Temperature acclimation effects on growth, respiration and enzyme activities in an arctic and a temperate isolate of Cenococcum geophilum Fr
Growth and respiration of ectomycorrhizal fungi associated with dwarf shrubs likely contribute significantly to carbon cycling in arctic tundra soils dominated by this vegetation type. Despite their importance little is known about how these fungi might respond to global warming. While previous studies have shown that some ectomycorrhizal fungi can tolerate or retain viability across a fairly wide range of temperatures little is known regarding their metabolic responses to temperature shifts. The present study was undertaken to examine the comparative physiological responses in vitro of arctic and temperate isolates of a common ectomycorrhizal fungus to shifts in growth temperature. Isolates of Cenococcum geophilum from Alaska and Maryland were grown at 12 and 20ºC in liquid culture to study the process of temperature acclimation. Measurements on each isolate at the two growth temperatures included linear growth rates, dry weight accumulation, oxygen consumption and the specific activities of the soluble enzymes glucokinase, phosphoglucose isomerase, 6-phosphogluconate dehydrogenase and glucose-6-phosphate dehydrogenase. In addition the response to growth temperature of glucokinase kinetic parameters (Ea, Km and Vmax) were also evaluated. Increasing the growth temperature tended to decrease the length of the observed lag phase; however, the growth rates in dry weight at 20ºC closely paralleled those at 20ºC. Both isolates demonstrated lower respiration rates when grown at 20 versus 12ºC. Thus for oxygen uptake each isolate demonstrated a phenotypic response known as ideal rate compensation. That is growth at 20ºC resulted in oxygen uptake rates at 20ºC that were similar to those measured at 12ºC for 12ºC-grown mycelium. This strategy, if common in ectomycorrhizal fungi, would reduce the expected carbon demand placed on the host and decrease the amount of carbon dioxide released by respiration in response to anticipated increases in soil temperatures. Differences in overall respiration rates by 12 and 20ºC grown mycelium could not be explained by changes in activities of soluble enzymes examined. Likewise few differences were observed in glucokinase kinetic parameters associated with growth temperature. Examination of soluble enzyme activity ratios as influenced by growth temperature suggests the potential exists to alter relative fluxes through primary metabolic pathways and warrants further investigation. Future studies of temperature acclimation should examine a wider range of ectomycorrhizal fungi and employ techniques such as DNA microarrays and metabolomics.
Arctic; Cenococcum geophilum; ectomycorrhizal fungi; enzyme activity; glycolysis; Km; oxygen uptake; pentose phosphate pathway; respiration; temperature acclimation; tundra
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