| Summary: | I have designed an improved method of matric water potential regulation in which relative humidity is controlled by the temperature difference between a saturated atmosphere and a connected experimental chamber. Versatility, ease of operation, and lack of interference with the gaseous composition of the experimental atmosphere are the major advantages of this system over other methods such as salt solutions. The water potential control system was used to investigate the role of thallus structure in CO$ sb2$ uptake of two hot desert lichen species under water stress. Photosynthesis of isolated lichen phycobiont cells at low water potentials was also examined. Within the lichen thallus, matric water potentials higher than ambient appear to result from physical properties of the fungal matrix. Increased water availability supports photosynthesis by the enclosed phycobiont at low ambient phycobiont (Trebouxia sp.) photosynthesize well under water stress. The combination of increased internal water availability and desiccation-tolerant phycobiont cells results in the survival of these lichens in an extremely arid climate. The two species occupy different microhabitats as a consequence of differences in CO$ sb2$ uptake under saturating conditions and hydrophobicity. The role of the endolithic habitat (enclosure) in the water economy of the cryptoendolithic microbial communities from the Ross Desert (Antarctica) and the Negev Desert (Israel) was examined. Colonized sandstone rocks adsorb or condense water vapor in pore spaces. This water supports photosynthesis by the Ross Desert cryptoendolithic lichen community but does not create conditions suitable for photosynthesis of Negev Desert cryptoendolithic cyanobacteria. The difference between the two communities is that cyanobacteria are similar to other prokaryotes: water potentials approaching that of liquid water are necessary for bacterial metabolism whereas eukaryotes typically function under water stress. Source: Dissertation Abstracts International, Volume: 49-03, Section: B, page: 0635. Major Professor: E. Imre Friedmann. Thesis (Ph.D.)--The Florida State University, 1987. I have designed an improved method of matric water potential regulation in which relative humidity is controlled by the temperature difference between a saturated atmosphere and a connected experimental chamber. Versatility, ease of operation, and lack of interference with the gaseous composition of the experimental atmosphere are the major advantages of this system over other methods such as salt solutions. The water potential control system was used to investigate the role of thallus structure in CO$ sb2$ uptake of two hot desert lichen species under water stress. Photosynthesis of isolated lichen phycobiont cells at low water potentials was also examined. Within the lichen thallus, matric water potentials higher than ambient appear to result from physical properties of the fungal matrix. Increased water availability supports photosynthesis by the enclosed phycobiont at low ambient phycobiont (Trebouxia sp.) photosynthesize well under water stress. The combination of increased internal water availability and desiccation-tolerant phycobiont cells results in the survival of these lichens in an extremely arid climate. The two species occupy different microhabitats as a consequence of differences in CO$ sb2$ uptake under saturating conditions and hydrophobicity. The role of the endolithic habitat (enclosure) in the water economy of the cryptoendolithic microbial communities from the Ross Desert (Antarctica) and the Negev Desert (Israel) was examined. Colonized sandstone rocks adsorb or condense water vapor in pore spaces. This water supports photosynthesis by the Ross Desert cryptoendolithic lichen community but does not create conditions suitable for photosynthesis of Negev Desert cryptoendolithic cyanobacteria. The difference between the two communities is that cyanobacteria are similar to other prokaryotes: water potentials approaching that of liquid water are necessary for bacterial metabolism whereas eukaryotes typically function under water stress.
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