Linking above- and belowground responses to global change at community and ecosystem scales

Cryptic belowground organisms are difficult to observe and their responses to global changes are not well understood. Nevertheless, there is reason to believe that interactions among above- and belowground communities may mediate ecosystem responses to global change. We used grassland mesocosms to manipulate the abundance of one important group of soil organisms, arbuscular mycorrhizal (AM) fungi, and to study community and ecosystem responses to CO₂ and N enrichment. Responses of plants, AM fungi, phospholipid fatty acids and community-level physiological profiles were measured after two growing seasons. Ecosystem responses were examined by measuring net primary production (NPP), evapotranspiration, total soil organic matter (SOM), and extractable mineral N. Structural equation modeling was used to examine the causal relationships among treatments and response variables. We found that while CO₂ and N tended to directly impact ecosystem functions (evapotranspiration and NPP, respectively), AM fungi indirectly impacted ecosystem functions by influencing the community composition of plants and other root fungi, soil fungi and soil bacteria. We found that the mycotrophic status of the dominant plant species in the mesocosms determined whether the presence of AM fungi increased or decreased NPP. Mycotrophic grasses dominated the mesocosm communities during the first growing season, and the mycorrhizal treatments had the highest NPP. In contrast, nonmycotrophic forbs were dominant during the second growing season and the mycorrhizal treatments had the lowest NPP. The composition of the plant community strongly influenced soil N, and the community composition of soil organisms strongly influenced SOM accumulation in the mesocosms. These results show how linkages between above- and belowground communities can determine ecosystem responses to global change.     

The Antigens Associated with the Cell Walls of Members of the Genus Pseudomonas

Three antigens were associated with the cell walls of pseudomonads. A highly antigenic, strain-specific antigen of high molecular weight and protein or lipoprotein in nature, occurred as an envelope around the cells. It could be washed off the cells closely associated with carbohydrate material but its antigenicity was not dependent on the carbohydrate present. Another antigen, common to all strains tested, was situated below the first antigen. This was less antigenic than the strain-specific antigen and was polysaccharide or lipopolysaccharide in nature. A second common antigen was the mucopeptide of the cell walls. This had an antigenicity similar to that of the second antigen and was dependent on both the carbohydrate and polypeptide components of the macromolecule. There appears to be some correlation between these findings and the structure of cell walls of pseudomonads are shown by electron microscopy.     

The Structure and Function of the Spore Outer Membrane in Dormant and Germinating Spores of Bacillus megaterium

Attempts to demonstrate the presence of the spore outer membrane in mature, dormant spores of a strain of Bacillus megaterium are described. The outer, integument, layers of this organism were found to contain one-third of the total spore cytochrome content, several enzymes of the electron transport chain (specifically NADH oxidase, dehydrogenase, cytochrome c reductase and NADPH dehydrogenase) and a large number of polypeptides extractable with sodium dodecylsulphate in the presence of dithiothreitol and protease inhibitors. These all suggest the presence of a membraneous element. Electron microscopic evidence is presented on the structure of the dormant integument enzymes. Changes in the integument enzymes and in the gel electrophoresis profile of the extractable integument polypeptides which occur during spore gemination, are described and compared with those that take place in the spore inner membrane. The heat sensitivity of the integument enzymes is compared with that of the inner membrane enzymes and the implications for theories of spore heat resistance discussed.