Lichen Mutualism PDF

Lichens: Mutualistic, Tripartate & Omnipresent… Dr Helen Carney Week 9: Microbiology Session Overview • Lichen overview: the relationship, morphology & biodiversity • Biochemical symbiosis • Air pollution & lichens • Field work on campus: local lichen biogeography Morphological diversity The...

Lichens: Mutualistic, Tripartate & Omnipresent… Dr Helen Carney Week 9: Microbiology Session Overview • Lichen overview: the relationship, morphology & biodiversity • Biochemical symbiosis • Air pollution & lichens • Field work on campus: local lichen biogeography Morphological diversity The Symbiotic Composition Mycobiont Fungal species: heterotrophic Photobiont A photosynthetic single celled microorganism autotrophic e.g. algae or cyanobacterium Mutualistic: fitness of both partners is increased by the symbiosis Lichen The Symbiotic Composition Mycobiont Fungal species: heterotrophic Pro/eukary otic partner A third partner in the symbiosis: tripartite Photobiont A photosynthetic single celled microorganism autotrophic e.g. algae or cyanobacterium Mutualistic: fitness of both partners is increased by the symbiosis Lichen Individual Symbiont Morphology • Mycobiont (Eukaryotic) • Fungal hyphae branch growth at tip • Photobiont (Eukaryotic / Prokaryotic) • Cyanobacterium: • Algae: Morphology • Thallus: defined as undifferentiated vegetative tissue with no specialized organ system that makes up a multicellular organism • Vegetative tissues that grow asexually • Thallus is the main visible body of the lichen • Thallus morphology is usually heavily influenced by the mycobiont species • Categorised into growth forms depending on structural characteristics Growth Forms Increasing freestanding ability / vertical positioning from substrate Fruticose Foliose Squamulose Crustose Generalised lichen vegetative structure • Stratified thallus – organised, layered structures within the lichen: Upper cortex (top to bottom): Pruina layer of inorganic deposits e.g. calcium oxalate Outer epicortex comprised of secretions, dead cells Epinecral layer: dead cells, etc Outer-facing, protective layer of the lichen. Dense layer of agglutinated fungal filaments. Usually, no photobiont cells in this layer. Protective role that prevents too much sunlight penetrating to the photobiont, regulates gas diffusion and protects inner layers. Upper cortex: Peltigera rufescens, sunny habitat (left) with epinecral layer; shady habitat (right) without layer Generalised lichen vegetative structure • Stratified thallus – organised, layered structures within the lichen: Photobiont layer: Fungal filaments interspersed with photobiont cells. Less densely packed than cortex to allow gas exchange and water / air circulation to photosynthetic cells. Cotton-like layer of interwoven long hyphal filaments with high internal air space. Generalised lichen vegetative structure • Stratified thallus – organised, layered structures within the lichen: Medulla layer: Cotton-like layer of interwoven long hyphal filaments with high internal air space. Fungal hyphae coated with deposits of crystalline secondary products that give them hydrophobic properties. This ensures that air space remains present by repelling water in wet periods to allow photosynthetic gas exchange to photobiont cells. Generalised lichen vegetative structure • Stratified thallus – organised, layered structures within the lichen: Lower cortex layer: Foliose and Fruticose lichens have a lower cortex layer due to their leaf like structural formation. Protects the lower side of the leaf structures in the same way as the upper cortex. Not present on crustose and squamulose lichens which are attached directly to surface at the medulla layer. Generalised lichen vegetative structure • Stratified thallus – organised, layered structures within the lichen: Attachments / appendages: Foliose and Fruticose lichens attach to surfaces via rhizines; strongly conglutinated hyphae that make branching structures. Such rhizines can attach to the surface, or penetrate deeply into the substrate onto which a lichen is growing. Secrete extracellular gelatinous materials to allow attachment to surfaces as a glue-like material. Crustose Lichens • Flatter, closely attached to surface • Epilithic / endolithic – stone & rock (surface / sub-surface) • Endophloedic – leaves / stems • Medula layer attached directly to substrate and can penetrate into surface • Lithocortex term used to describe thicker inorganic (usually calcium) deposits on upper cortex • Endolithic lichens contribute to biodeterioration of stone/rock formations including historic monuments Squamulose Lichens • Sub-group of crustose lichens, flat but with leaf-like areolated thallus structures that may detach from surface • Appear to have scales • No lower cortex layer, contrary to foliose lichens Foliose Lichens Leaf-like with flat surfaces. Upper and lower cortex, with appendage attachments to surfaces Lobe structured thalli (leaf-like) Circular structured thalli with central anchor point (umbilicate) Lobe umbilicate Lobe Fruticose Lichens Shrub / hair-like structure Superficially resemble plants Can have vertical stalk structures (podetia) Flatter, strap-like thalli Finer, beard-like structure High surface area that supports gas diffusion and photosynthesis, prevents water-logging Mycobiont attributes: lichenised fungi • Heterotrophic: • Require organic carbon as C source • Predominantly ecologically obligate, physiologically facultative: • Can be cultured on their own • In nature, rarely found without their symbiont • Acquire carbon from living algae / cyanobacterial cells Lichenised / nonlichenised fungi (those that can / cannot form the symbiosis) Definitions Symbiotic / aposymbiotic (fungi in a symbiosis / fungi capable but not in a symbiosis) Mycobiont attributes: lichenised fungi • Lichenised cytology: cells do not differ from non-lichenised fungi • Not a cellular difference • Only approx. 25 % lichens form 3D structures such as foliose / fruticose – most are crustose and flat / with close substrate contact • Mycobiont forms ‘housing’ structure that leads to adequate lighting & gas exchange for photobiont Mycobiont diversity • Photoautotrophic • Fix atmospheric carbon in form of CO2 and utilise sunlight as their energy source • Carry out photosynthesis • Require light, water and carbon dioxide to be provided • Usually water-born taxa • Synthesise glucose and use this to synthesise own organic molecules Photobiont attributes Symbiotic Exchange Need a balance between CO2 assimilation and CO2 respiration to ensure growth. Photobiont Releases: • Organic carbon CO2 atmosphere Minerals leached from substrate Requires: • CO2 • Water • Minerals (N, P, K etc) • Sunlight • Shelter Mutualistic: fitness of both partners is increased by the symbiosis Mycobiont Requires • Organic Carbon • Minerals Water from • Water environment Releases: • CO2 • Organic carbon Facilitates: • Sunlight • Shelter Carbohydrate exchange: Green Algae (phycobionts) Polyhydric sugar alcohol (polyol) E.g. erythritol, ribitol, sorbitol Rapidly taken up by mycobiont and converted to mannitol which cannot be utilised by photobiont • Cyanobacteria (cyanobionts) • Gylcan • Converted to glucose by mycobiont enzymes • • • • Lichen Carbon Sinks: 50 % of CO2 assimilated during photosynthesis is used as part of respiration, remaining make up thallus cell walls Other C compounds: polyols support osmotic pressure thallus; secondary metabolites. Tripartite symbioses • Green algae & cyanobacterial photobionts present • Cyanobacterial partner responsible for N-fixation Lichens as bioindicators of air pollution • Field work on campus • Compare lichen biodiversity between car parks and green areas • Split into small groups • TWO different car / traffic areas; TWO different green areas • Photograph ALL lichens in designated area • Return to lecture room and identify • Determine if patterns can be seen