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Ice Nucleation in Lichens – Part III

2012/03/13

by Bruce Moffett

An estimation of 10 14 tonnes of lichen biomass worldwide was made by Lynn Margulis in a popular science book (Margulis 1998).  Other estimates are 10 9 tonnes of  lichens on rocks (Schwartzman pers. Com.) and, when averaged over an entire year, more lichen than leaf  biomass on deciduous trees. There are around 10 5 high temperature ice nuclei (IN) g-1 so this is not an insignificant source of IN.

I have measured 130g of lichen falling from a single tree in a three week period. Reindeer and other ungulates (as well as smaller beasties like snails and springtails) use them as a significant part of their diet so there is a great deal of lichen biomass.

It has been speculated that bacteria in clouds may make a contribution to the formation of ice and so initiate precipitation.  However (other than a patent taken out by Tom Kieft concerning weather modification) there has been little thought given to how lichens might do the same. They disperse by air and are very resistant to desiccation and UV. They have been exposed to space for 60 orbits and been unaffected.  By initiating freezing at  temperatures at which the vapour pressure difference between water and ice is at its maximum, lichen provide exactly the type of nuclei which will drive the Bergeron- Findeisen process. Therefore lichen derived IN, if they become airborne, are excellent candidates for inducing precipitation.

Often the question asked is- Do they get into the air?  The answer is yes. In an aerobiological monitoring programme carried out on Signy Island in the Maritime Antarctic, lichen soredia were the most abundant airborne propagules with a size range of 30μm to 100 μm (Marshall 1996). Tormo et al. (2001) found lichen propagules in urban air in Spain.

Lichens might not even need to be levitated but simply fall into clouds! The Puy De Dome site in France at 1465 metres often gets maritime clouds coming in from the west.  There are mountains 40 Km to the south west (Puy de l’Angle) which are around 270m higher.  The picture below shows several species of lichen on a single fence post on this summit which could potentially contribute biological IN to the cloud water harvested on the Puy De Dome.  
P1030098

 Several lichen species on a wooden post at Puy de l’Angle

On a more general note lichens have been found in the Himalayas at 0ver 7000m and so there are a number of sites where lichens are already in or above clouds. 

Unfortunately lichen grow very slowly – often only a few mm per year.  In addition there are very few lichen DNA sequences in the databases and so the molecular approach used successfully to identify bacteria in cloud water  is unlikely to pick up their signature.

So what could we do to test the hypothesis that lichen contribute to cloud processes?  One way would be to investigate the gene(s) in the lichen which produce the ice nuclei and then design molecular tools to look specifically for these in cloud water and precipitation. Alternatively when several full lichen genomes are sequenced (2 for each “species” as they are a symbiosis of a fungus and a photosynthetic partner) we might be able to use data mining tools to home in on candidate genes.

References

Margulis L. (1998) The symbiotic planet: a new look at evolution. Basic Books, New York.

Marshall W.A. (1996) Aerial dispersal of lichen soredia in the maritime Antarctic. New Phytologist  134:523-530.

Tormo R. Recio D. Silva I. and Munoz A.F. (2001) A quantitative investigation of airborne algae and lichen soredia obtained from pollen traps in South West Spain, European Journal of Phycology 36:385-390.

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