Light screening in lichen cortices can be quantified by chlorophyll fluorescence techniques for both reflecting and absorbing pigments

Abstract

Lichens, representing mutualistic symbioses between photobionts and mycobionts, often accumulate high concentrations of secondary compounds synthesized by the fungal partner. Light screening is one function for cortical compounds being deposited as crystals outside fungal hyphae. These compounds can non-destructively be extracted by 100% acetone from air-dry living thalli. Extraction of atranorin from Physcia aipolia changed the lichen colour from pale grey to green in the hydrated state, whereas acetone-rinsed and control thalli were all pale grey when dry. Removal of parietin from Xanthoria parietina changed the colour of desiccated thalli from orange to grey. Colour changes were quantified by reflectance measurements. By a new chlorophyll fluorescence method, screening was assessed as the decrease in incident irradiance (PAR) necessary to reach identical effective quantum yields of PSII (Phi(PSII)) in acetone-rinsed and control thalli. Thereby, we estimated a screening efficiency due to cortical atranorin crystals at 61, 38, and 40% of blue, green and red light, respectively, whereas parietin screened 81, 27 and 1% of these wavelength ranges. Removal of atranorin caused similar levels of increased photoinhibition for P. aipolia in blue, green and red light, whereas parietin-deficient thalli of X. parietina exhibited increased photoinhibition with decreasing wavelengths. Atranorin possibly prevents water from entering the spaces between the hyphae in the cortex. The air-filled cavities with white atranorin crystals reflect excess light, whereas the yellow compound parietin absorbs excess light. Thereby, both atranorin and parietin play significant photoprotective roles for symbiotic green algae, but with compound-specific screening mechanisms.

Lichen Specific Thallus Mass and Secondary Compounds Change across a Retrogressive Fire-Driven Chronosequence

“…In our study system, light transmission through the forest canopy is greatest on the small islands [31] and on these islands P. sulcata increases its production of atranorin and chloroatranorin per unit area, which should confer better protection against light. As such, atranorin has previously been shown to reduce photoinhibition [52].…”

A potential trade-off with stictic acid improves ascospore viability inXanthoparmelia cumberlandia

“…The major CBSM produced by A. centrifuga are alectoronic acid, atranorin and usnic acid; X. cumberlandia produces stictic, norstictic and usnic acids; and X. viriduloumbrina produces usnic, consalazinic and salazinic acids. Usnic acid and atranorin are thought to have light screening properties (Armaleo et al 2008;BeGora & Fahselt 2001;McEvoy et al 2006;Solhaug et al 2009Solhaug et al , 2010, the stictic acid complex is thought to have antiherbivory properties (Asplund et al 2009;Gauslaa 2005;Lawrey 1980) and the salazinic acid complex is thought to have antioxidant properties (Gaikwad et al 2012). These three species of lichens colonize the rock surface and overlap in their distributions (Brodo et al 2001;Hale 1990;Lendemer 2005;Thomson 1984), exhibiting different life history strategies (Deduke et al in press).…”

Carbon based secondary compounds do not provide protection against heavy metal road pollutants in epiphytic macrolichens

“…The more than 1050 different CBSCs found in lichens (Molnár and Farkas, 2010), are located outside the wall of fungal hyphae (Fahselt, 1994). Most lichens have some CBSCs in the medulla that deter lichen-feeders and additionally one CBSC in their upper cortex protecting underlying photobionts from excess solar radiation (Solhaug et al, 2010). So far, few have studied functional relationships between these compounds and elemental composition in field experiments.…”