A peculiar odour that occurs in marshy areas and cowsheds is due to gas produced by
Archaebacteria
Marshy areas are water logged areas where there is growth of grasses, reeds, sedges etc.
Since these areas are water logged the amount of oxygen in the soil is less, allowing the growth of anaerobic bacteria like methanogens.
The methanogens act on the organic matter releasing methane and hydrogen sulfide.
Methane is called marsh gas and causes a peculiar smell.
Cowsheds contain a lot of cow dung that contain methanogens as these bacteria reside in the intestine of ruminants.
Methanogens in the dung act on the organic matter of the dung releasing methane causing the smell.
Methanogens are prokaryotic organisms that belong to the domain Archaea and grow in anaerobic conditions.