, M. and Weidner, T. (2016) Ice-nucleating bacteria control the order and dynamics of interfacial water. Science advances. 2(4), e1501630.
  3. Morris, C.E., Sands, D.C., Vinatzer, B.A., Glaux, C., Guilbaud, C., Buffiere, A., Yan, S., Dominguez, H. and Thompson, B.M. (2008) The life history of the plant pathogen Pseudomonas syringae is linked to the water cycle. The ISME journal. 2(3), 321–334.
  4. Prasanth, M., Nachimuthu, R., Gothandam, K.M., Kathikeyan, S. and Shanthini, T. (2015) Pseudomonas syringae: an overview and its future as a “rain making bacterium”. International Research Journal of Biological Sciences. 4(2), 70–77.
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