Preliminary characterization of microbial functional diversity using sole-C-source utilization profiles in Tremoal do Pedrido mire (Galicia, NW Spain) .

Peatlands are a global carbon sink. It has been estimated that Northern peatlands store 257-342 Gt of carbon. Most carbon is in the form of peat organic matter whose transformation is largely dependent on microbial activity, and this in turn has a major influence on carbon cycling. The presence of two main physico-chemical environments in the peat deposits -the upper, aerobic, acrotelm and the lower, anaerobic catotelm- suggests that there could be differences in both the microbial activity and communities’ composition with depth. In this preliminary study we analyzed the depth variation in microbial community functional diversity using sole–carbon–source utilization profiles, in a raised bog from Galicia (NW Spain). Substrate utilization was quantified by measuring the absorbance at 590 nm of the colour developed by the reduction of the tetrazolium dye contained in each carbon source. Substrates consumption was followed for 10 days and expressed as average well colour development (AWCD) and microbial diversity (Shannon-Wiener index, H). The highest activity (AWCD 1.6-2.0) and microbial diversity (H 3.3) were found at the surface (upper 2 cm) of the peatland. No substrate was used below a depth of 52 cm. Principal components analysis showed three main depth records of degradation: i) substrates (N-compounds, carbohydrates, carboxylic acids) only used at the surface of the peatland, ii) substrates (mostly carboxylic acids) used at the surface and at 46-48 cm, and iii) substrates (N-compounds and polymers) used from the surface to a depth of 48-52 cm. The overall kinetics of substrate utilization showed four patterns: i) asymptotic, ii) exponential (although of low activity), iii) linear, and iv) no reaction over the whole incubation period. Some differences were observed, both in intensity of substrate degradation and total time of reaction, when substrates were grouped according to the results of principal component analysis. These findings suggest a “stratification” of the microbial communities that may be controlled by the varying geochemical conditions (humidity, temperature, acidity, nutrient- and oxygen availability) with depth, and that the acrotelm/catotelm boundary is an effective barrier for oxidative degradation of the organic matter in peatlands.