The importance of dissimilatory nitrate reduction to ammonium ( DNRA ) in the nitrogen cycle of coastal ecosystems

Author Posting. © The Oceanography Society, 2013. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 26, no. 3 (2013): 124–131, doi:10.5670/oceanog.2013.54.


DNr a pathWay BiOchemiStry
Recent biochemical and genetic studies yield a great deal of information about the enzymes and genes involved in DNRA and the organisms capable of carrying out fermentative DNRA.
A periplasmic nitrate reductase complex (NapAB) mainly catalyzes the initial reduction of nitrate to nitrite. Nitrite reduction to ammonium is mediated by a pentaheme cytochrome C nitrite reductase (NrfA) and is carried out without producing any intermediate N compound (Einsle et al., 1999

R ates of DNR a iN estuaRiNe aND Coastal eNViRoNmeNts
One of the earliest reports on the occurrence of DNRA in the environment came from a study of estuarine sediment using 15 N tracers (Buresh and Patrick, 1978 (Tobias et al., 2003;Porubsky et al., 2008Porubsky et al., , 2009 (Porubsky et al., 2008). This contrasts with studies in systems with higher nitrate availability where DNRA is more important (Rysgaard et al., 1996).
The predominance of DNRA can hold under both light and dark conditions (Dunn et al., 2012), and, surprisingly, variations in DNRA rates and light show no consistent pattern.

Studies that consider mineralization
and DNRA indicate that while DNRA can be significant relative to denitrification, it typically remains small relative to NH 4 produced from decomposition (Tobias et al. 2001a(Tobias et al. , 2003Porubsky et al., 2011). As was the case for unvegetated sediment, the DNRA contribution to nitrate reduction ranges from < 3% to > 60-99%. To date, 30% of the studies reported DNRA rates in marshes and mangroves that exceed measured denitrification rates at some sites or times (Neubauer et al., 2005;Koop-Jackobsen and Giblin, 2010;Uldahl, 2011;Fernandes et al., 2012), and half the studies report DNRA rates that account for 25-50% of the total nitrate reduction.  Christensen et al. (2000) found rates of DNRA were three to seven times higher below fish cages than in nearby reference sediment. Beggiatoa mats were present below the fish cage and were presumed to be responsible for the high rates of DNRA. Absolute rates of DNRA were considerably higher in mangroves receiving effluent from shrimp ponds than in nearby sites, and DNRA was two to three times more important as a nitrate reduction pathway than denitrification (Molnar et al., 2013).
Early researchers proposed that high organic carbon/nitrate ratios favor DNRA over denitrification (Tiedje et al., 1982). Recent experiments (Streminska et al., 2012) and models (Algar and Vallino, in press) support this general view, and they suggest that there is a positive covariance between anthropogenic organic carbon loads and DNRA (Burgin and Hamilton, 2007;Ferrón et al., 2009). For instance, high dissolved organic carbon to nitrate ratios favored DNRA over denitrification in sediment at the Georgia Coastal Ecosystems LTER (Porubsky et al., 2008). However, DNRA rates across different ecosystems, and within marshes (Tobias et al. 2001a) and mangroves (Rivera-Monroy et al., 1995;Molnar et al., 2013), cannot currently be predicted based upon carbon stocks alone. Shifts in nitrate reduction pathways are likely to be influenced less by carbon quantity as by quality or lability, which is more difficult to assess. In addition, chemolithoautotrophic DNRA might be favored by increased carbon loading, which produces more sulfide.  refereNceS Algar, C.K., and J.J. Vallino. In press. Predicting nitrate reduction pathways in coastal sediments. Applied Environmental Microbiology. An, S., and W.S. Gardner. 2002