16S rRNA gene transcript numbers of total Bacteria, and selected bacterial taxa (Clostridia [Group I], Planctomycetaceae, and two uncultivated taxa of Bacteroidetes) decreased more in anoxic than in oxic cellulose-supplemented soil microcosms in the presence of both herbicides. Collectively, the results suggested that the metabolism of anaerobic cellulose-degrading Bacteria was impaired by typical in situ herbicide concentrations, whereas in situ concentrations did not impair metabolism of aerobic cellulose- and cellobiose-degrading soil Bacteria. Cellulose is metabolized by diverse aerobic
and anaerobic, cellulolytic and saccharolytic microorganisms in soils (Falkowski et al., 2002; Lynd PS-341 et al., 2002; Wei et al., 2009; Schellenberger et al., 2010). Increasing Bleomycin application of herbicides over the past decades in agriculture has resulted in accumulation of herbicide residues in soils that may affect microbial metabolism (Wainwright, 1978; Thorstensen et al., 2001; Chowdhury et al., 2008; Hiller et al., 2008). Herbicides can be degraded in soils (Müller et al., 2001; Gonzales et al., 2006), although, their degradation is slow compared with that of natural organic compounds (such as sugars or amino acids) and is primarily aerobic (Harrison et al., 1998; Knauber et al., 2000; Liu et al., 2010). Bentazon [3-isopropyl-1H2,1,3-benzothiadiazin-4(3H)-one-2,2-dioxide; pKa = 3.28]
is a control agent of broadleaf weeds in agricultural crop plantations. It inhibits the photosynthetic electron flow in plants, and interacts with membranous proteins, which leads to an inhibition of ATPase (Hull & Cobb, 1998). Therefore, bentazon inhibits growth of pure cultures of various soil bacteria (e.g. Actinobacteria, rhizobia, cyanobacteria), and reduces dinitrogen
fixation and nitrogen mineralization in soils (Cernakova et al., 1991; Galhano et al., 2009). MCPA (4-chloro-2-methylphenoxyacetic Histone demethylase acid; pKa = 3.73) is also used as a control agent of broadleaf weeds, and acts as a plant growth hormone analog. MCPA enters the cytoplasm in the acidic form by diffusion, which causes a dissipation of the transmembrane proton-motive force (Cabral et al., 2003). The toxic effect of MCPA on cell growth has been observed with pure cultures of yeast, Pseudomonas putida and Vibrio fischeri (Ahtiainen et al., 2003; Cabral et al., 2003). Therefore, the toxic effects of these herbicides on cellulose-degrading soil Bacteria have been addressed in the current study. Soil from a wheat-planted agricultural cropland (Germany; 48°30.0′N, 11°20.7′E; sampled June 2009) was used (Table 1) to prepare soil microcosms. Cellulose- and cellobiose-supplemented soil microcosms were incubated at 15 °C in darkness. Two different experiments were set up. Microcosms with wet soil were supplemented with cellulose paper sheets.