Green lichens have been shown to attain positive net photosynthesis in the presence of water vapour while blue-green lichens require liquid water (Lange et al. 1986). This behaviour is confirmed not only for species with differing photobionts in the genusPseudocyphellaria but for green and blue-green photobionts in a single joined thallus (photosymbiodeme), with a single mycobiont, and also when adjacent as co-primary photobionts. The different response is therefore a property of the photobiont. The results are consistent with published photosynthesis/water content response curves. The minimum thallus water content for positive net photosynthesis appears to be much lower in green lichens (15% to 30%, related to dry weight) compared to blue-greens (85% to 100%). Since both types of lichen rehydrate to about 50% water content by water vapour uptake only green lichens will show positive net photosynthesis. It is proposed that the presence of sugar alcohols in green algae allow them to retain a liquid pool (concentrated solution) in their chloroplasts at low water potentials and even to reform it by water vapour uptake after being dried. The previously shown difference in δC values between blue-green and green lichens is also retained in a photosymbiodeme and must be photobiont determined. The wide range of δC values in lichens can be explained by a C carboxylation system and the various effects of different limiting processes for photosynthetic CO fixation. If carboxylation is rate limiting, there will be a strong discrimination ofCO, at high internal CO partial pressure. The resulting very low δC values (-31 to-35‰) have been found only in green lichens which are able to photosynthesize at low thallus water content by equilibraiton with water vapour. When the liquid phase diffusion of CO becomes more and more rate limiting and the internal CO pressure decreases, theC content of the photosynthates increases and less negative δC values results, as are found for blue-green lichens.

CO exchange rate in relation to thallus water content (WC, % of dry weight) was determined for 22 species of lichens, mainly members of the genera Pseudocyphellaria and Sticta, from a temperate rainforest, Urewere National Park, New Zealand. All data were obtained in the field, either using a standard technique in which the lichens were initially wetted (soaked or sprayed, then shaken) and allowed to slowly dry, or from periodic measurements on samples that were continuously exposed in their natural habitat. A wide range of WC was found, with species varying from 357 to 3360% for maximal WC in the field, and from 86 to 1300% for optimal WC for photosynthesis. Maximal WC for lichens, wetted by the standard technique, were almost always much less than the field maxima, due to the presence of water on the thalli. The relationships between CO exchange rate and WC could be divided into four response types based on the presence, and degree, of depression of photosynthesis at high WC. Type A lichens showed no depression, and Type B only a little at maximal WC. Type C had a very large depression and, at the highest WC, CO release could occur even in the light. Photosynthetic depression commenced soon after optimal WC was reached. Type D lichens showed a similar depression but the response curve had an inflection so that net photosynthesis was low but almost constant, and never negative, at higher WC. There was little apparent relationship between lichen genus or photobiont type and the response type. It was shown that high WC does limit photosynthetic CO uptake under natural conditions. Lichens, taken directly from the field and allowed to dry under controlled conditions, had net photosynthesis rates that were initially strongly inhibited but rose to an optimum, before declining at low WC. The limiting effects of high WC were clearly shown when, under similar light conditions, severe photosynthetic depression followed a brief, midday, rain storm. Over the whole measuring period the lichens were rarely at their optimal WC for photosynthesis, being mostly too wet or, occasionally, too dry. Photosynthetic performance by the lichens exposed in the field was similar to that expected from the relationship between the photosynthetic rate and WC established by the standard procedure.

Measurements were made of net rates of CO2 assimilation in lichens at various ambient concentrations of CO2 in air and in helox (79% He, 21% O2). Because of the faster rate of CO2 diffusion in the pores of lichen thalli when filled with helox than when filled with air, a given net rate of assimilation was achieved at a lower ambient concentration of CO2 in helox. The differences were used to estimate resistances to diffusion through the gas-filled pore systems in lichens. The technique was first tested with five lichen species, and then applied in a detailed study with Ramalina maciformis, in which gas-phase resistances were determined in samples at four different states of hydration and with two irradiances. By assuming, on the basis of previous evidence, that the phycobiont in R. maciformis is fully turgid and photosynthetically competent at the smallest hydration imposed (equilibration with vapour at 97% relative humidity), and that, with this state of hydration, diffusion of CO2 to the phycobiont takes place through continuously gas-filled pores, it was possible also to determine both the dependence of net rate of assimilation in the phycobiont on local concentration of CO2 in the algal layer, and, with the wetter samples, the extents to which diffusion of CO2 to the phycobiont was impeded by water films. In equilibrium with air of 97% relative humidity, the thallus water content being 0.5 g per g dry weight, the resistance to CO2 diffusion through the thallus was about twice as large as the resistance to CO2 uptake within the phycobiont. Total resistance to diffusion increased rapidly with increase in hydration. At a water content of 2 g per g it was about 50 times as great as the resistance to uptake within the phycobiont and more than two-thirds of it was attributable to impedance of transfer by water. The influences of water content on rate of assimilation at various irradiances are discussed. The analysis shows that the local CO2 compensation concentration of the phycobiont in R. maciformis is close to zero, indicating that photorespiratory release of CO2 does not take place in the alga, Trebouxia sp., under the conditions of these experiments.

Hypolithic microbial communities (i.e. cryptic microbial assemblages found on the undersides of translucent rocks) are major contributors of carbon input into the oligotrophic hyper-arid desert mineral soils of the Eastern Antarctic Dry Valleys. Here we demonstrate, for the first time, that hypolithic microbial communities possess both the genetic capacity for nitrogen fixation (i.e. the presence of nifH genes) and the ability to catalyse acetylene reduction, an accepted proxy for dinitrogen fixation. An estimate of the total contribution of these communities suggests that hypolithic communities are important contributors to fixed nitrogen budgets in Antarctic desert soils.

A lichen growing in a continental Antarctic region with low temperatures and strong irradiance in summer was investigated for evidence of photoinhibition. Field experiments with Umbilicaria aprina from a sheltered site with heavy snowpack showed no effects of photoinhibition when the lichen was exposed to strong sun irradiance for nearly 11 h a day. This was evident from CO exchange and simultaneous chlorophyll a fluorescence measurements. CO exchange was also not affected if quartz glass allowing greater UV penetration, was used as a lid for the cuvette. The dependency of net photosynthesis on photosynthetic photon flux density suggests that the lichen is photophilous.