Biological Strategies Of Lichen Symbionts To The Toxicity Of Lead (Pb)

Lichens are symbiotic organisms, originated by mutualistic associations of heterotrophic fungi (mycobiont), photosynthetic partners (photobionts) which can be either cyanobacteria (cyanobionts) or green microalgae (phycobionts), and bacterial consortia. They are poikilohydric organisms without cuticles or nutrient absorption organs adapted to anhydrobiosis. They present a large range of tolerance to abiotic stress (UV radiation, extreme temperatures, high salinity, mineral excess, etc.) and prosper all around the Earth, especially in harsh habitats, including Antarctica and warm deserts. Their biodiversity is widely used as a bioindicator of environmental quality due to this diversity of tolerance in different species, and they are included in air Pb monitoring programmes worldwide. Their ability to bioaccumulate environmental substances, including some air pollutants and heavy metals, makes them excellent passive biomonitors of Pb. Heavy metal tolerance is related to diverse mechanisms: cell walls and exclusion systems (such as extracellular polymeric substances), intracellular chelators and an extraordinary antioxidant and repair capacity. But recent data show that the most powerful mechanism is related with the upregulation of mutual systems by symbiosis.