Abstract
Despite its potential to provide new nitrogen (N) to the environment, knowledge on benthic dinitrogen (N2) fixation remains relatively sparse and its contribution to the marine N budget, is regarded as minor. Benthic N2 fixation is often observed in organic-rich sediments coupled to heterotrophic metabolisms, such as sulfate reduction. In the present study, benthic N2 fixation together with sulfate reduction and other heterotrophic metabolisms were investigated at six station between 47 and 1108 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 N2 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 N2 fixation could be linked to sulfate- and iron-reducing bacteria. Molecular analysis of the key functional marker gene for N2 fixation, nifH, confirmed the presence of sulfate- and iron-reducing diazotrophs. High N2 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 N2 fixation in Mauritanian sediments remains unidentified or that a more complex interaction of control mechanisms exists. N2 fixation rates in Mauritanian sediments were 2.7 times lower than those from the anoxic Peruvian OMZ.
Highlights
Dinitrogen (N2) fixation is the dominant source of new bioavailable nitrogen (N) in the marine environment (Brandes and Devol, 2002)
Our findings add to the growing knowledge of benthic N cycling in upwelling regions and will aid in our understanding of potential environmental factors that control benthic diazotrophs
This result was supported by molecular analysis of the nifH gene, which confirmed the presence of several sulfate-reducing bacteria related to Desulfovibrio spp
Summary
Dinitrogen (N2) fixation is the dominant source of new bioavailable nitrogen (N) in the marine environment (Brandes and Devol, 2002). While most studies on marine N2 fixation have focused on pelagic environments (e.g., Zehr and Ward, 2002; Galloway et al, 2004; Riemann et al, 2010; Löscher et al, 2014 and references therein), benthic N2 fixation gained renewed attention only recently with a few studies demonstrating active N2 fixation in sediments and identifying diazotrophs by nifH gene analysis (Fulweiler et al, 2007; Bertics et al, 2010, 2013; Gier et al, 2016). Previous studies identified the availability of organic matter as major control on benthic microbial processes (Jørgensen, 1983; Howarth et al, 1988; Fulweiler et al, 2007; Bertics et al, 2013). Benthic N2 fixation and organic matter have been found to correlate in different habitats, such as sediments within the high-productive Peruvian upwelling region (Gier et al, 2016) and coastal sediments inhabited by the bioturbating ghost shrimp Neotrypaea californiensis (Bertics et al, 2010)
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