Effects of Light and Dark on the Ultrastructure of Lichen Algae

Abstract

Quantitative ultrastructural observations have been made on the algal cells (Trebouxia) in two lichens, Parmelia sulcata and P. laevigata, stored for 48 h in the dark or under a light/dark regime. The response of the alga was found to differ in these lichens. In P. sulcata the dark treatment caused a decrease in starch grains, lipid-rich pyrenoglobuli and peripheral cytoplasmic storage bodies and an increase in pyrenoid and chloroplast protein bodies. The algae in P. laevigata contained little starch and no chloroplast protein bodies. However, after dark treatment, starch, cytoplasmic storage bodies and pyrenoid dimensions sometimes declined, while pyrenoglobuli numbers increased. Some of these apparent changes depended upon the units used for calculating the cross-sectional áreas of structures, e.g. absolute units, percentage of cell wall, protoplast or chloroplast cross-sectional área. Chloroplast área increased in the dark (as a % of cell wall área) in both species while mitochondria were larger in the dark in P. sulcata but not in P. laevigata. Ultrastructural changes were not clearly correlated with changes in photosynthetic and respiratory rates. These results directly support the suggestion that some intra-cellular structures are energy-generating reserves the dimensions of which can rapidly change.

New Ultrastructural and Physiological Features of the Thallus in Antarctic Lichens

“…In the Antarctic lichens we studied, starch grains of various sizes occurred in the form of transparent areas around the pyrenoid or in the vicinity of mitochondria. In Brown et al's (1988) work, Parmelia sulcata thalli kept in the dark contained no starch supplies, while those under day/night conditions had a significant amount of starch. Tarhanen et al (1997) observed a rise in the starch content of algal cells under increased ozone, and also suggested that lipid metabolism is facilitated in the presence of ozone.…”

“…Few or no lipid droplets were found in the thallus of Hypogymnia physodes collected in winter, whereas thalli collected in summer contained a considerable amount of lipids (Fiechter and Honneger, 1988). Brown et al (1988) reported decreased content of lipid reserve materials under dark conditions in Parmelia sulcata and P. laevigata, and suggested that the lipid material gathered in photobionts is not the first to be exhausted in a situation unfavorable to lichens. The main metabolite used up in response to stress is starch.…”

“…Describing the ultrastructure of the Trebouxia photobiont in the thalli of ten lichen species examined in situ under conditions of thallus hydration and desiccation, Jacobs and Ahmadjian (2007) reported that starch grains occurred in all the chloroplasts, and lipid-containing globules occurred in both hydrated and desiccated conditions in the pyrenoids. The location of the pyrenoid and the electron-dense matrix can change during drought (Brown et al, 1988). According to Ahmadjian, (1993), the amount of pyrenoglobuli in the pyrenoid depends on light intensity: they are more numerous under low lighting.…”

“…According to Ahmadjian, (1993), the amount of pyrenoglobuli in the pyrenoid depends on light intensity: they are more numerous under low lighting. According to Brown et al (1988) and Ahmadjian (1993) they function as a type of reserve material and, depending on the light stress, time of year or hydration level of the thallus, can change their size, amount and location.…”

“…Antarctic lichens are adapted to life under extremely low temperature, dehydration and solar radiation (Kappen et al, 1987;1995). They carry out photosynthesis with the minimum of light (Brown et al, 1988), at temperatures below 0°C or even below the temperature of ice nucleation in the cellular fluids of the thallus. Lichens stimulate ice deposition in intercellular spaces, just as vascular plants do.…”