The tropospheric bacteria of rain and snow
Nowadays there is more evidence that the bacteria found in the high troposphere (8-15 km) could influence the density of clouds and rain.
Firstly, we must remind that the troposphere is the lowest part of the atmosphere, and the 8-15 km layer is the high troposphere, near the tropopause that borders the stratosphere, above the Mount Everest. Here there are some of the highest clouds.
So, in a recent study (DeLeón-Rodríguez et al, 2013) it has been shown that the viable bacteria (by epifluorescence microscopy and quantitative PCR) at a 10 km altitude (samples taken above the Caribbean Sea and the Atlantic West) represent 20% of the particles with size between 0.25 and 1 mm, and bacteria are at least 10 times more abundant than fungi, with numbers of 105 per m3, with a 60% of viable cells. This suggests that bacteria are an important and underestimated fraction of microparticles of atmospheric aerosols, even at higher concentrations than lower altitudes.
The authors have analyzed the bacteria by pyrosequencing (Roche 454) the rRNA genes. They have seen that the tropospheric microbiome has a good variety of bacterial taxa that vary dynamically according to the atmospheric turbulence and in the presence of hurricanes. Some of the most abundant bacteria found are those using compounds C1-C4 (e.g., oxalic acid) present in the atmosphere, so these bacteria are metabolically active at these altitudes. This reinforces the idea of the active role of bacteria in the troposphere, and that there are not only inert spores (fungal) floating through the air.
In this sense, this metagenomic analysis also confirms the presence of bacteria that are able to catalyze the formation of ice crystals and hence the cloud condensation. This process of nucleation (ice nucleation, IN) occurs when the water molecules coalesce around a seed particle, for example dust. Depending on the temperature, these complexes can grow to become water droplets or ice, leading to the formation of rain or snow. Given that the high troposphere dust particles are scarce, it is evident the role of bacteria in this phenomenon.
One of the key roles in the nucleation of ice (IN) by bacteria is that they catalyze ice formation at temperatures close to 0°C, unlike the formation of ice nuclei by the inorganic particles, which is done at temperatures lower, below -10°C, and without any core particle the ultra-pure water freezes at -40°C.
Ice nucleation by bacteria has been reproduced in the laboratory (Christner et al, 2008) with samples of rain and snow from around the world (Canada, USA, Pyrenees, Alps and Antarctica), showing that in the samples treated with lysozyme (which hydrolyzes bacterial cell wall) or treated with heat, the IN activity was reduced almost 100% at a temperature of -5°C. Therefore, bacteria are responsible of the IN at these relatively high temperatures.
The bacteria most commonly associated with the IN activity are species associated with plants, such as Pseudomonas syringae or Xanthomonas campestris, which also often have been detected in atmospheric aerosols and clouds. P. syringae has also been found in the hail stones.
The phenomenon of IN by P. syringae was already observed in 1974 (Maki et al.) and after it has been shown (Gurian-Sherman & Lindow 1993) that IN strains of this species and others have in the outer membrane of the cell wall, as a active IN, a protein of 180 kDa, composed of repeats of a consensus octapeptide. This protein forms a planar arrangement that traps water molecules producing a mold for ice formation.
This feature makes that these bacteria are responsible for most of frost damage in plants, besides than P. syringae is pathogen of many plants at room temperature by the production of a compound (coronatin) who keeps the stomata open, causing the bacterial invasion of plant tissues (Nigel Chaffey, 2012).
Tomato leaf infected with Pseudomonas syringae (Alan Collmer, Cornell University/Wikimedia Commons)
Coming back to the frost damage, most plants can withstand up to -5°C without much damage if these bacteria are absent, but the presence of the IN protein-forming bacteria such as P. syringae in numbers of only 1000 cells by g of plant increases dramatically the damage by freezing. These damages also facilitate the penetration of bacteria and infection.
Frozen plant (MO Plants& Maureen Gilmer)
This feature of ice nucleation by P. syringae is also utilized for the production of artificial snow. Although this can be made usually by the forced expansion of a pressurized mixture of water and air under appropriate conditions of temperature and humidity (e.g. ≤ 2°C at 20% humidity, or ≤ -2°C at 60%), snow production is favoured by the addition of nucleation agents, which can be inorganic, organic or the mentioned bacterial protein.
Coming back to the clouds, we must remind that bacteria are far less the sole agents of nucleation forming condensation droplets resulting in rain or snow. The cloud condensation nuclei, CCN, also called cloud seeds, can be very different types of microparticles of sizes around 0.1 – 1 mm. When this aerosol of microdroplets is condensed, it forms drops of 0.02 mm in the clouds, which give falling raindrops of 2 mm.
The microparticles are mostly of natural origin such as dust, sea salt, volcanic sulphates or organic microparticles result of the oxidation of volatile compounds. Some of these may be of industrial origin, as well as soot and other particles resulting from combustion. Another important biological source of CCN is the aerosols of sulphate and methanosulphate produced from dimethyl sulphur, which is made by phytoplankton in the oceans.
Anyway, despite atmospheric microbiology is still in its infancy, as we have seen there are more and more data on the importance of bacteria and other microorganisms on bioprecipitation of rain and snow. To find out more about their role, research must go beyond the description of the abundance of microorganisms in the atmosphere, and to understand the biological, physical and chemical properties of the transport processes involved. This will require interdisciplinary approach seemingly different disciplines such as oceanography, bacterial genetics and physics of the atmosphere, for example.
Chaffey N. (2012) COR, nice one, Mr Microbe !. AoB Blog.
Christner B. et al. (2008) Geographic, seasonal, and precipitation chemistry influence on the abundance and activity of biological ice nucleators in rain and snow. PNAS 105, 48, 18854-18859.
DeLeón-Rodríguez N. et al. (2013) Microbiome of the upper troposphere: species composition and prevalence, effects of tropical storms, and atmospheric implications. PNAS 110, 7, 2575-2580.
Gurian-Sherman D. & S.E. Lindow (1993) Bacterial ice nucleation: significance and molecular basis. FASEB J. 7, 14, 1338-1343.
Hardy J. (2008) The rain-making bacteria. Micro-Bytes.
Maki L.R. et al.(1974) Ice nucleation induced by Pseudomonas syringae. App!. Microbiol. 28, 456-460.
Morris C.E. et al. (2011) Microbiology and atmospheric processes: research challenges concerning the impact of airborne micro-organisms on the atmosphere and climate, Biogeosciences 8, 17-25