Intracellular urease activity in the lichen Cladonia verticillaris, and its implication for toxicity

• Urea is commonly used as a nitrogen fertilizer in crops.
• Several lichens can grow in the vicinity of these crops and capture the excess of urea employed during fertilization.
• Many lichen species produce urease that hydrolyzes urea to release ammonia to the atmosphere.
• Urea hydrolysis inside the lichen thallus increases ammonia concentration that can produce damage in the photosynthetic apparatus of the photobiont.
• Subcellular location of urease in the thallus can provide a mechanism to avoid ammonia toxicity.

Urea is currently used as a nitrogen fertilizer in many plant cultures, such as sugar cane. Several lichen species grow in the edges of the fields fertilized with urea. This implies that the hydrolysis of an excess of urea by soil bacteria or by the lichens themselves would increase the concentration of ammonia in the lichen thallus to a level that may be toxic to the photobiont. However, Cladonia verticillaris produces urease through positive feedback by urea supplied from the medium. This urease is partially secreted to the media or retained on the external surface of algal cells, as demonstrated herein by an adequate cytochemical reaction. This implies that ammonia produced by urea hydrolysis will be immediately dissolved in the water filling the intercellular spaces on the thallus. A possible protection mechanism against eventual ammonia toxicity, derived from the results described here, is also discussed.