Abstract
Abstract. Recent studies demonstrate that marine N2 fixation can be carried out without light by heterotrophic N2 fixers (diazotrophs). However, direct measurements of N2 fixation in aphotic environments are relatively scarce. Heterotrophic as well as unicellular and colonial photoautotrophic diazotrophs are present in the oligotrophic Gulf of Aqaba (northern Red Sea). This study evaluates the relative importance of these different diazotrophs by combining biogeochemical models with time series measurements at a 700 m deep monitoring station in the Gulf of Aqaba. At this location, an excess of nitrate, relative to phosphate, is present throughout most of the water column and especially in deep waters during stratified conditions. A relative excess of phosphate occurs only at the water surface during nutrient-starved conditions in summer. We show that a model without N2 fixation can replicate the observed surface chlorophyll but fails to accurately simulate inorganic nutrient concentrations throughout the water column. Models with N2 fixation improve simulated deep nitrate by enriching sinking organic matter in nitrogen, suggesting that N2 fixation is necessary to explain the observations. The observed vertical structure of nutrient ratios and oxygen is reproduced best with a model that includes heterotrophic as well as colonial and unicellular autotrophic diazotrophs. These results suggest that heterotrophic N2 fixation contributes to the observed excess nitrogen in deep water at this location. If heterotrophic diazotrophs are generally present in oligotrophic ocean regions, their consideration would increase current estimates of global N2 fixation and may require explicit representation in large-scale models.
Highlights
Biological nitrogen fixation refers to the conversion of dinitrogen gas (N2) via reduction to ammonium (NH4) into bioavailable forms of nitrogen by a specialized group of microbes containing the nitrogenase enzyme complex
Multi-year periods of accumulation of nutrients in deep waters were observed from (i) the beginning of the series to the end of 2006, (ii) after the winter of 2008 until February 2012 and (iii) after the winter of 2013 until the end of the series. These periods are bookended by winters with extremely deep mixing events in 2007, 2008, 2012 and 2013 during which nutrient concentrations are nearly homogenized in the entire water column
We implemented and optimized biogeochemical models that represent a range of different assumptions about diazotrophy in a 700 m deep pelagic station from the northern Gulf of Aqaba
Summary
Biological nitrogen fixation refers to the conversion of dinitrogen gas (N2) via reduction to ammonium (NH4) into bioavailable forms of nitrogen by a specialized group of microbes containing the nitrogenase enzyme complex. The size of the oceanic reservoir of bioavailable nitrogen, and the ocean’s capacity for exporting carbon to depth, is controlled by the balance between removal of fixed nitrogen by denitrification and input by N2 fixation (Falkowski, 1997; Haug et al, 1998; Deutsch et al, 2007; Gruber and Galloway, 2008; Fennel et al, 2005). “new nitrogen”) into the euphotic zone (Eppley and Peterson, 1979). These new nitrogen inputs determine the amount of “new production”, which is directly related to the exported fraction. The supply of new nitrogen can occur through several mechanisms, including microbially mediated N2 fixation, diapycnal mixing injecting deep nitrate (NO3) into the surface, lateral transport, atmospheric sources and riverine input. While the injection of deep NO3 is often regarded as the dominant source of new nitrogen that drives the seasonal cycle of marine primary production, there is significant interest in quantifying the contribution of N2 fixation to primary production, in oligotrophic areas (Karl, 2002; Zehr and Ward, 2002; Capone et al, 2005; Luo et al, 2012)
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