Cyanolichens

Abstract

CYANOLLICHENS Amar N. Rai and Birgitta Bergman Amar N. Rai (corresponding author; e-mail: rai_amar Ohotmail.com), Biochemistry Department, North Eastern Hill University, Shillong-793022, India; Birgitta Bergman, Department of Botany, Stockholm University, SE-10691 Stockholm, Sweden. INTRODUCTION This short review provides an overview of some physiological and biochemical aspects of cyano lichens. For more details of lichens in general and the aspects covered here in particular: see Ahmadjian (1993), Galun (1988), Nash (1996), Rai (1990) and Rai et al. (2000). Of all known fungi, 20% form lichens (13,500 species). Most are ascomycetes (98%), 1.6% deuteromycetes and 0.4% basidiomycetes. About 10% of lichen species are bipartite and 3-4% tripartite cyanolichens. The latter contain a green alga (phycobiont) as the primary photobiont. Sometimes both bipartite and tnrpartitemorpho types occur in the same thallus (chimeroid thallus). Lichens have a worldwide distribution, occurrng in the highest, hottest, coldest, wettest and driest habitats, yet they are extremely sensitive to pollution. Lichens have a stable and organised thallus morphology quite distinct from any of the sym bionts. They appear as crustose (small lobes or scales; e.g. Collema), foliose (flat and dorsiventral lobes; e.g. Peltigera) or fruticose (round or flat thalli, upright or hanging down from the substra tum; e.g. Stereocaulon). In the thallus, the partners remain extracellular to each other and can be isolated and grown in culture (facultative symbio sis), but the symbiosis remains stable under a var iety of environmental conditions in nature. The partners involved undergo balanced and synchron ised growth and development. This prevents any one partner outgrowing the other(s). Close physi cal contacts occur between the cyanobiont and the mycobiont. The cell walls are thinner (less sheath material), and specialised mycobiont hyphae and haustoria, showing transfer cell ultrastructure, have been noted. As the bulk of the thallus consists of mycobiont, the thallus interior ismicroaerobic. A lichen thallus may develop from propagules (phyllidia, isidia, soredia, and hormocystangia) of a pre-existing thallus or from fresh synthesis. The latter involves fresh acquisition of the cyanobiont from the environment. Similar modes of cyanobiont acquisition also apply to cephalodia development. THE CYANOBIONT A wide range of cyanobionts is found in lichen symbioses. Most common are the heterocystous fonns Nostoc, Scytonema, Calothrix and Fischerella (including Hyphomorpha, Stigonema and Mastigocladus). Several unicellular forms are also reported. These include Gloeocapsa (also Chroococ cus), Gloeothece and Synechocystis (alsoAphanocapsa). In bipartite lichens, cyanobionts are either dispersed throughout the thallus (e.g. Collema) or occur as a distinct layer below the upper cortex (e.g. Peltigera canina). In tripartite lichens, the cyanobionts occur in cephalodia. The latter occur at the upper' surface of the thallus (external cephalodia; e.g. in P. aphthosa) or inside the medulla (internal cephalodia; e.g. Nephroma arcticum). Intemal cephalodia could be close to the lower surface of the thallus (e.g. in P. venosa). Entry of the cyanobiont for development of inter nal cephalodia is from the lower surface of the thallus, but occasionally, when the cyanobiont enters from above, the phycobiont layer is pressed deep into the medulla. In tripartite lichens, there is never a direct contact between the cyanobiont and the phycobiont. Lectins of mycobiont origin have been impli cated in the recognition of the cyanobiont by a mycobiont. However, direct observations, lectin binding experiments and tRNALeU intron analysis all indicate a broad or low cyanobiont-mycobiont specificity. Different lichen species can have the same cyanobiont, and different cyanobionts have been reported among cephalodia of a single lichen thallus. Different Nostoc strains have been found in different lichen species from the same site, whereas different lichen species from distant places had the same Nostoc strain. In chimeroid thalli, both bipart ite and tnrpartitemorphotypes are reported to have the same cyanobiont strain. Overall, there is a great deal of cyanobiont diversity among lichens, and much of itmay be due to themode of cyanobiont acquisition during the development of the lichen thallus and/or cephalodia. STRUCTURAL-FUNCTIONAL CHANGES In the lichen thallus, cyanobionts show structural functional changes that permit close interaction and the development of nutrient exchange be tween the partners. The filamentous habit tends to become aseriate, and sometimes a filamentous cyanobiont appears unicellular. Other changes...