Nitrogen-limited mangrove ecosystems conserve N through dissimilatory nitrate reduction to ammonium

Abstract

Earlier observations in mangrove sediments of Goa, India have shown denitrification to be a major pathway for N loss1. However, percentage of total nitrate transformed through complete denitrification accounted for <0–72% of the pore water nitrate reduced. Here, we show that up to 99% of nitrate removal in mangrove sediments is routed through dissimilatory nitrate reduction to ammonium (DNRA). The DNRA process was 2x higher at the relatively pristine site Tuvem compared to the anthropogenically-influenced Divar mangrove ecosystem. In systems receiving low extraneous nutrient inputs, this mechanism effectively conserves and re-circulates N minimizing nutrient loss that would otherwise occur through denitrification. In a global context, the occurrence of DNRA in mangroves has important implications for maintaining N levels and sustaining ecosystem productivity. For the first time, this study also highlights the significance of DNRA in buffering the climate by modulating the production of the greenhouse gas nitrous oxide.

Dissimilatory nitrate reduction to ammonium conserves nitrogen in anthropogenically affected subtropical mangrove sediments in Southeast China

“…Anammox (NH 4 + + NO 2 − → N 2 ) also removes bioavailable nitrogen, and this process widely exists in coastal basins as well as pelagic oceans (including water bodies, sediments, mangrove sediments, and even Arctic ice sheets) (Brandes et al, 2007). Moreover, DNRA converts NO 3 − to NH 4 + , which is more bioavailable; thus, it conserves nitrogen in the ecosystem (Bernard et al, 2015;Fernandes et al, 2012;Giblin et al, 2013). Thus, DNRA reduces the loss of nutrients caused by denitrification through this nitrogen recycling process and is an important mechanism to produce reusable nitrogen, Marine Pollution Bulletin xxx (2016) xxx-xxx which assures high productivity of the mangrove system (Fernandes et al, 2012).…”

“…Available nitrogen forms include nitrate (NO 3 − ), nitrite (NO 2 − ), ammonia (NH 4 + ), and organic nitrogen (ON) (Brandes et al, 2007). The mangrove system can convert inorganic nitrogen to gaseous N 2 by nitrogen burial, physical transport, denitrification, anammox, and microbial activities (Vitousek et al, 1997) and conserve nitrogen through the DNRA process (Bernard et al, 2015;Fernandes et al, 2012). The denitrification process reduces NO 3 − and NO 2 − to N 2 and N 2 O, respectively; therefore, it removes nitrogen that was originally stabilized in mangroves.…”

Nutrient gradients in Panamanian estuaries: effects of watershed deforestation, rainfall, upwelling, and within-estuary transformations

“…9) suggested (1) no evident differences associated with differences in watershed deforestation, a result confirming the erasure of watershed imprints down-estuary; (2) ammonium concentration deviations in creased down-estuary to salinities of about 15 ‰, implying within-estuary insertions (by regeneration or, less likely, by dissimilatory reduction of nitrate to ammonium; Fernandes et al 2012) during transit down-estuary; (3) deviations in ammonium and nitrate concentrations leaving estuaries seemed largely positive (more points above the dashed line) at salinities > 25 ‰ (Fig. 9), leaving the same visual impression as Figs.…”

“…5, Table 4). The mid-estuary high suggested considerable release of ammonium into the water column, probably by action of ammonium regeneration from mangrove sediments (Mwa shote & Jumba 2002, Ferguson et al 2004, Alongi et al 2005, Fernandes et al 2010, Smith et al 2011, and perhaps some influence of dissimilatory nitrate reduction to ammonium (Fernandes et al 2012).…”

Phylogenetic Diversity of Diazotrophs along an Experimental Nutrient Gradient in Mangrove Sediments

“…Despite their high productivity, mangroves are often nutrient limited, particularly for nitrogen [5][6][7][8]. Mangroves ability to conserve nutrients as a whole ecosystem relies largely on the mineralization of organic matter by active microorganisms, an important component for the productivity of the ecosystem [9][10][11]. Diazotrophs (nitrogen fixing bacteria) are a physiologically and phylogenetically diverse microbial group responsible for most nitrogen input in the mangrove ecosystem [12][13][14].…”