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Hobbie, E.A., L.S. Watrud, S. Maggard, T. Shiroyama and P.T. Rygiewicz. 2003. Carbohydrate use and assimilation by litter and soil fungi assessed by carbon isotope and BIOLOGŪ assays. Soil Biology and Biochemistry 35:303-311. WED-01-137

Soil fungi are integral to decomposition in forests and other habitats, yet identifying probable functional roles of different taxa is problematic. Here, we compared carbohydrate assimilation patterns derived from stable isotope analyses on cultures with patterns of metabolic activity measured on BIOLOGŪ SF-P plates for 12 taxa of soil- and litter-inhabiting saprotrophic fungi isolated from Douglas-fir (Pseudotsttga menziesii) ecosystems. To determine the relative assimilation of carbon from malt extract versus sucrose by 13C stable isotope analyses, we cultured fungi with malt extract (consisting primarily of glucose and maltose) plus either C3- or C4- derived sucrose as carbon sources. Rhodotorula graminis and F. oxysporum assimilated the highest proportion of sucrose, a Mortierella isolate and an unidentified sterile isolate (FPC 341) assimilated the lowest proportion of sucrose, and remaining cultures assimilated similar and intermediate proportions of sucrose. On Biolog plates, low metabolic activity of Mortierella and FPC 341 on sucrose and R. graminis and F. oxysporum on maltose were qualitatively consistent with isotopic results. Assimilation of sucrose calculated isotopically was correlated with the ratio of sucrose; maltose activity calculated from Biolog assays (r2 = 0.45, P = 0.0145, n = 12). Metabolic activity on Biolog plates for six other common soil carbohydrates were also determined: glucose, fructose, galactose, cellobiose, lactose, and glycogen. Metabolic activity was greatest overall 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. Our results indicated that stable isotope studies and Biolog assays may provide complementary information to characterize metabolic potential of fungi in forest litter and soil.

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