Benthic Dinitrogen Fixation Traversing the Oxygen Minimum Zone Off Mauritania (NW Africa)

Abstract

Despite its potential to provide new nitrogen (N) to the environment, knowledge on benthic dinitrogen (N 2 ) fixation remains relatively sparse, and its contribution to the marine N budget is regarded as minor. Benthic N 2 fixation is often observed in organic-rich sediments coupled to heterotrophic metabolisms, such as sulfate reduction. In the present study, benthic N 2 fixation together with sulfate reduction and other heterotrophic metabolisms were investigated at six station between 47 and 1,108 m water depth along the 18 • N transect traversing the highly productive upwelling region known as Mauritanian oxygen minimum zone (OMZ). Bottom water oxygen concentrations ranged between 30 and 138 µM. Benthic N 2 fixation determined by the acetylene reduction assay was detected at all stations with highest rates (0.15 mmol m −2 d −1 ) on the shelf (47 and 90 m water depth) and lowest rates (0.08 mmol m −2 d −1 ) below 412 m water depth. The biogeochemical data suggest that part of the N 2 fixation could be linked to sulfate-and iron-reducing bacteria. Molecular analysis of the key functional marker gene for N 2 fixation, nifH, confirmed the presence of sulfate-and iron-reducing diazotrophs. High N 2 fixation further coincided with bioirrigation activity caused by burrowing macrofauna, both of which showed high rates at the shelf sites and low rates in deeper waters. However, statistical analyses proved that none of these processes and environmental variables were significantly correlated with benthic diazotrophy, which lead to the conclusion that either the key parameter controlling benthic N 2 fixation in Mauritanian sediments remains unidentified or that a more complex interaction of control mechanisms exists. N 2 fixation rates in Mauritanian sediments were 2.7 times lower than those from the anoxic Peruvian OMZ.

Widespread nitrogen fixation in sediments from diverse deep‐sea sites of elevated carbon loading

“…However, other environmental differences may also explain the difference. Two recent studies measuring nitrogenase activity within oxygen minimum zone (OMZ) sediments off the coast of Peru and northwest Africa reported integrated rates 1-2 orders of magnitude higher than those observed here (70-1025 m water depth, acetylene reduction assay) (Gier et al, 2016;Gier et al, 2017); one notable difference in the sediments of the Peruvian OMZ is the nearly 10 times greater organic carbon content compared to those investigated here.…”

“…(, )]. The recent pair of reports of benthic N 2 fixation rates in OMZs reported opposite trends in N 2 fixation rates with organic carbon content of sediments, suggesting this complexity extends to the deep‐sea environment (Gier et al ., , ). At methane seeps, including as seen here at Hydrate Ridge and in previous studies off‐shore Costa Rica and in the Eel River Basin, methane appears to consistently stimulate N 2 fixation (Dekas et al ., , , ).…”

“…The distribution of N 2 fixation with depth in the sediment was not overall negatively correlated with in situ pore water ammonium concentrations (Fig. ), consistent with previous reports (Gier et al ., , ). However, a negative correlation was evident during the time‐course analysis, when ammonium concentrations in the incubations were shown to change with time (Figs and S5–S7).…”

“…The relationship between diazotrophy and carbon loading in shallow marine sediments is complex, with increased carbon typically but not always increasing diazotrophy [reviewed in Capone (1988) and revisited by Fulweiler et al (2007Fulweiler et al ( , 2008]. The recent pair of reports of benthic N 2 fixation rates in OMZs reported opposite trends in N 2 fixation rates with organic carbon content of sediments, suggesting this complexity extends to the deep-sea environment (Gier et al, 2016(Gier et al, , 2017. At methane seeps, including as seen here at Hydrate Ridge and in previous studies off-shore Costa Rica and in the Eel River Basin, methane appears to consistently stimulate N 2 fixation (Dekas et al, 2009(Dekas et al, , 2016.…”

“…However, the deep sea is among the most under-studied habitats on Earth, and nitrogen cycling on the deep seafloor remains enigmatic. Although marine sediments are traditionally thought to be replete with bioavailable nitrogen due to the breakdown of organic matter, multiple lines of evidence in recent years have suggested or demonstrated N 2 fixation (diazotrophy) in deep-sea sediments: microbiological culturing (Mehta and Baross, 2006), detection of nif genes and transcripts (Mehta et al, 2003;Dang et al, 2009;Dekas et al, 2009;Miyazaki et al, 2009;Dang et al, 2013;Dekas et al, 2014Dekas et al, , 2016Reese et al, 2018), isotopic modelling (Wankel et al, 2015), acetylene reduction assays (Hartwig and Stanley, 1978;Gier et al, 2016Gier et al, , 2017 and both bulk and single-cell measurements of 15 N 2 assimilation (Dekas et al, 2009(Dekas et al, , 2016. Despite this growing body of evidence, few direct rate measurements of N 2 fixation are available, limiting our ability to assess the quantitative significance deep-sea diazotrophy and its biogeochemical controls.…”