Anammox in the Environment

The linkage of anammox activity with the removal of fixed inorganic nitrogen in natural systems was first confirmed in the Black Sea suboxic water column (Kuypers et al. 2003). Since then, anammox has been shown to be a significant contributor to nitrogen losses in a variety of environments, responsible for 19-35% of the nitrogen loss in an anoxic coastal bay (Dalsgaard et al. 2003) and the majority of N removal in one of the most productive regions of the world's oceans, the Benguela upwelling oxygen minimal zone (Kuypers et al. 2005). These sites exhibit characteristics of oxygen minimum zones, which are thought to be responsible for 30-50% of global N removal (Brandes and Devol 2002). Evidence for the anammox reaction in sediments or soils is generally first determined by the pore-water N profile. Anoxic zones where there is a concomitant reduction in both nitrite/nitrate and ammonium represent the initial conditions necessary for anammox activity (Fig. 10.3). The maximum reported contribution of anammox is 67-79%, occurring in sediments at a depth of 700 m (Engstrom et al. 2005), which led to the hypothesis that relative anammox contributions increase with depth. However, current evidence suggests that anammox accounts for between 13 and 51% of total N2 production in deep ocean sediments (ca. 3,000 m).

Ammonium is typically abundant in anoxic systems, provided by organic matter oxidation. Nitrate reducing or aerobic ammonium oxidizing bacteria provide the nitrite necessary for the anammox reaction. As such, organic matter availability is

Fig. 10.3 Experimental layout of the isotope pairing technique used for estimating anammox and denitrification activities

thought to be a major factor influencing the relative significance of anammox to total N2 production. Greater organic matter availability creates a higher demand by denitrifiers for NO2- and NO3-, and less NO2- is liberated for anammox consumption. As such, anammox contributed 0-9% of total N2 production in subtropical mangrove sediments (Meyer et al. 2005), 8% in estuarine sediments (Trimmer et al. 2003), and less than 2% in eutrophic shallow coastal bay sediments (Thamdrup and Dalsgaard 2002). Although sediment reactivity has been negatively correlated with anammox contribution to total N2 production (Trimmer et al. 2003), absolute anammox rates appear to peak at sites with intermediate reactivity (Engstrom et al. 2005). As such, a strict relationship between anammox activity and organic matter availability is not firmly established. Additionally, due to the slow growth of anam-mox and their inhibition by low concentrations of O2 (if the "marine" anammox respond the same as the "freshwater" species), environmental stability may be an important controlling factor of anammox activity.

16S rRNA sequences identical or closely related to the marine anammox have been found widely distributed in marine systems, freshwater lakes, and subtropical wetlands (Penton and Tiedje 2006). However, relatively few studies have investigated anammox activity in natural freshwater systems, although the "freshwater" anammox bacteria are the most intensively studied due to their implementation in wastewater treatment bioreactors. Schubert et al. (2006) reported an anammox contribution of 13% in the largest freshwater anoxic lake in the world, Lake Tanganyika. Anammox 16S rRNA gene sequences with > 96% sequence identity to Candidatus Scalindua brodae were identified in the anoxic water column, and anammox cells were enumerated using FISH. Molecular analysis was used to assess the diversity of the anammox population in the Xinyi River (China) (Zhang et al. 2007). Sequences, obtained by targeted PCR, exhibited 16S nucleotide identities of 95% to Candidatus Brocadia anammoxidans and 95% to the Candidatus Scalindua species, including the sequence obtained from the Lake Tanganyika study. These findings suggest that more diverse anammox communities may exist in freshwater habitats, compared to the multitude of marine studies that indicate a single, dominant anammox ecotype.

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