QUESTION: What is the peculiar smell of the earth after the first shower?
T.S. Chellamal Anni, Chennai
ANSWER 1: The characteristic earthy odour of soil is caused by the production of a series of streptomycete metabolites called geosmins.
These
substances are sesquiterpenoid compounds and unsaturated compound of
carbon, oxygen and hydrogen. The geosmins first discovered has the
chemical name trans-1, 10-dimethyl-trans-9-decalol; however, other
volatile products produced by certain species of Streptomyces may also
be responsible for the characteristic smell.
An unforgettable attribute of the streptomycetes is the musty odour they emit, an odour reminiscent of freshly turned soil.
Streptomyces
are primarily soil micro-organisms requiring a lower potential for
growth. The most significant environmental adaptation of the
Streptomyces group is their ability to withstand dessication. Geosmins
are also produced by some cyanobacteria.
Dr. I. M. Sarawad
Regional Agricultural Research Station
Bijapur, Rajasthan
ANSWER 2:
The piquant, musky odour that hangs in the air emanates from an odorous
chemical buried in the soil called `geosmin' (literally, earth smell).
The
smell is given off by Streptomyces bacteria, a genus belonging to the
Actinomycetales order of Gram-positive eubacteria, also called
actinomycetes. The soil normally contains a multitude of environmental
saprophytic fungi.
Actinomycetes,
a type of filamentous bacteria grow in soil when conditions are damp
and warm. When the soil dries out, the bacteria produces the geosmin
spores in the soil. Rain hitting the ground kicks up an aerosol of water
and soil and spores into the air, where they are easier to smell. (just
like an aerosol air freshener).
We breathe in fine particles of soil containing the bacteria.
K. Kamalakkannan, Doha, Qatar
ANSWER 3:
A pleasant smell after the first shower is because of a group of
filamentous bacteria Actinomycetes found in the soil. They grow well in
soil when the conditions are damp and warm.
When
the soil is too hot, the bacteria are not able to tolerate the
dessication, so it produces spores as survival strategies. The spores
remain invulnerable for years and are resistant to dessication and heat.
During the rainfall, the spores are taken up in the air by the force of
wind and suspended in the air as aerosol. When we breathe the air,
which contains spores, we are able to feel the earthy "after the rain
smell". Geosmine (dimethyl-9-decalols) is the microbial product found in
the spores is responsible for the pleasant smell.
Dr. P. Mariappan
JJ College of Arts and Science
Pudukkottai, Tamil Nadu
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