Extensive investigations made in the past two decades on lichen photosynthesis in relation to water content have shown two features of particular interest: first, the depression of net photosynthesis at high water contents, suprasaturation (i.e. the lichen contains more water than necessary to saturate photosynthesis), and, second, the ability of green algal lichens to regain photosynthetic activity by uptake of water from humid air. Evidence from several investigators is presented to confirm that both phenomena are now well substantiated through laboratory investigations. It has been questioned whether these features do actually occur in nature and, if they do, to what extent. Recent work is summarized that demonstrates that for many of the lichens studied suprasaturation is of major importance and can result in depressed photosynthesis for around a third of the time that the lichens are photosynthetically active. Reactivation of photosynthesis of green algal lichens by high humidity is also, apparently, very common in some environments, for example, humid temperate rainforests, occurring almost every night. It is possible that the dominance of green algal lichens, rather than cyanobacterial species, in these habitats is a result of their ability to utilize water vapour. If so, then the phenomenon must have major ecological importance for lichen productivity. In general, laboratory studies seem to be able to predict extremely well the behaviour of lichens in their natural habitat.

Hydration‐dependent photosynthetic production of lichens: what do laboratory studies tell us about field performance?

Photosynthetic and respiratory CO2 gas exchange was measured under controlled climate conditions in the laboratory in two epiphytic lichens, Lobaria pulmonaria and Platismatia glauca, with the aim of modelling their net productivity using field microclimate data. For both, the thallus water content (WC) and the light intensity had the greatest impact on photosynthesis. L. pulmonaria had optimum net photosynthesis (NP) at WCs between 75–175% of the thallus dry weight (DW), while P. glauca required a WC of c. 85% for maximal NP without depression at higher WCs. Both species reached light compensation of NP at 5–10 µmol photons m−2 s−1 and were saturated at 100–150 µmol photons m−2 s−1. Respiratory CO2 loss corresponded to 35–40% of gross photosynthesis at 85–100% WC and 15° C, in both species. Growth of the two species were followed in transplanted thalli during a 16-month period at two contrasting sites, a forest edge adjacent to a 15 year old clear-cut and within the interior of a mature Picea abies forest. At these sites, the microclimate parameters; light, temperature, relative humidity (RH) and thallus WC were also monitored. Judged from the microclimate data, the lichens were active for 13–19% of the time with thallus WC monitoring, where >60% of the active time occurred in darkness. When photosynthetically active, the edge transplants received a 2–3 times higher light dose and were active for a longer accumulated time compared to the interior transplants. The field microclimate data in conjunction with the laboratory data predicted a 4 times higher DW yield of the edge transplants compared to the interior transplants. However, the DW yield of L. pulmonaria was overestimated at the edge and underestimated for P. glauca in the interior by our model. Possible reasons for these discrepancies and the validity of using laboratory data and microclimate monitoring to predict growth rates of lichens under varying field conditions are discussed.

Growth and vitality of epiphytic lichens

Filamentous cyanobacteria are the main primary producers in biological desert sand crusts The cells are exposed to extreme environmental conditions including temperature, light, and diurnal desiccation/rehydration cycles We have studied the kinetics of activation of photosynthesis during rehydration of the cyanobacteria, primarily Microcoleus sp, within crust samples collected in the Negev desert, Israel We also investigated their susceptibility to photoinhibition Activation of the photosynthetic apparatus, measured by fluorescence kinetics, thermoluminescence, and low temperature fluorescence emission spectra, did not require de novo protein synthesis Over 50% of the photosystem II (PSII) activity, assembled phycobilisomes, and photosystem I (PSI) antennae were detected within less than 5 min of rehydration Energy transfer to PSII and PSI by the respective antennae was fully established within 10 to 20 min of rehydration The activation of a fraction of PSII population (about 20%‐30%) was light and temperature-dependent but did not require electron flow to plastoquinone [was not inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea] The cyanobacteria within the crusts are remarkably resistant to photoinhibition even in the absence of protein synthesis The rate of PSII repair increased with light intensity and with time of exposure Consequently, the extent of photoinhibition in high-light-exposed crusts reached a constant, relatively low, level This is in contrast to model organisms such as Synechocystis sp strain PCC 6803 where PSII activity declined continuously over the entire exposure to high illumination Ability of the crust’s organisms to rapidly activate photosynthesis upon rehydration and withstand photoinhibition under high light intensity may partly explain their ability to survive in this ecosystem

Activation of Photosynthesis and Resistance to Photoinhibition in Cyanobacteria within

& We tested the hypothesis that changed microclimate at induced forest edges causes reduced growth of epiphytic lichens. Two foliose, green algal lichens were transplanted to the lower canopy of a mature Picea abies forest at six distances (2, 6.25, 12.5, 25, 50 and 100 m) from a clearcut. The biomass growth in Platismatia glauca (6.2% in 16 months) was 41% higher than in Lobaria pulmonaria (4.4%). We found no growth reduction near the forest edge. In contrast, the highest growth in both species occurred within 12 m from the edge. Further, fluorescence and chlorophyll measurements showed that lichen vitality was unaffected by distance from edge. The light intensity was 4.3 times higher at the edge than in the interior during the growing season, but there were only minor differences in air temperature and relative humidity. Monitoring of thallus water content revealed clear differences in both number and length of wetting and drying cycles. However, the total time with water content sufficient for photosynthetic activity was only slightly higher at the edge. The data thus indicate that our gradient in microclimate was too small to significantly affect lichen growth, and that lichens are largely metabolically inactive when large edge-interior contrasts in microclimate occur. Lichen response to forest edge microclimate results from intricate interactions among several biotic and abiotic factors. Linking data on lichen growth, microclimate and thallus water content with physiological measurements provides a framework for future studies of the mechanisms behind abiotic edge effects.

I. Responses to microclimate along a forest edge-interior gradient

Retention patches as potential refugia for bryophytes and lichens in managed forest landscapes

Diel patterns of CO2-exchange for six lichens from a temperate rain forest in New Zealand

Atmospheric CO2 levels were determined (at 2 m height) in the rainforest and in a clearing outside the forest, during spring (November) 1991, Urewera National Park, New Zealand. CO2 levels within the forest were 30 ppm higher and showed a more variable diel pattern (range up to 70 ppm) than outside the forest. CO2 levels were generally higher at night than during the day. Detailed measurements were made at several sites at a depth of 25 mm in the phylloplane of three moss species and under, or between, the thalli of four lichen species. Mean levels were 50% (moss phylloplane) and 10% (lichen thalli) higher than the levels in the clearing and, in 80% of sites, also higher than air within the rainforest. The diel pattern of the CO2 concentration at each of the sites was not predictable from measurements of CO2 in the bulk air of the forest. High levels of CO2 may be important in elevating photosynthetic rates of mosses and, to a lesser extent, lichens in the field.

https://www.tandfonline.com/doi/pdf/10.1080/0028825X.1994.10410474

Diel changes of atmospheric CO2 concentration within, and above, cryptogam stands in a New Zealand temperate rainforest

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Diel patterns of CO2-exchange for six lichens from a temperate rain forest in New Zealand

Diel changes of atmospheric CO2 concentration within, and above, cryptogam stands in a New Zealand temperate rainforest