Moussab/ORKG-training-evaluation-set huggingface dataset:
Research field: Oceanography
Title: 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.
Extracted statement — Sampling year: 2005
Research field: Oceanography
Title: NITROGEN SOURCES FOR NEW PRODUCTION IN THE NE INDIAN OCEAN
Abstract: Productivity measurements were carried out during spring 2007 in the northeastern (NE) Indian Ocean, where light availability is controlled by clouds and surface productivity by nutrient and light availability. New productivity is found to be higher than regenerated productivity at most locations, consistent with the earlier findings from the region. A comparison of the present results with the earlier findings reveals that the region contributes significantly in the sequestration of CO2 from the atmosphere, particularly during spring. Diatomdominated plankton community is more efficient than those dominated by other organisms in the uptake of CO2 and its export to the deep. Earlier studies on plankton composition suggest that higher new productivity at most locations could also be due to the dominance of diatoms in the region.
Extracted statement — Sampling year: 2007
Research field: Oceanography
Title: Latitudinal distribution of &lt;i&gt;Trichodesmium&lt;/i&gt; spp. and N&lt;sub&gt;2&lt;/sub&gt; fixation in the Atlantic Ocean
Abstract: Abstract. We have determined the latitudinal distribution of Trichodesmium spp. abundance and community N2 fixation in the Atlantic Ocean along a meridional transect from ca. 30° N to 30° S in November–December 2007 and April–May 2008. The observations from both cruises were highly consistent in terms of absolute magnitude and latitudinal distribution, showing a strong association between Trichodesmium abundance and community N2 fixation. The highest Trichodesmium abundances (mean = 220 trichomes L−1,) and community N2 fixation rates (mean = 60 μmol m−2 d−1) occurred in the Equatorial region between 5° S–15° N. In the South Atlantic gyre, Trichodesmium abundance was very low (ca. 1 trichome L−1) but N2 fixation was always measurable, averaging 3 and 10 μmol m2 d−1 in 2007 and 2008, respectively. We suggest that N2 fixation in the South Atlantic was sustained by other, presumably unicellular, diazotrophs. Comparing these distributions with the geographical pattern in atmospheric dust deposition points to iron supply as the main factor determining the large scale latitudinal variability of Trichodesmium spp. abundance and N2 fixation in the Atlantic Ocean. We observed a marked South to North decrease in surface phosphate concentration, which argues against a role for phosphorus availability in controlling the large scale distribution of N2 fixation. Scaling up from all our measurements (42 stations) results in conservative estimates for total N2 fixation of ∼6 TgN yr−1 in the North Atlantic (0–40° N) and ~1.2 TgN yr−1 in the South Atlantic (0–40° S).
Extracted statement — Sampling year: 2007
Research field: Oceanography
Title: 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.
Extracted statement — Sampling year: 2007
Research field: Oceanography
Title: Latitudinal distribution of &lt;i&gt;Trichodesmium&lt;/i&gt; spp. and N&lt;sub&gt;2&lt;/sub&gt; fixation in the Atlantic Ocean
Abstract: Abstract. We have determined the latitudinal distribution of Trichodesmium spp. abundance and community N2 fixation in the Atlantic Ocean along a meridional transect from ca. 30° N to 30° S in November–December 2007 and April–May 2008. The observations from both cruises were highly consistent in terms of absolute magnitude and latitudinal distribution, showing a strong association between Trichodesmium abundance and community N2 fixation. The highest Trichodesmium abundances (mean = 220 trichomes L−1,) and community N2 fixation rates (mean = 60 μmol m−2 d−1) occurred in the Equatorial region between 5° S–15° N. In the South Atlantic gyre, Trichodesmium abundance was very low (ca. 1 trichome L−1) but N2 fixation was always measurable, averaging 3 and 10 μmol m2 d−1 in 2007 and 2008, respectively. We suggest that N2 fixation in the South Atlantic was sustained by other, presumably unicellular, diazotrophs. Comparing these distributions with the geographical pattern in atmospheric dust deposition points to iron supply as the main factor determining the large scale latitudinal variability of Trichodesmium spp. abundance and N2 fixation in the Atlantic Ocean. We observed a marked South to North decrease in surface phosphate concentration, which argues against a role for phosphorus availability in controlling the large scale distribution of N2 fixation. Scaling up from all our measurements (42 stations) results in conservative estimates for total N2 fixation of ∼6 TgN yr−1 in the North Atlantic (0–40° N) and ~1.2 TgN yr−1 in the South Atlantic (0–40° S).
Extracted statement — Sampling year: 2008
Research field: Oceanography
Title: Chemolithoautotrophic production mediating the cycling of the greenhouses gases N&lt;sub&gt;2&lt;/sub&gt;O and CH&lt;sub&gt;4&lt;/sub&gt; in an upwelling ecosystem
Abstract: Abstract. Coastal upwelling ecosystems with marked oxyclines (redoxclines) present high availability of electron donors that favour chemoautotrophy, leading in turn to high N2O and CH4 cycling associated with aerobic NH4+ (AAO) and CH4 oxidation (AMO). This is the case of the highly productive coastal upwelling area off Central Chile (36° S), where we evaluated the importance of total chemolithoautotrophic vs. photoautotrophic production, the specific contributions of AAO and AMO to chemosynthesis and their role in gas cycling. Chemoautotrophy (involving bacteria and archaea) was studied at a time-series station during monthly (2002–2009) and seasonal cruises (January 2008, September 2008, January 2009) and was assessed in terms of dark carbon assimilation (CA), N2O and CH4 cycling, and the natural C isotopic ratio of particulate organic carbon (δ13POC). Total Integrated dark CA fluctuated between 19.4 and 2.924 mg C m−2 d−1. It was higher during active upwelling and represented on average 27% of the integrated photoautotrophic production (from 135 to 7.626 mg C m−2d−1). At the oxycline, δ13POC averaged -22.209‰ this was significantly lighter compared to the surface (-19.674‰) and bottom layers (-20.716‰). This pattern, along with low NH4+ content and high accumulations of N2O, NO2- and NO3- within the oxycline indicates that chemolithoautotrophs and specifically AA oxydisers were active. Dark CA was reduced from 27 to 48% after addition of a specific AAO inhibitor (ATU) and from 24 to 76% with GC7, a specific archaea inhibitor, indicating that AAO and maybe AMO microbes (most of them archaea) were performing dark CA through oxidation of NH4+ and CH4. AAO produced N2O at rates from 8.88 to 43 nM d−1 and a fraction of it was effluxed into the atmosphere (up to 42.85 μmol m−2 d−1). AMO on the other hand consumed CH4 at rates between 0.41 and 26.8 nM d−1 therefore preventing its efflux to the atmosphere (up to 18.69 μmol m−2 d−1). These findings show that chemically driven chemoautotrophy (with NH4+ and CH4 acting as electron donors) could be more important than previously thought in upwelling ecosystems and open new questions concerning its future relevance.
Extracted statement — Sampling year: 2008
Research field: Oceanography
Title: First direct measurements of N2 fixation during a Trichodesmium bloom in the eastern Arabian Sea
Abstract: We report the first direct estimates of N2 fixation rates measured during the spring, 2009 using the 15N2 gas tracer technique in the eastern Arabian Sea, which is well known for significant loss of nitrogen due to intense denitrification. Carbon uptake rates are also concurrently estimated using the 13C tracer technique. The N2 fixation rates vary from ∼0.1 to 34 mmol N m−2d−1 after correcting for the isotopic under‐equilibrium with dissolved air in the samples. These higher N2 fixation rates are consistent with higher chlorophyll a and low δ15N of natural particulate organic nitrogen. Our estimates of N2 fixation is a useful step toward reducing the uncertainty in the nitrogen budget.
Extracted statement — Sampling year: 2009
Research field: Oceanography
Title: First direct measurements of N2fixation during aTrichodesmiumbloom in the eastern Arabian Sea: N2FIXATION IN THE ARABIAN SEA
Abstract: We report the first direct estimates of N2 fixation rates measured during the spring, 2009 using the 15N2 gas tracer technique in the eastern Arabian Sea, which is well known for significant loss of nitrogen due to intense denitrification. Carbon uptake rates are also concurrently estimated using the 13C tracer technique. The N2 fixation rates vary from ∼0.1 to 34 mmol N m−2d−1 after correcting for the isotopic under‐equilibrium with dissolved air in the samples. These higher N2 fixation rates are consistent with higher chlorophyll a and low δ15N of natural particulate organic nitrogen. Our estimates of N2 fixation is a useful step toward reducing the uncertainty in the nitrogen budget.
Extracted statement — Sampling year: 2009
Research field: Oceanography
Title: Chemolithoautotrophic production mediating the cycling of the greenhouses gases N&lt;sub&gt;2&lt;/sub&gt;O and CH&lt;sub&gt;4&lt;/sub&gt; in an upwelling ecosystem
Abstract: Abstract. Coastal upwelling ecosystems with marked oxyclines (redoxclines) present high availability of electron donors that favour chemoautotrophy, leading in turn to high N2O and CH4 cycling associated with aerobic NH4+ (AAO) and CH4 oxidation (AMO). This is the case of the highly productive coastal upwelling area off Central Chile (36° S), where we evaluated the importance of total chemolithoautotrophic vs. photoautotrophic production, the specific contributions of AAO and AMO to chemosynthesis and their role in gas cycling. Chemoautotrophy (involving bacteria and archaea) was studied at a time-series station during monthly (2002–2009) and seasonal cruises (January 2008, September 2008, January 2009) and was assessed in terms of dark carbon assimilation (CA), N2O and CH4 cycling, and the natural C isotopic ratio of particulate organic carbon (δ13POC). Total Integrated dark CA fluctuated between 19.4 and 2.924 mg C m−2 d−1. It was higher during active upwelling and represented on average 27% of the integrated photoautotrophic production (from 135 to 7.626 mg C m−2d−1). At the oxycline, δ13POC averaged -22.209‰ this was significantly lighter compared to the surface (-19.674‰) and bottom layers (-20.716‰). This pattern, along with low NH4+ content and high accumulations of N2O, NO2- and NO3- within the oxycline indicates that chemolithoautotrophs and specifically AA oxydisers were active. Dark CA was reduced from 27 to 48% after addition of a specific AAO inhibitor (ATU) and from 24 to 76% with GC7, a specific archaea inhibitor, indicating that AAO and maybe AMO microbes (most of them archaea) were performing dark CA through oxidation of NH4+ and CH4. AAO produced N2O at rates from 8.88 to 43 nM d−1 and a fraction of it was effluxed into the atmosphere (up to 42.85 μmol m−2 d−1). AMO on the other hand consumed CH4 at rates between 0.41 and 26.8 nM d−1 therefore preventing its efflux to the atmosphere (up to 18.69 μmol m−2 d−1). These findings show that chemically driven chemoautotrophy (with NH4+ and CH4 acting as electron donors) could be more important than previously thought in upwelling ecosystems and open new questions concerning its future relevance.
Extracted statement — Sampling year: 2009
Research field: Oceanography
Title: N2 Fixation in the Eastern Arabian Sea: Probable Role of Heterotrophic Diazotrophs
Abstract: Biogeochemical implications of global imbalance between the rates of marine dinitrogen (N2) fixation and denitrification have spurred us to understand the former process in the Arabian Sea, which contributes considerably to the global nitrogen budget. Heterotrophic bacteria have gained recent appreciation for their major role in marine N budget by fixing a significant amount of N2. Accordingly, we hypothesize a probable role of heterotrophic diazotrophs from the 15N2 enriched isotope labelling dark incubations that witnessed rates comparable to the light incubations in the eastern Arabian Sea during spring 2010. Maximum areal rates (8 mmol N m-2 d-1) were the highest ever observed anywhere in world oceans. Our results suggest that the eastern Arabian Sea gains ~92% of its new nitrogen through N2 fixation. Our results are consistent with the observations made in the same region in preceding year, i.e., during the spring of 2009.
Extracted statement — Sampling year: 2010
Research field: Oceanography
Title: Evidence of active dinitrogen fixation in surface waters of the eastern tropical South Pacific during El Niño and La Niña events and evaluation of its potential nutrient controls: N2FIXATION IN THE ETSP
Abstract: Biological N2 fixation rates were quantified in the Eastern Tropical South Pacific (ETSP) during both El Niño (February 2010) and La Niña (March–April 2011) conditions, and from Low‐Nutrient, Low‐Chlorophyll (20°S) to High‐Nutrient, Low‐Chlorophyll (HNLC) (10°S) conditions. N2 fixation was detected at all stations with rates ranging from 0.01 to 0.88 nmol N L−1 d−1, with higher rates measured during El Niño conditions compared to La Niña. High N2 fixations rates were reported at northern stations (HNLC conditions) at the oxycline and in the oxygen minimum zone (OMZ), despite nitrate concentrations up to 30 µmol L−1, indicating that inputs of new N can occur in parallel with N loss processes in OMZs. Water‐column integrated N2 fixation rates ranged from 4 to 53 µmol N m−2 d−1 at northern stations, and from 0 to 148 µmol m−2 d−1 at southern stations, which are of the same order of magnitude as N2 fixation rates measured in the oligotrophic ocean. N2 fixation rates responded significantly to Fe and organic carbon additions in the surface HNLC waters, and surprisingly by concomitant Fe and N additions in surface waters at the edge of the subtropical gyre. Recent studies have highlighted the predominance of heterotrophic diazotrophs in this area, and we hypothesize that N2 fixation could be directly limited by inorganic nutrient availability, or indirectly through the stimulation of primary production and the subsequent excretion of dissolved organic matter and/or the formation of micro‐environments favorable for heterotrophic N2 fixation.
Extracted statement — Sampling year: 2010
Research field: Oceanography
Title: Aphotic N2 Fixation in the Eastern Tropical South Pacific Ocean
Abstract: We examined rates of N2 fixation from the surface to 2000 m depth in the Eastern Tropical South Pacific (ETSP) during El Niño (2010) and La Niña (2011). Replicated vertical profiles performed under oxygen-free conditions show that N2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 µmol N m−2 d−1 in 2010 and 24±14 to 118±87 µmol N m−2 d−1 in 2011. In the aphotic layers, volumetric N2 fixation rates were relatively low (<1.00 nmol N L−1 d−1), but when integrated over the whole aphotic layer, they accounted for 87–90% of total rates (euphotic+aphotic) for the two cruises. Phylogenetic studies performed in microcosms experiments confirm the presence of diazotrophs in the deep waters of the Oxygen Minimum Zone (OMZ), which were comprised of non-cyanobacterial diazotrophs affiliated with nifH clusters 1K (predominantly comprised of α-proteobacteria), 1G (predominantly comprised of γ-proteobacteria), and 3 (sulfate reducing genera of the δ-proteobacteria and Clostridium spp., Vibrio spp.). Organic and inorganic nutrient addition bioassays revealed that amino acids significantly stimulated N2 fixation in the core of the OMZ at all stations tested and as did simple carbohydrates at stations located nearest the coast of Peru/Chile. The episodic supply of these substrates from upper layers are hypothesized to explain the observed variability of N2 fixation in the ETSP.
Extracted statement — Sampling year: 2010
Research field: Oceanography
Title: Evidence of active dinitrogen fixation in surface waters of the eastern tropical South Pacific during El Niño and La Niña events and evaluation of its potential nutrient controls: N2FIXATION IN THE ETSP
Abstract: Biological N2 fixation rates were quantified in the Eastern Tropical South Pacific (ETSP) during both El Niño (February 2010) and La Niña (March–April 2011) conditions, and from Low‐Nutrient, Low‐Chlorophyll (20°S) to High‐Nutrient, Low‐Chlorophyll (HNLC) (10°S) conditions. N2 fixation was detected at all stations with rates ranging from 0.01 to 0.88 nmol N L−1 d−1, with higher rates measured during El Niño conditions compared to La Niña. High N2 fixations rates were reported at northern stations (HNLC conditions) at the oxycline and in the oxygen minimum zone (OMZ), despite nitrate concentrations up to 30 µmol L−1, indicating that inputs of new N can occur in parallel with N loss processes in OMZs. Water‐column integrated N2 fixation rates ranged from 4 to 53 µmol N m−2 d−1 at northern stations, and from 0 to 148 µmol m−2 d−1 at southern stations, which are of the same order of magnitude as N2 fixation rates measured in the oligotrophic ocean. N2 fixation rates responded significantly to Fe and organic carbon additions in the surface HNLC waters, and surprisingly by concomitant Fe and N additions in surface waters at the edge of the subtropical gyre. Recent studies have highlighted the predominance of heterotrophic diazotrophs in this area, and we hypothesize that N2 fixation could be directly limited by inorganic nutrient availability, or indirectly through the stimulation of primary production and the subsequent excretion of dissolved organic matter and/or the formation of micro‐environments favorable for heterotrophic N2 fixation.
Extracted statement — Sampling year: 2011
Research field: Oceanography
Title: Aphotic N2 Fixation in the Eastern Tropical South Pacific Ocean
Abstract: We examined rates of N2 fixation from the surface to 2000 m depth in the Eastern Tropical South Pacific (ETSP) during El Niño (2010) and La Niña (2011). Replicated vertical profiles performed under oxygen-free conditions show that N2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 µmol N m−2 d−1 in 2010 and 24±14 to 118±87 µmol N m−2 d−1 in 2011. In the aphotic layers, volumetric N2 fixation rates were relatively low (<1.00 nmol N L−1 d−1), but when integrated over the whole aphotic layer, they accounted for 87–90% of total rates (euphotic+aphotic) for the two cruises. Phylogenetic studies performed in microcosms experiments confirm the presence of diazotrophs in the deep waters of the Oxygen Minimum Zone (OMZ), which were comprised of non-cyanobacterial diazotrophs affiliated with nifH clusters 1K (predominantly comprised of α-proteobacteria), 1G (predominantly comprised of γ-proteobacteria), and 3 (sulfate reducing genera of the δ-proteobacteria and Clostridium spp., Vibrio spp.). Organic and inorganic nutrient addition bioassays revealed that amino acids significantly stimulated N2 fixation in the core of the OMZ at all stations tested and as did simple carbohydrates at stations located nearest the coast of Peru/Chile. The episodic supply of these substrates from upper layers are hypothesized to explain the observed variability of N2 fixation in the ETSP.
Extracted statement — Sampling year: 2011