Bacteria in the sky, making it rain, snow, and hail

A post by Anne Jefferson

Even though we all think of the freezing point of water as 0 °C, very pure water remains a liquid until about -40 °C. Water crystallizes to ice in the presence of tiny nucleation particles in the atmosphere. These particles can be sea spray, soot, dust … and bacteria.

Bacteria are particularly good at ice nucleation (IN), causing it to occur at temperatures as high as -2 °C. As Ed Yong described 3 years ago:

Ice-forming bacteria like Pseudomonas syringae rely on a unique protein that studs their surfaces. Appropriately known as ice-nucleating protein, its structure mimics the surface of an ice crystal. This structure acts as a template that forces neighbouring water molecules into a pattern which matches that of an ice lattice. By shepherding the molecules into place, the protein greatly lowers the amount of energy needed for ice crystals to start growing.

USDA photo of ice on flowers

Ice on fruit tree flowers. There because of bacteria? (USDA photo)

The fact that bacteria like P. syringae nucleate ice crystals has been known for decades. They can be used for gee-whiz science demonstrations, and, at a much larger scale, as one method for creating artificial snow. On the flip side, the presence of P. syringae is also also makes plants more likely to be frost damaged at temperatures just below freezing. Only in the last several years, though, has the role of bacteria in producing precipitation from the atmosphere begun to be appreciated.

First, Brent Christner and colleagues discovered that every freshly fallen snow sample they collected, even in Antarctica, contained these ice nucleating bacteria. In the resulting 2008 Science paper, they noted:

The samples analyzed were collected during seasons and in locations (e.g., Antarctica) devoid of deciduous plants, making it likely that the biological IN we observed were transported from long distances and maintained their ice-nucleating activity in the atmosphere…our results indicate that these particles are widely dispersed in the atmosphere, and, if present in clouds, they may have an important role in the initiation of ice formation, especially when minimum cloud temperatures are relatively warm.

Then researchers in the Amazon rainforest discovered that primary biological aerosol (PBA) particles, including plant fragments, fungal spores…and yes, bacteria, were a dominant contributor to ice nucleation in clouds above the rainforest. (Even the though the Earth surface is hot in the Amazon, high enough in the troposphere, it’s still below freezing.) As Pöschl and colleagues reported in Science in 2010:

Measurements and modeling of IN concentrations during AMAZE-08 suggest that ice formation in Amazon clouds at temperatures warmer than –25°C is dominated by PBA particles… Moreover, the supermicrometer particles can also act as “giant” [cloud condensation nuclei] CCN, generating large droplets and inducing warm rain without ice formation.

The latest contribution to the growing understanding of bacteria’s role in precipitation was recently presented at the American Society of Microbiology meeting. Alexander Michaud studied hailstones that fell on his Montana State University campus, and as reported by the BBC:

He analysed the hailstones’ multi-layer structure, finding that while their outer layers had relatively few bacteria, the cores contained high concentrations. “You have a high concentration of ‘culturable’ bacteria in the centres, on the order of thousands per millilitre of meltwater,” he told the meeting.

What all of this adds up to is that we now know that bacteria and other biological particles are prevalent in the atmosphere around the world and are stimulating multiple forms of precipitation. As a hydrologist, I think I can wrap my head around this. But what’s really wild is the feedback between biological productivity and precipitation and the possibility that the ice nucleating bacteria moving in the atmosphere may be an evolutionary trait.

Precipitation stimulated by ice nucleation above an ecosystem where the bacteria or other biological particles were emitted sustains the ecosystem that created those particles. As Pöschl et al write:

Accordingly, the Amazon Basin can be pictured as a biogeochemical reactor using the feedstock of plant and microbial emissions in combination with high water vapor, solar radiation, and photo-oxidant levels to produce [secondary organic aerosols] SOA and PBA particles (31, 32). The biogenic aerosol particles serve as nuclei for clouds and precipitation, sustaining the hydrological cycle and biological reproduction in the ecosystem.

Or, in discussion of the recent hailstone findings [from the BBC]:

Dr Christner, also present at the meeting, said the result was another in favour of the bio-precipitation idea – that the bacteria’s rise into clouds, stimulation of precipitation, and return to ground level may have evolved as a dispersal mechanism. … “We know that biology influences climate in some way, but directly in such a way as this is not only fascinating but also very important.”

Tara Smith lolbacteria

and ur rainz and ur hailz (image created by Tara Smith on the Aetiology blog)

Christner, B., Morris, C., Foreman, C., Cai, R., & Sands, D. (2008). Ubiquity of Biological Ice Nucleators in Snowfall Science, 319 (5867), 1214-1214 DOI: 10.1126/science.1149757

Pöschl U, Martin ST, Sinha B, Chen Q, Gunthe SS, Huffman JA, Borrmann S, Farmer DK, Garland RM, Helas G, Jimenez JL, King SM, Manzi A, Mikhailov E, Pauliquevis T, Petters MD, Prenni AJ, Roldin P, Rose D, Schneider J, Su H, Zorn SR, Artaxo P, & Andreae MO (2010). Rainforest aerosols as biogenic nuclei of clouds and precipitation in the Amazon. Science (New York, N.Y.), 329 (5998), 1513-6 PMID: 20847268

Categories: by Anne, climate science, hydrology, paper reviews
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Comments (5)

  1. motsfo says:

    Another reason to not let grandkids eat snow…..
    And i’ll never look at the pure downey flakes in the same way again…

  2. OrchidGrowinMan says:

    I had heard adout the “ICE-” Pseudomonas, and it still sounds completely wrong-headed to think that using it to reduce the amount of ice that forms OUTSIDE a plant somehow protects the INSIDE of the plant from cold damage: The freezing of water, whether facilitated so that supercooling can’t occur or not, releases heat, and prevents anything in intimate contact from falling below the freezing point until the freezing stops.
    Citrus growers spray water on their groves during cold-snaps, not because the water is warm, but because, as it freezes on the trees and fruits, it prevents them from being subjected to sub-freezing temperatures. Aside from the weight, ice on the surface is not as bad as ice in the tissues.
    It seems to me, you would want to use “ICE+” on plants (and maybe spraying too) to prevent supercooling and the tissues falling below (maybe FAR below) freezing.
    They had this well-publicized test of “ICE-.” I see a motive to report positive or at least “inconclusive” results, which makes me want to closely scrutinize the study, because I really can’t think of a plausible mechanism for the prevention of external freezing also preventing internal freezing, unless the basteria or their active components are actually inside the plants.

    It is well-known that dehydrated plants also resist cold damage, at least until dehydration itself becomes the main problem (because of low water transport in cold plants).

  3. morganism says:

    would still like to see someone study whether or not the kelp beds are throwing this bacteria up in the air out in the middle of the pacific.

    After seeing that picture of clouds forming over the sea lanes cut by a supertanker out in the middle of the sea, have always wondered if that is the main factor driving rainfall events in the western us.

    those giant kelp beds are now gone, cut up by boats bringing goods from China.

  4. Ann Willis says:

    My sister lives in Wisconsin and there have been times when she’s told me it’s too cold to snow. I always thought this to be such an unusual comment to make. But now after reading this post that comment makes sense. If it gets really cold would that kill the bacteria and then there would be no nucleus for the snow to form around?

    • Hi Ann,

      The “too cold to snow” saying is actually a myth. This video from Accuweather does a pretty good job explaining why that myth gets repeated – basically, the coldest temperatures (in the US at least) are when there is high pressure associated with Arctic air mass. High pressure means sunny skies – and no snow. But, of course, it can’t be too cold to snow – the highest latitudes and the highest elevations (i.e., the coldest places) are snow-covered.

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