Carbon-dioxide exchange in lichens: determination of transport and carboxylation characteristics

Abstract

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.

Effects of bryophyte and lichen cover on permafrost soil temperature at large scale

“…To estimate the availability of CO 2 for bryophytes and lichens, the model computes diffusion of CO 2 from the atmosphere into the organisms. The CO 2 diffusivity is decreasing with increased water content of the organisms due to narrowing of diffusion pathways and formation of water films (Cowan et al, 1992). In addition to CO 2 and PAR, the rate of modelled photosynthesis depends on surface temperature, which is calculated by JSBACH from the surface energy balance.…”

“…We set D CO 2 ,sat to a value of 0.01 [mol m −2 s −1 ] based on Williams and Flanagan (1998) and vary this value by multiplying it by the factors 0.5 and 2.0. We choose this form of variation since D CO 2 ,sat shows relatively large natural variation from around 5 × 10 −4 to 2 × 10 −2 [mol m −2 s −1 ] (Williams and Flanagan, 1998;Cowan et al, 1992); consequently, a linear variation would not be adequate (Porada et al, 2013). It should be noted that variation in D CO 2 ,sat represents an extension to the original bryophyte and lichen model, described in Porada et al (2013).…”

Estimating global carbon uptake by lichens and bryophytes with a process-based model

“…The diffusivity of the thallus for CO 2 , which is inversely related to water saturation since water leads to a swelling of cells and thus to a narrowing of the diffusion pathways (Cowan et al, 1992); 2. The water potential, which increases from −∞ at zero water saturation to a maximum value of 0 at a certain threshold saturation.…”

“…w D CO 2 is estimated using the data points in Fig. B10, while w D CO 2 ,min and w D CO 2 ,max are taken from the literature (Cowan et al, 1992).…”

Tansley Review No. 117

“…The presence of a CCM in the photobiont is correlated with low CO # compensation points (Cowan et al, 1992 ;Palmqvist 1993 ;Palmqvist et al, 1994a ;Ma! guas et al, 1995 ;Smith & Griffiths 1996) and a decreased sensitivity of lichen photosynthesis to O # (Palmqvist, 1993).…”