Photosynthetic Depression at High Thallus Water Contents in Lichens: Concurrent Use of Gas Exchange and Fluorescence Techniques with a Cyanobacterial and a Green Algal Peltigera Species

Abstract

Lichens, being poikilohydric, have varying thallus water contents (We) and show a complex interaction between net photosynthesis (NP) and we. NP can be depressed at low WC (desiccation effects) and, in some species, also at high we. In the latter case the depression is normally ascribed to increased CO 2 diffusion resistances through water blockage. Recently, an earlier explanation, that the depression at high WC is due to recycling of CO 2 from increased dark respiration processes (DR),has been given renewed prominence.The two explanations were distinguished by the concurrent use of gas exchange and chlorophyll fluorescence techniques to investigate NP: WC relationships in the lichens Peftigera feucophlebia (green algal) and P.neckeri (cyanobacterial). Both species had a distinct optimal WC for NP with depressed values at low and high We. The maximal quantum yield for both CO 2 fixation (initial slope of light response curves of NP) and photosystem II (fluorescence signals of dark-adapted thalli) was depressed only at low WC and remained high at optimal and greater we. In contrast, the relative electron transport rate (ETR, derived from fluorescence signals of thalli in the light) tracked NP and was depressed at low and high We. The depression of both NP and ETR at high WC (not that at low We) could be prevented by using elevated external CO 2 concentrations. A single, linear relationship was found between all values of gross photosynthesis (NP + DR) and ETR regardless of external CO 2 concentration or We.Our results show that, for these lichens, the depression in NP at high WC is a real fall in photosynthetic rate of the photobionts and is not due to recycling of CO 2 , The removal of the depression in NP and ETR at high WC by using elevated external CO 2 levels allows us to conclude that an additional CO 2 diffusion resistance is present.

The Influence of Biological Soil Crusts on Soil Characteristics along a High Arctic Glacier Foreland, Nunavut, Canada

“…Oversaturation with water is a well-described phenomenon severely limiting photosynthetic CO 2 exchange in lichens by increasing CO 2 diffusion resistance (Lange et al, 1993). This phenomenon can be more easily detected with quantitative gas exchange measurements than with chlorophyll fluorescence measurements (Lange et al, 1996), a fact confirming the method applied here. Optimal thallus water content and maximum photosynthesis rates can only be found during the following drying (Lange, 2003b).…”

Dehydration-induced responses of primary photosynthetic processes and spectral reflectance indices in Antarctic Nostoc commune

“…the photosynthetizing poikilohydric organism contains more water than necessary to saturate photosynthesis) and well described for lichens (see e.g. Lange et al 1996Lange et al , 2001). In the following phase (RWC from 80 to 30%), the atmospheric CO 2 diffusion into the colony is enhanced and an increased  PSII can be explained as a coaction of increased CO 2 intake and fixation by the colony (when considering an equilibrium between photochemical and biochemical processes).…”

“…We hypothesized that, similarly to cyanolichens (Lange et al 1996), water suprasaturation effect resulting in inhibition of photosynthetic processes could be detected at high thallus hydration. We, therefore, also hypothesized that optimum hydration of N. commune colony would be less than full hydration, i.e.…”

Earth as a Tool for Astrobiology—A European Perspective

“…This time-consuming observation is necessary because of the dominance of very slow growing species present, for example, in extreme Mars-analogue field sites. In parallel, measurements on irradiation, temperature, relative humidity, methane/CO 2 production or consumption and oxygen production are also performed (Lange et al 1970(Lange et al , 1996. Such investigation on planetary analogue field sites on Earth provides fundamental information on life's behavior and its capacity to influence or change its environment.…”