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CARBOHYDRATE USE AND ASSIMILATION BY LITTER AND SOIL FUNGI ASSESSED BY CARBON ISOTOPES AND BIOLOG ASSAYS
Citation:
Hobbie, E. A., L S. Watrud, S. Maggard, T Shiroyama, AND P T. Rygiewicz. CARBOHYDRATE USE AND ASSIMILATION BY LITTER AND SOIL FUNGI ASSESSED BY CARBON ISOTOPES AND BIOLOG ASSAYS. SOIL BIOLOGY AND BIOCHEMISTRY. Elsevier Science Ltd, New York, NY, 35:303-311, (2003).
Description:
Soil fungi are integral to decomposition in forests, yet identification of probable functional roles of different taxa is problematic. Here, we compared carbohydrate assimilation patterns derived from stable isotope analyses on cultures with those produced from cultures on Biolog? SP-F plates for 12 taxa of soil- and litter-inhabiting saprophytic fungi isolated from Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) ecosystems. To determine the relative assimilation of malt extract versus sucrose by 13C stable isotope analyses, we cultured fungi with malt extract plus either C3- or C4-derived sucrose as carbon sources. Rhodotorula graminis and Fusarium oxysporum assimilated the highest proportion of sucrose, a Mortierella isolate and the unidentified sterile isolate FPC 341 assimilated the lowest proportion of sucrose, and the remainder of the cultures assimilated similar and intermediate proportions of sucrose. We then used Biolog SF-P plates to determine the metabolic activity of the fungi on eight carbohydrates similar to those present in the isotopic study: glucose, fructose, galactose, maltose, sucrose, cellobiose, lactose, and glycogen. In general, metabolic activity was greatest on maltose and glucose and lowest on fructose. Two of the isolates (Aspergillus flavus and F. oxysporum) had higher metabolic activity on the glucose-containing disaccharide cellobiose than on glucose, strongly suggesting preferential uptake of cellobiose compared to glucose and suggesting the potential ability to use cellulose. The high metabolic activity of these cultures on galactose, a primary constituent of hemicellulose, also suggested cellulolytic capabilities. With metabolic activity normalized among cultures, the Mortierella isolate and the unidentified sterile isolate FPC 341 had the lowest metabolic activity on sucrose, results generally consistent with assimilation patterns calculated isotopically. Low metabolic activities of R. graminis and F. oxysporum on maltose in Biolog assays were qualitatively consistent with isotopic results. The small assimilation of maltose in these two cultures when sucrose was also present suggested that sucrose inhibited maltose uptake. Assimilation of sucrose as calculated isotopically was correlated with the ratio of sucrose : maltose assimilation as calculated from Biolog assays (r2=0.45, p=0.0145, n=12). These results indicate that stable isotope studies and Biolog methodologies may provide complementary information to characterize functional roles of fungi in forest litter and soil.