Biological N2O Fixation in the Eastern South Pacific Ocean and Marine Cyanobacterial Cultures

Abstract

Despite the importance of nitrous oxide (N2O) in the global radiative balance and atmospheric ozone chemistry, its sources and sinks within the Earth’s system are still poorly understood. In the ocean, N2O is produced by microbiological processes such as nitrification and partial denitrification, which account for about a third of global emissions. Conversely, complete denitrification (the dissimilative reduction of N2O to N2) under suboxic/anoxic conditions is the only known pathway accountable for N2O consumption in the ocean. In this work, it is demonstrated that the biological assimilation of N2O could be a significant pathway capable of directly transforming this gas into particulate organic nitrogen (PON). N2O is shown to be biologically fixed within the subtropical and tropical waters of the eastern South Pacific Ocean, under a wide range of oceanographic conditions and at rates ranging from 2 pmol N L−1 d− to 14.8 nmol N L−1 d−1 (mean ± SE of 0.522±1.06 nmol N L−1 d−1, n = 93). Additional assays revealed that cultured cyanobacterial strains of Trichodesmium (H-9 and IMS 101), and Crocosphaera (W-8501) have the capacity to directly fix N2O under laboratory conditions; suggesting that marine photoautotrophic diazotrophs could be using N2O as a substrate. This metabolic capacity however was absent in Synechococcus (RCC 1029). The findings presented here indicate that assimilative N2O fixation takes place under extreme environmental conditions (i.e., light, nutrient, oxygen) where both autotrophic (including cyanobacteria) and heterotrophic microbes appear to be involved. This process could provide a globally significant sink for atmospheric N2O which in turn affects the oceanic N2O inventory and may also represent a yet unexplored global oceanic source of fixed N.

Climate relevant trace gases (N2O and CH4) in the Eurasian Basin (Arctic Ocean)

“…The 15 N-N 2 experiments were also positive at most of the sampling stations, indicating new input of N into the system, which may support the hypothesis that N 2 fixation is an extremely versatile process, being led by both autotrophic (Cyanobacteria) and heterotrophic bacteria in Arctic Ocean brine and seawater. Thus, the existence of N-fixing microorganisms in this marine region may have an effect on the N 2 O inventory, which could be used as an alternative substrate, as suggested by Farías et al (2013). Microorganisms that carry out biological N 2 O fixation could produce N 2 O depletion and simultaneous under-saturation in these marine environments.…”

Biological oceanography, biogeochemical cycles, and pelagic ecosystem functioning of the east central South Pacific Gyre: focus on Easter Island and Salas y Gomez Island

“…Chlorophyll and CDOM exhibit very tightly coupled seasonal co-variations, suggesting that seasonal variations in sea surface chlorophyll might not reflect true Farías, et al (2013). biomass variations. Instead variations in the depth of the mixed layer and level of mixing might cause more unbleached CDOM and higher pigmented cells to be transported towards the surface in the winter, whereas cells and CDOM near the surface in the summer have been exposed to solar bleaching for longer on average (Morel et al, 2010).…”

Dissolved greenhouse gases (nitrous oxide and methane) associated with the naturally iron-fertilized Kerguelen region (KEOPS 2 cruise) in the Southern Ocean

“…This suggests that there is a close relationship between phytoplankton, the only producers of DMSP (Yoch, 2002), and microbial communities which may be recycling DMS. Phyto-and bacterioplankton relationships control DMS turnover, which could result in several mechanisms of DMSP/DMS degradation (Simó et al, 2002;VilaCosta et al, 2006) and produce CH 4 (Damm et al, 2010;Florez-Leiva et al, 2013;Weller et al, 2013). These publications show that phytoplankton species composition and biomass in different bloom phases, as well as eddy dynamics, were important determinants of CH 4 saturation and emission.…”

“…Christaki et al (2014) showed that the highest bacterial production rates (up to 110 mg C m −2 d −1 ) and the greatest abundance of heterotrophic bacteria were associated with stations where the phytoplankton bloom was developed (TEW-7 and A3-2). Recent evidence indicates that methylotrophs are candidates for mediated CH 4 generation using methylated compounds such as DMSP and DMS (Florez-Leiva et al, 2013;Weller et al, 2013). Among these heterotrophic microorganisms DMS degradation can be ascribed to methylotrophic bacteria (Vissher et al, 1994) that derive energy from the conversion of methyl into other products, as well as using S as a source of methionine biosynthesis (Kiene et al, 1999).…”

“…N 2 O is mainly produced during the first step of nitrification (the aerobic oxidation of NH (Codispoti and Cristiensen, 1985) or assimilatory N 2 O reduction to NH + 4 (Farias et al, 2013). CH 4 , on the other hand, is mainly formed by methanogens during anaerobic OM degradation (Wuebble and Hayboe, 2002) or by methylotrophs during CH 4 formation derived from transformations of methyl compounds such as methylphosphonate (MPn; Karl et al, 2008), dimethylsulfoniopropionate (DMSP; Damm et al, 2010) and dimethylsulfide (DMS; Florez-Leiva et al, 2013). In addition, CH 4 can be consumed (oxidized) via aerobic methanotrophy (Hanson and Hanson, 1996).…”