However, based on the normalized signal intensity, only vanilate demethylase genes showed a significant increase (p < 0.05) under eCO2 (Additional file 10). The details about this gene are described in Additional file 5. The above results clearly indicate that microbial CO2 fixation may increase, and that microbial degradation and utilization of labile C substrates (e.g., starch, cellulose) may also increase at eCO2, but the degradation of recalcitrant C (e.g., lignin) may not be stimulated by eCO2. Responses

of N cycling genes to eCO2 Sixteen enzymes/genes involved in different N cycling processes were selected in GeoChip 3.0 to target important N cycling processes, such as N2 fixation, nitrification, and denitrification. #click here randurls[1|1|,|CHEM1|]# Based on the total signal intensity detected, significant changes were observed in nifH and nirS, but not other N cycling genes. N2 fixation is exclusively performed by prokaryotes, and nifH encoding the iron protein of Milciclib in vitro N synthase complex, nitrogenase, is the most widely used functional gene marker for N2 fixation [29] and also a phylogenetic marker for nifH-containing organisms [30]. A total of 147 nifH gene variants were detected with 92 shared by both aCO2 and eCO2 samples, 41 unique to eCO2, and 15 unique to aCO2 samples. The total normalized signal intensity of these detected nifH genes was significantly (p < 0.05) higher at

eCO2 than that at aCO2. Ten gene variants were significantly (p < 0.05) increased, and five were significantly decreased at eCO2. More than 69% of the nifH genes detected were affiliated with uncultured or unidentified microorganisms, and five (44829093, 12001884, 780709, 89512880, and 3157614) had >3.0% of the total nifH gene signal intensity. For 13 significantly increased nifH gene variants, ten were from the uncultured or unidentified bacteria, Farnesyltransferase and three (116697525, 2897667, and 148568718) were derived

from Syntrophobacter fumaroxidans MPOB, Paenibacillus macerans, and Roseiflexus sp. RS-1, respectively. Similarly, for five significantly decreased genes detected, three were from unidentified marine eubacterium and unidentified bacteria, and two (77463858 and 138897063) were derived from Rhodobacter sphaeroides 2.4.1 and Geobacillus thermodenitrificans NG80-2, respectively (Figure 4). It is also noted that nine of the top ten abundant genes were from uncultured or unidentified bacteria (Figure 4). Figure 4 The top ten abundant and other significantly changed nifH genes. The number of the probes detected from eCO2 and aCO2 were presented following the bars in parentheses. The statistical significant results of response ratio were shown in front of the GenBank accession number of the probes (**p < 0.05, *p < 0.10). NifH has been classified into five distinct evolutionary groups [31].