Anammox in Permafrost

Although the anammox process has not been investigated in permafrost soils, "marine" anammox 16S rRNA sequences have been identified in Siberian frozen alluvial sandy loam, deposited in the Middle Pleistocene Epoch 300,000-400,000 years ago in the Cape Svyatoi Nos tundra zone on the Laptev Sea coast (Penton and Tiedje 2006). Rysgaard and Glud (2004) found that anammox was responsible for up to 19% of total N2 production in a Greenland Sea ice floe, but was not detectable in annual sea ice, perhaps due to increased stability. Both aerobic and anaerobic processes in microzones were found to occur simultaneously in brine pockets. This raises the possibility that anammox contributes to N2 removal in permafrost soils.

Due to the use of N2O as a common measure of denitrification and nitrification in permafrost soils, the anammox contribution to nitrogen losses remains an enigma. Ma and colleagues (2007) have reported that a reduction in ammonia concentrations may not be linked to nitrous oxide production in Canadian permafrost soils. Uptake by plants was listed as a possible cause, though they noted that a concomitant nitrate reduction was not observed. The anammox pathway is another possibility that would describe the uptake of ammonia that was not recorded in the N2O emissions. Other evidence for an active N microbial consortium comes from the reported presence of "unstable" ammonia-oxidizing bacteria in Arctic permafrost (Vorobyova et al. 1997). The presence of active nitrifiers at low but finite O2 concentrations in Vostok ice was inferred by Sowers (2001), and a novel cold-adapted nitrite oxidizing bacterium was isolated from a Siberian permafrost sample (Alawi et al. 2007). Low oxygen concentrations, anaerobic microsites and slow water transport, coupled with low organic matter availability, are ideal conditions for anammox bacteria to outcompete denitrifiers for available nitrified NO2- in permafrost. If anammox do indeed contribute to N losses in permafrost brine channels, system stability is a key issue that may affect activity on an annual or over an extended warming trend. However, the use of 15 N isotopic measures, such as variations of the isotope pairing technique, and molecular methods are necessary to assess the viability and response of the anammox and nitrogen cycling community as a whole to ecosystem changes.

Permafrost melting increases water activity and mixing, resulting first in increased O2 availability, which should theoretically negatively impact the anaerobic anammox community. The "explosive microbial growth" following permafrost thawing (Vorobyova et al. 1997) with high available SOM and no mineralization constraints (Uhlirova et al. 2007) would result in competition for available NO2- by denitrifiers. However, the use of 15N isotopic measures, such as variations of the isotope pairing technique, and molecular methods are necessary to assess the viability and response of the anammox and nitrogen cycling community as a whole to ecosystem changes.

Was this article helpful?

0 0

Post a comment