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Vol. 17 No. 4 - October 2011

Lichens - A Potential Organism For Sustainable Agriculture

By: Rajesh Bajpai and Dalip K Upreti^*

When most people see lichens growing on trees, rocks, electric poles and decaying wood they believe, they are plants. In facts, lichens and the partners from three different kingdoms are both taxonomically and physiologically a very diverse group, which makes them interesting from both ecological and biotechnological point of view. They are the stable vegetative products of a mutually beneficial symbiotic relationship between a fungus and cyanobacteria or green alga, capable of producing food by photosynthesis.

This highly successful strategy for nutrition, transferring carbon from the photosynthetic ‘producer’ to the fungal ‘user’, allows lichen associations to become ‘plantlike’, and thereby to exploit a much wider range of terrestrial habitats than would otherwise be available to fungi alone.

In general, three major life forms of lichen thallus are recognized: crustose (crust-like biofilm), foliose (leaf-like), and fruticose (branched tree-like, shrubby and pendulous) thalli. A fourth type, gelatinous thallus, is restricted to some cyanobacterial lichens. Even without roots, lichens can efficiently extract nutrients (Phosphorus, Magnesium, Calcium, Potassium, Sulfur, and Iron) from recalcitrant surfaces. Lichens often grow in habitats with extreme light, dryness, or temperature, which are less favorable or unsuitable for higher plants. Both mycobiont and the algal photobiont may participate in seasonal photoacclimation in green algal lichens. The light and desiccation tolerance is greater in the lichen symbiosis than in its isolated partners. Lichens adapted to open habitats tolerate extreme desiccation and UV exposure via their screening cortical pigments by preventing the formation of or by scavenging free radicals. Lichen thalli are poikilohydrous, which means that their water status passively follows the atmospheric humidity and the presence of water, rapidly activates lichen metabolism. The continuation of water metabolism, green algae in lichens are able to activate their photosynthesis with water vapor while cyanobacteria in lichens need liquid water. Due to this reason green algal lichens survive in dryer habitats than cyanobacterial lichens. Some cyanobacterial lichen species with gelatinous polysaccharides-containing thalli and green algal lichens with cushious water-storing thalli are able to extend their daily metabolism compared to thin, easily drying lichen species.

Lichens and their natural products have a long history for their use in decorations, brewing/distilling, perfumery, dyeing industries, food, pollution monitoring and climate change. Lichens are an integral part of all ecosystem, they are often responsible for either fixing or capturing essential nutrients from the air.

A famous lichenologist Trevor Goward, says that “Lichens are fungi that have discovered agriculture” (Dayan & Romagni 2001). This statement elaborates that the lichens are common primary successional species, preparing a previously barren landscape for other plants and organisms to exploit. Lichens secondary metabolites in particular the weak acids, begin the breakdown of sedimentary and igneous rocks, providing a loose soil matrix in which other nonvascular and vascular plants can root. With modern technology, the potential of discovering and utilizing useful metabolites in lichens has increased simultaneously and may open new path of research. 

            The success of modern agriculture can be attributed, in part, to the advance in the chemical control of pests, weeds and to prevent insect and microbial damage to crops by chemical means, accompanied by the use of fertilizer, as helped to drive the ‘green revolution’ of the last 5-6 decades (Dayan & Romagni 2001). However, public concern over the effects of xenobiotic compounds such as pesticide, fertilizers on the environment and human health have caused a dramatic change in the attitude toward synthetic pesticides as well as fertilizers. Natural products are now being considered as an alternative to the arsenal of synthetic compounds. This paper provides a brief note on the potential use of lichens in production of biopesticide and biofertilizers in near future for sustainable agriculture.  

Ecologically, lichens are also important as major nitrogen-fixers in grassland and forest ecosystems. Lichens, especially those that have the capacity of fixing nitrogen, grow a good deal more quickly than is generally imagined, making them important as ‘biological fertilizers’, some producing 2–4 cm linear or radial growth per year in a short burst of winter growth (cyano lichens). In this role lichens are important in colonising disturbed habitats – perhaps investigation into possible practical applications of lichens in habitat restoration could be useful.

The cyanolichens such as Collema, Leptogium, Lobaria, Peltigera, Cococarpia and Peltula are abundantly found in India. All are containing Nostoc cyanobacteria in their thallus except Cococarpia and Peltula having different phycobiont like Sytonema and Anacystis respectively. They have large thallus (foliose) except Peltula (Squamulose) and have great capacity to fix atmospheric nitrogen and leading to maintain the ecosystem. Lichen communities are therefore vital primary producers in the cycling of carbon and nitrogen nutrients in our forest and grassland ecosystems, and in addition, they can tell us a great deal about the health of our environment.  About 78.084% of nitrogen is freely found in the atmosphere. It is a single gas in the atmosphere that cannot be used directly by either plants or animals and must be concentrated to a reduced state in order to be useful for higher plants and animals.  In the atmosphere, it exists in inert nitrogen form and must be converted before it becomes useful in the soil. Some microorganisms (nitrogen fixing bacteria) can utilize atmospheric nitrogen to manufacture nitrogenous compounds for use in their own cells. This process called biological nitrogen fixation, requires a great deal of energy. Therefore, free living organisms that perform the reaction, such as Azotobacter generally fix little nitrogen each year (less than 20 lb/acre) because food energy is usually scarce. Most of this fixed nitrogen is released for use by other organisms upon death of the organism. Bacteria such as Rhizobia receive much food energy from legume plants can fix much more nitrogen per year (100 lb/acre) a number of unrelated plant species such as Alnus, Lichen, Myrica and Gunnera also seem to capture minute nitrogen (Hermann et al. 2007).  

Osborne & Sprint (2002), highlight the ecological significance of cyanobacteria in lichens and their impact in nitrogen cycling in nutrient-poor environments where nitrogen leaks from growing and degrading lichens. Prevention of desertification and restoration of desertificated lands could be aided by focusing land restoration on biological soil crusts comprised of mosses and nitrogen fixing cyanobacterial lichens (Bowker et al. 2005). The latter may also play an important role in increasing soil water-holding capacity and nutrient availability. It is proposed that lichen fragments as well as culture could be combined with plant seeds and adapted to extremes and sand-fixing liquid mixture, which is sprayed on to desertificated land (Yang 2002). The lichen thallus can be cultured in a stainless steel bioreactor with specific growth medium. After some time (abundant biomass produced) it is taken out from bioreactor and mixed with some carrier and supplied to the farmers.  The biotechnological aspects of lichens that is recombinant DNA technology, identification of resistant gene from phyocobiont and its inoculation may be useful for future. The use of lichen cyanobacteria coupled with various crops may improve the soil health of the agricultural field. In developing countries like India the conventional agricultural practices are more common and need advancement. Leading to these context this type of low cost fertilizer input to the field is beneficial for the farmers in reducing the dependency on synthetic fertilizers.

In other aspects lichens are also well known organism to produce about more than 850 different types of secondary metabolites. These metabolites are derived from three pathways such as, Acetate polymalonate pathway, Mevalonic acid pathway and Shikimic acid pathway. The common metabolites atranorin, zeorin, parietin, norstictic, lecanoric and usnic acid are the most frequently occurring secondary metabolite in lichens. Majority of these chemicals are produced by the species of lichen genera Lecanora, Parmotrema, Heterodermia, Cladonia, Xanthoparmelia, Lepraria and Diploschistes.

            At present most of the secondary metabolites are successfully targeted against various human pathogens in India and abroad. The medicinal aspects of lichens has turned to new directions after the untiring research work done by Japanese workers, who studied the anti-tumor and anti-HIV activity of lichens (Upreti & Chatterjee, 2001). But the crop pathogenic activity has been neglected. Lichen metabolites have the capacity to develop defense against several pathogens such as anti fungal, anti viral, anti bacterial and anti cancer are proven as best biocontrol agents (Table-1). In agricultural practices the lichen metabolites (mycobiont culture/ lichen fermentation) may be used against several crop plant pathogens like Fusarium, Alternaria, Phytopthora, Albugo etc as well as other bacterial and viral pathogens. These metabolites may be used during seed sowing (mixed with seeds) or spraying over seedlings it develops defense at rhizosphere as well as phyllosphere region of the crop plant.

Now the lichenological application in agriculture may proven a better option in near future for crop improvement and safe environment. This type of study needs more research in this way against targeted organism and make commercialized products (lichenoproducts) for sustainable agriculture and eco-friendly environment.

TABLE 1 Targeted lichen metabolites

^*Lichenology Laboratory, CSIR-National Botanical Research Institute, Rana Pratap Marg, Lucknow-226001, India. E-mail: [email protected] / [email protected] 

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.