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Reconstructing N2-fixing cyanobacterial blooms in the Baltic Sea

beyond observations using 6- and 7-methylheptadecane in sediments as specific biomarkers

Abstract.

"Summer cyanobacterial blooms represent a threat to the Baltic Sea ecosystem, causing deoxygenation of the bottom water and the spread of the so-called dead zones. The history of the Baltic Sea cyanobacterial blooms is known from in situ and satellite observations since the early 1980s but is still not well understood. [...]"

Source: Biogeosciences
Authors: Jérôme Kaiser et al.
DOI: 10.5194/bg-17-2579-2020

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Changing perspectives in marine nitrogen fixation

Abstract.

"Biological dinitrogen (N2) fixation, the reduction of atmospheric N2 to ammonia, is important for maintaining the fertility of the oceans by providing biologically useful nitrogen to support primary organic matter production (i.e., carbon dioxide fixation). N2 fixation offsets the removal of combined nitrogen by microbial denitrification and anaerobic ammonium oxidation (anammox) and export to the deep sea. For several decades, there has been a lack of consensus as to whether losses of N through microbial removal pathways are balanced by biological nitrogen fixation, along with other inputs such as atmospheric nitrogen deposition and terrestrial runoff. [...]"

Source: Science
Authors: Jonathan P. Zehr1 and Douglas G. Capone
DOI: 10.1126/science.aay9514

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No nitrogen fixation in the Bay of Bengal?

Abstract.

"The Bay of Bengal (BoB) has long stood as a biogeochemical enigma, with subsurface waters containing extremely low, but persistent, concentrations of oxygen in the nanomolar range which – for some, yet unconstrained, reason – are prevented from becoming anoxic. One reason for this may be the low productivity of the BoB waters due to nutrient limitation and the resulting lack of respiration of organic material at intermediate waters. [...]"

Source: Biogeosciences
Authors: Carolin R. Löscher et al.
DOI: 10.5194/bg-17-851-2020

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Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Abstract.

"The availability of the micronutrient iron (Fe) in surface waters determines primary production, N2 fixation, and microbial community structure in large parts of the world's ocean, and thus it plays an important role in ocean carbon and nitrogen cycles. Eastern boundary upwelling systems and the connected oxygen minimum zones (OMZs) are typically associated with elevated concentrations of redox-sensitive trace metals (e.g., Fe, manganese (Mn), and cobalt (Co)), with shelf sediments typically forming a key source. [...]"

Source: Biogeosciences
Authors: Insa Rapp et al.
DOI: 10.5194/bg-16-4157-2019

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Marine nitrogen fixers mediate a low latitude pathway for atmospheric CO2 drawdown

Abstract.

"Roughly a third (~30 ppm) of the carbon dioxide (CO2) that entered the ocean during ice ages is attributed to biological mechanisms. A leading hypothesis for the biological drawdown of CO2 is iron (Fe) fertilisation of the high latitudes, but modelling efforts attribute at most 10 ppm to this mechanism, leaving ~20 ppm unexplained [...]"

Source: Nature Communications 
Authors: Pearse J. Buchanan et al.
DOI: 10.1038/s41467-019-12549-z

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Dinitrogen fixation across physico‐chemical gradients of the Eastern Tropical North Pacific oxygen deficient zone

Abstract.

"The Eastern Tropical North Pacific (ETNP) Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hotspots for reactive nitrogen (Nr) removal processes, ODZs generate conditions proposed to promote Nr inputs via dinitrogen (N2) fixation. In this study, we quantified N2 fixation rates by 15N‐tracer bioassay across oxygen, nutrient and light gradients within and adjacent to the ODZ. [...]"

Source: Global Biogeochemical Cycles
Authors: C.R. Selden et al.
DOI: 10.1029/2019GB006242

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Rates and pathways of N2 production in a persistently anoxic fjord: Saanich Inlet, British Columbia

Abstract.

"Marine oxygen minimum zones (OMZs) support 30-50% of global fixed-nitrogen (N) loss but comprise only 7% of total ocean volume. This N-loss is driven by canonical denitrification and anaerobic ammonium oxidation (anammox), and the distribution and activity of these two processes vary greatly in space and time. Factors that regulate N-loss processes are complex, including organic matter availability, oxygen concentrations, and NO2- and NH4+ concentrations. [,,,]"

Source: Frontiers in Marine Science
Authors: Céline C. Michiels et al.
DOI: 10.3389/fmars.2019.00027

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High denitrification and anaerobic ammonium oxidation contributes to net nitrogen loss in a seagrass ecosystem in the central Red Sea

Abstract.

"Nitrogen loads in coastal areas have increased dramatically, with detrimental consequences for coastal ecosystems. Shallow sediments and seagrass meadows are hotspots for denitrification, favoring N loss. However, atmospheric dinitrogen (N2) fixation has been reported to support seagrass growth. Therefore, the role of coastal marine systems dominated by seagrasses in the net N2 flux remains unclear. Here, we measured denitrification, anaerobic ammonium oxidation (anammox), and N2 fixation in a tropical seagrass (Enhalus acoroides) meadow and the adjacent bare sediment in a coastal lagoon in the central Red Sea. [...]"

Source: Biogeosciences
Authors: Neus Garcias-Bonet et al.
DOI: 10.5194/bg-15-7333-2018

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Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Abstract.

"The availability of the micronutrient iron (Fe) in surface waters determines primary production, N2 fixation and microbial community structure in large parts of the world's ocean, and thus plays an important role in ocean carbon and nitrogen cycles. Eastern boundary upwelling systems and the connected oxygen minimum zones (OMZs) are typically associated with elevated concentrations of redox-sensitive trace metals (e.g. Fe, manganese (Mn) and cobalt (Co)), with shelf sediments typically forming a key source. Over the last five decades, an expansion and intensification of OMZs has been observed and this trend is likely to proceed. [...]"

Source: Biogeosciences
Authors: Insa Rapp et al.
DOI: 10.5194/bg-2018-472

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Dissolved Organic Matter Influences N2 Fixation in the New Caledonian Lagoon (Western Tropical South Pacific)

Abstract.

"Specialized prokaryotes performing biological dinitrogen (N2) fixation (“diazotrophs”) provide an important source of fixed nitrogen in oligotrophic marine ecosystems such as tropical and subtropical oceans. In these waters, cyanobacterial photosynthetic diazotrophs are well known to be abundant and active, yet the role and contribution of non-cyanobacterial diazotrophs are currently unclear. The latter are not photosynthetic (here called “heterotrophic”) and hence require external sources of organic matter to sustain N2 fixation.  [...]"

Source: Frontiers in Marine Science
Authors: Mar Benavides et al.
DOI: 10.3389/fmars.2018.00089

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