This result is consistent with a number of other studies that have found no link between function (including measurements of denitrification rate and denitrifying enzyme

activity) and denitrifier gene copy number using QPCR [13, 25–27]. #AG-881 cost randurls[1|1|,|CHEM1|]# We previously suggested that, in the absence of NO3- addition, denitrifiers in our microcosms used other electron acceptors for respiration when NO3- was not available [17], since denitrifiers are known to use other respiratory pathways [see review 10]. There were proportionally higher EGTs in the iron acquisition and metabolism category in the –N metagenome, and the specific EGT match was to a TonB-dependent receptor (Table 1). TonB-dependent receptors are a category of energy-coupling proteins, which are known to be involved in iron uptake by members of the genus Pseudomonas[28, 29], and there is some evidence that one specific TonB-dependent receptor is involved in dissimilatory iron reduction by Shewanella oneidensis[30]. This suggests that the microbial community in the –N microcosms contained a greater number of organisms capable of acquiring iron and, perhaps, utilizing it for energy, which may have been a potential survival strategy in

the absence of the NO3- addition. To our knowledge, LY3039478 research buy evidence to support this hypothesis Carnitine palmitoyltransferase II is sparse (but see Hauck et al. [31], who found that denitrifiers can also perform anaerobic ferrous iron oxidation). It is accepted, however, that denitrifying organisms primarily perform aerobic respiration and then switch to denitrification under anoxic conditions where NO3- supply is sufficient [32]. There is a category available through MG-RAST for respiration genes. There were close to 400 EGT matches from the two metagenomes to this category for genes involved in both aerobic and anaerobic respiratory pathways.

However, there were no proportional changes in respiration EGT abundance between the +NO3- and the –N conditions (data not shown), likely because the microcosms were made anoxic prior to the metagenome creation, which could negate any advantage to aerobic organisms in either treatment. Though we did not observe proportional changes for EGTs involved in a known alternative respiratory pathway for denitrifiers, the observed proportional increase in iron acquisition and metabolism EGTs in the –N metagenome suggests that iron might be biogeochemically important under anoxic N-limited conditions. Another possible reason for lack of denitrifier EGT treatment response is that denitrifiers may have been in low abundance compared to other microbial groups, making changes to their population undetectable relative to the background population numbers.