Nitrogen Fixation in Denitrified Marine Waters

Abstract

Nitrogen fixation is an essential process that biologically transforms atmospheric dinitrogen gas to ammonia, therefore compensating for nitrogen losses occurring via denitrification and anammox. Currently, inputs and losses of nitrogen to the ocean resulting from these processes are thought to be spatially separated: nitrogen fixation takes place primarily in open ocean environments (mainly through diazotrophic cyanobacteria), whereas nitrogen losses occur in oxygen-depleted intermediate waters and sediments (mostly via denitrifying and anammox bacteria). Here we report on rates of nitrogen fixation obtained during two oceanographic cruises in 2005 and 2007 in the eastern tropical South Pacific (ETSP), a region characterized by the presence of coastal upwelling and a major permanent oxygen minimum zone (OMZ). Our results show significant rates of nitrogen fixation in the water column; however, integrated rates from the surface down to 120 m varied by ∼30 fold between cruises (7.5±4.6 versus 190±82.3 µmol m−2 d−1). Moreover, rates were measured down to 400 m depth in 2007, indicating that the contribution to the integrated rates of the subsurface oxygen-deficient layer was ∼5 times higher (574±294 µmol m−2 d−1) than the oxic euphotic layer (48±68 µmol m−2 d−1). Concurrent molecular measurements detected the dinitrogenase reductase gene nifH in surface and subsurface waters. Phylogenetic analysis of the nifH sequences showed the presence of a diverse diazotrophic community at the time of the highest measured nitrogen fixation rates. Our results thus demonstrate the occurrence of nitrogen fixation in nutrient-rich coastal upwelling systems and, importantly, within the underlying OMZ. They also suggest that nitrogen fixation is a widespread process that can sporadically provide a supplementary source of fixed nitrogen in these regions.

Active nitrogen-fixing heterotrophic bacteria at and below the chemocline of the central Baltic Sea

“…In the anoxic sample, the mean rate was 0.44±0.26 nmol l À 1 day À 1 of which 0.24 ± 0.26 nmol l À 1 day À 1 was in the o10 mm size fraction. These rates are comparable to those of hypoxic waters in the Southern Californian Bight (average 0.07 nmol l À 1 day À 1 ; Hamersley et al, 2011) and the Peruvian OMZ (average 1.27 nmol l À 1 day À 1 ; Fernandez et al, 2011). In the same water sample, transcription of the D0CY3 cluster was 3.2 Â 10 4 copies l À 1 (Figure 4a) strongly suggesting that these anaerobic bacteria were largely responsible for the N 2 fixation at 200 m.…”

“…Such areas, representing zones with large N-losses through anammox and heterotrophic denitrification (Lam and Kuypers, 2011), can be possible sites for heterotrophic N 2 fixation (Zehr et al, 2006;Riemann et al, 2010). Concordantly, N 2 fixation rates and diverse putative heterotrophic diazotrophs were recently reported from hypoxic waters in the eastern tropical South Pacific (Fernandez et al, 2011) and the Southern Californian Bight (Hamersley et al, 2011). Similarly, nifH genes and transcripts related to heterotrophs were reported from the Arabian Sea OMZ (Jayakumar et al, 2012).…”

“…On the 18 July 2011 at 119 m, EQF91 transcripts were detected but below the limit of quantification (Figure 4). The ALHOU cluster was related to Pseudomonas stutzeri (96% aa similarity) and a prevalent cluster from the South Pacific OMZ (93% aa similarity; HM801245; Fernandez et al, 2011). The abundance of ALHOU reached up to 3.0 Â 10 6 copies l À 1 .…”

“…Similarly, N 2 fixation has been measured off Peru and in a meromictic lake at measurable NH 4 þ levels (Halm et al, 2009;Fernandez et al, 2011). In the laboratory, NH 4 þ downregulates or shuts down nitrogenase expression (for example, Kanemoto and Ludden, 1984;Klugkist and Haaker, 1984).…”

Significant N2 fixation by heterotrophs, photoheterotrophs and heterocystous cyanobacteria in two temperate estuaries

“…New data on nitrogenase reductase (nifH) gene composition continue to expand the potential biome of marine N 2 fixation both in terms of latitude and depth (Mehta et al, 2003;Farnelid et al, 2011;Fernandez et al, 2011;Hamersley et al, 2011) and diverse putative heterotrophic diazotrophs have been reported from temperate estuaries (for example, Affourtit et al, 2001;Jenkins et al, 2004;Farnelid et al, 2009), yet these organisms may thrive due to other traits than diazotrophy (Short and Zehr, 2007). They do, however, seem to fix N in estuarine sediments (Fulweiler et al, 2013) and heterotrophic nifH gene expression accompanies N 2 fixation in the mesohaline deep waters of the Baltic Sea , indicating that these organisms could potentially be important diazotrophs in estuarine pelagic waters as well.…”