How dry is dry? Molecular mobility in relation to thallus water content in a lichen

Lichens can withstand extreme desiccation to water contents of ≤ 0.1 g H2O g–1 DW, and in the desiccated state are among the most extremotolerant organisms known. Desiccation-tolerant life-forms such as seeds, mosses and lichens survive ‘vitrification’, that is the transition of their cytoplasm to a ‘glassy’ state, which causes metabolism to cease. However, our understanding of the mechanisms of desiccation tolerance is hindered by poor knowledge of what reactions occur in the desiccated state. Using Flavoparmelia caperata as a model lichen, we determined at what water contents vitrification occurred upon desiccation. Molecular mobility was assessed by dynamic mechanical thermal analysis, and the de- and re-epoxidation of the xanthophyll cycle pigments (measured by HPLC) was used as a proxy to assess enzyme activity. At 20 °C vitrification occurred between 0.12–0.08 g H2O g−1 DW and enzymes were active in a ‘rubbery’ state (0.17 g H2O g−1 DW) but not in a glassy state (0.03 g H2O g−1 DW). Therefore, desiccated tissues may appear to be ‘dry’ in the conventional sense, but subtle differences in water content will have substantial consequences on the types of (bio)chemical reactions that can occur, with downstream effects on longevity in the desiccated state.

The lifestyle of lichens in soil crusts

Lichens are one of the common dominant biota in biological soil crusts (biocrusts), a community that is one of the largest in extent in the world. Here we present a summary of the main features of the lifestyle of soil crust lichens, emphasizing their habitat, ecophysiology and versatility. The soil crust is exposed to full light, often to high temperatures and has an additional water source, the soil beneath the lichens. However, despite the open nature of the habitat the lichens are active under shady and cooler conditions and avoid climate extremes of high temperature and light. In temperate and alpine habitats they can also be active for long periods, several months in some cases. They show a mixture of physiological constancy (e.g. similar activity periods and net photosynthetic rates) but also adaptations to the habitat (e.g. the response of net photosynthesis to thallus water content can differ for the same lichen species in Europe and the USA and some species show extensive rhizomorph development). Despite recent increased research, aspects of soil crust ecology, for example under snow, remain little understood.

Photosynthetic Performance of Two Closely Related Umbilicaria Species in Central Spain: Temperature as a Key Factor

Net photosynthesis (NP) and dark respiration (DR) of thalli of the lichen species Umbilicaria grisea and U. freyi growing together in the same habitat the Sierra de Guadarrama, central Spain, were measured under controlled conditions in the laboratory and under natural conditions in the field over a range of photosynthetic photon flux densities (PPFD), thallus temperatures and thallus water contents. Laboratory experiments revealed that the photosynthetic response to PPFD at optimum thallus water content is very similar in both species. The light compensation points of NP increased from PPFD of c. 20 µmol m−2 s−1 at 0°C up to c. 100 µmol m−2 s−1 PPFD at 25°C. In both species light saturation was not reached up to 700 µmol m−2 s−1 PPFD except at 0°C. By contrast, the temperature dependence of CO2 gas exchange differed substantially between U. grisea and U. freyi. Both species gave significant rates at 0°C. Optimal temperatures of NP were always higher in U. grisea at various PPFD levels if the samples were kept at optimal thallus water content. NP showed maximal rates at 95% dw in U. grisea and 110% dw in U. freyi respectively. In U. grisea a much stronger depression of NP was observed with only 5% of maximal NP reached at 180% dw. At all PPFD and temperature combinations U. freyi showed higher rates of NP and more negative rates of DR if calculated on a dry weight basis. This was also true under natural conditions at the same site, when U. freyi was always more productive than U. grisea. The differences in the photosynthetic response to temperature between both species correlated well with the different distribution patterns of both species. The possibility of genetic control of the physiological performance of these species and its influence on their distribution patterns and autecology is discussed.

Continuous in Situ Recording of the Photosynthetic Activity of Antarctic Lichens—established Methods and a New Approach

A new method is described that records the periods of photosynthetic and respiratory activity in lichen thalli in situ by measuring the actinic chlorophyll fluorescence response of the photobiont. The technique is based on a pulse amplitude modulation fluorometer that was specially developed for continuous and unattended measurements in the Antarctic. In Livingston Island, South Shetland Islands, the periods of metabolic activity of Usnea antarctica were recorded simultaneously with the microclimatic parameter in its natural environment. In one series of diurnal measurements the fluorescence response signal was treated as a qualitative on/off signal for photosynthetic activity. In a parallel series the diurnal courses of net photosynthesis and dark respiration of rain-moistened thalli of U. antarctica were measured by means of a CO2/H2O porometer together with gravimetric water content determination. Both methods showed marked coincidence from low water content to water saturation of the thalli. In the laboratory both the actinic fluorescence response and the photosynthetic rate as a function of thallus water content were investigated in Caloplaca regalis. In the laboratory experiments the CO2 exchange cuvette contained a balance for gravimetric water content control as well as the glass-fibre optics of the fluorescence analysis equipment. The experiments revealed a quantitative relationship between actinic fluorescence and photosynthetic rate within the range of optimum to limiting low thallus water content. The method described is suitable for measurements of lichen activity in field microclimate studies and should allow continuous unattended year-round measurements of lichen activity, which are needed in remote regions such as Antarctica.

The ecology of Ramalina menziesii. VI. Laboratory responses of net CO2 exchange to moisture, temperature, and light

The response of net CO2 exchange to thallus water content, thallus temperature, and photosynthetically active radiation was measured in the laboratory for two morphologically different forms of Ramalina menziesii collected from a coastal and an inland habitat in central California. Equations describing the response curves are fitted to the data and compared statistically for the two sites during two seasons. Significant differences were present for all responses both in summer and winter but were more pronounced for net photosynthesis than for dark respiration. The main differences between the two forms were in the absolute rates of net photosynthesis; a maximum of 6.2 was measured for the inland form but only 3.6 mg∙g−1∙h−1 for the coastal form. Chlorophyll contents were also different between the two forms, indicating that chlorophyll is the likely cause for the difference in net photosynthetic rates. Net photosynthetic rates were higher at low temperatures during winter than during summer, but otherwise seasonal variations in the gas exchange responses were relatively minor. Both forms of the lichen are light saturated at quantum fluxes greater than 200 μE∙m−2∙s−1. Both show an optimum temperature for maximum CO2 exchange at 25 °C, well above the mean operating temperature of R. menziesii in the field.

The ecology of Ramalina menziesii. IV. In situ photosynthetic patterns and water relations of reciprocal transplants between two sites on a coastal–inland gradient

The lichen Ramalina menziesii shows a clear coast–inland gradient in morphology. To investigate whether functional differences in the field were imposed by the environment or were relatively stable phenotypic characteristics, reciprocal transplants were made between coastal and inland habitats in central California. Patterns of gross photosynthesis and thallus water content, together with the driving environmental variables, were measured simultaneously on indigenous and transplanted samples. The comparison showed that thallus water content of the two morphological forms was not significantly different when atmospheric humidity was low. In a humid atmosphere, however, the coastal samples maintained a slightly higher hydration level than the inland samples. This resulted in slightly higher photosynthetic rates when water content was low. At higher hydration levels, the inland samples photosynthesized at a higher rate than the coastal samples. It is possible that these differences are an effect of increased salt load in coastal thalli.