Transfer of diazotrophic nitrogen into the pelagic food web: the role of essential and non-essential amino acids
What is the quality of nitrogen from N2-fixation by cyanobacteria (diazotroph nitrogen) for the marine ecosystem once it has been transformed into bioavailable molecules like amino acids? The annual input of nitrogen by nitrogen fixation has been estimated to be 110 Tg for the global ocean (Gruber and Sarmiento 1997, Capone 2001). Depending on the pathway and bioavailable form by which diazotroph nitrogen enters the food web, it may facilitate autotrophic or hetreotrophic growth and thus cause a carbon export from the euphotic zone, or bacterial production and respiration in the euphotic zone (Fig. 1, Mulholland 2007). A compound specific isotope analysis (CSIA) allows to track the qualitative and quantitative „end-to-end“ transfer of 15N marked N2 into the amino acids of cyanobacteria and further into the essential and non-essential amino acids of their grazers by gaschromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS).
Fig. 1: Pathways of diazotrophic nitrogen into the heterotrophic food web. In the FONIP Project we investigate the primary uptake of diazotroph nitrogen by heterotrophs like copepods.
Besides field investigations we do controlled laboratory experiments with ecologically relevant model-organisms like the cyanobacterium Nodularia spumigena and its grazer the copepod Eurytemora affinis. For the first time we can quantify the transfer of newly formed amino acids from nitrogen fixation from the cyanobacteria into their grazers as well as detect the quality of the diazotrophic nitrogen for the zooplankton. The identification of the quality of diazotroph nitrogen as well as the quantification of its pathways into the marine ecosystem are fundamental prerequisites to predict the effects of increased or decreased diazotrophy in the context to global change like ocean acidification.
Capone, D. G., 2001. Current Opinion in Microbiology 4 (3): 341 – 348.
Gruber, N. and Sarmiento, J. L., 1997. Global Biogeochemical Cycles 11 (2): 235 – 266.
Mulholland, M. R., 2007. Biogeosciences, 4, 37–51.
NiFiM - Stickstofffixierung in der monsunbeeinflussten Flussfahne des Mekong
(2016 - 2018)
The South China Sea (SCS) is the largest marginal sea in the world, surrounded by densley populated coastal states like China, Indonesia, Philippines or Vietnam. Climate change and strong anthropogenic impacts (18 major damming for hydropower are planned, aquaculture is increasing) in the catchment area of River Mekong will change riverine loads with the consequence of changes in near coastal biogeochemistry. The speed and magnitude of anthropogenic changes in the Mekong basin make it very likely that the proposed field program will be one of the few opportunities to study the SCS before significant changes in nutrient and organic matter loading occur as a result of anthropogenic impact in the river basin. The present role of the Mekong for the productivity of the SCS in comparison to upwelling nutrients will be studied. Previous results from us have suggested a strong role of diazotroph-diatom associations (DDA) in the river plume even in the presence of high nitrate concentrations, but also unicellular and colony forming cyanobacteria like Trichodesmium are present. Nutrient concentrations and nutrient ratios have been suggested to play a major role for the abundance and activity of the nitrogen fixing species and will be a focus of this study. Nutrient concentrations will be measured and the uptake of nitrogen and carbon will be quantified in field samples and specific incubation experiments. Bulk fixation rates of various N-fixing groups can be determined with isotope ratio mass spectrometry and on a cellular basis studied with NanoSIMS technology. Moreover, US and Vietnamese scientists will contribute important information on species composition and metabolic activity of nitrogen fixing communities. The outcome of this project will provide a solid understanding of the role of the Mekong river plume and upwelling processes on the N-fixing organisms in the coastal SCS. Future changes in river biogeochemistry can thus be evaluated on the basis of these results.
Two cruises to the SCS are financed by an approved proposal of Schmidt’s Oceanographic Institute. Field sampling and experimental work on board is thus secured. Moreover, previous DFG funding established close collaboration with the Institute of Oceanography in Nha Trang so that the proposed work builds upon an established Vietnamese-German collaboration.
Follow 3 members of our working group on a cruise to the South China Sea
A Changing River: Measuring Nutrient fluxes to the South China Sea - LEG1 (03.06.-17.06.2016)
Contakt: Dr. Maren Voß
N sinks Baltic
"Quantification of Nitrogen sinks of the Baltic Sea and dependence on various environmental parameters" (N sinks Baltic), DFG funded (2007-2009)
The Baltic Sea is the largest brackish water body of the world and because of its enclosed
position and the hydrographical conditions very sensible to nutrient inputs. Whereas the
main sources of nitrogen are well known (nitrogen fixation and atmospheric deposition for the
central Baltic; direct and riverine inputs for the coastal areas) there is still a lack of
knowledge about the losses of reactive nitrogen via processes like denitrification and
anaerobic ammonium oxidation (anammox; fig. 1). The microbial processes in the water
column and the sediments are manifold and still less understood.
The DFG funded project "N sinks Baltic" deals with the quantification of these N losses. By means of the isotope pairing method the rates of denitrification and anammox are measured in different sediment types (mud, silt, sand) and the water column. The investigations are carried out at different seasons. Additional determinations of nutrient and oxygen profiles in the water column and the sediment as well as other sediment parameters should provide further information about the controlling factors.
By means of sediment maps the measured rates should be extrapolated to the whole area of the Baltic Sea.
Surface Ocean Processes in the Anthropocene
Surface Ocean Processes in the Anthropocene , BMBF funded (2007 - 2010)
Theme 2: Effect of Anthropocene CO2 levels on marine ecosystems and sea-to-air gas fluxes The atmosphere's composition determines the Earth's climate and habitability. This composition is, in turn, strongly determined by biological, physical and chemical processes occurring within the oceans. Increasingly, mankind is altering the composition of the atmosphere and therefore altering the interplay between the physical climate system and biogeochemistry. Such changes impact, in turn, the atmospheric composition, ocean ecosystems and climate via feedback processes. The SOPRAN (Surface Ocean Processes in the ANthropocene) project addresses three aspects of this interaction:
- How changing atmospheric composition (e.g. increased CO2, dust) affects the surface ocean ecosystem ?
- How climate-related changes in surface ocean processes (upwelling, mixing, light, biology) alter oceanic emissions to the atmosphere ?
- The mechanisms and rates of ocean-atmosphere material exchanges.
SOPRAN's focus is on processes operating within and close to the surface ocean, and their potential changes over the next century. The project is an integrated study of surface ocean response to global change, combining the insight of marine and atmospheric chemists, biological and physical oceanographers, as well as modelling on a range of scales. Shared activities include technological development and use of new floating mesocosms for studying global change effects on marine ecosystems, extension of physical modelling capabilities to study key biological and chemical processes in the surface layer, and the use of a new climate and biogeochemistry observatory in the Cape Verde Islands (in collaboration with UK, EU and Cape Verde investigators). The programme will deliver a better understanding of the role of the ocean in the climate system and an improved description of the effects of global change on the sensitive marine ecosystem. SOPRAN will furthermore:
The project involves 42 investigators from 12 partner institutions from all over Germany working in 23 sub-projects. The sub-projects are allocated to 4 inter-related Themes:
- the oceanic response to atmospheric dust
- effect of high CO2 on marine ecosystems and sea-toair gas fluxes
- production and emission of radiatively and chemically active gases in the Tropical Oceans
- inter-phase transfer at the sea surface.
These Themes are further interlinked by "Over-Arching Activities".Field work is concentrated in the Baltic Sea and the Eastern Tropical North Atlantic.
TRACES Ocean- Atmosphere- Land Impacts On Tropical Atlantic Ecosystems
WGL-Network: "TRACES Ocean- Atmosphere- Land Impacts On Tropical Atlantic Ecosystems", funded by the Leibniz Institutes (2006-2008)
TRACES is a joint research project investigating interactions within the ecosystem Atlantic
Ocean (an area that reacts critically towards global changes). Special focus is put on the
exchange of matter (in particular carbon an nitrogen fluxes) between land, ocean and atmosphere
within the tropical Atlantic Ocean. This region is of special importance for the global nitrogen
balance, because here the commonly occurring nitrogen fixation is fuelled by the input of its
limiting nutrients iron and phosphorous coming from terrestrial sources (dust and riverine input).
Aspects of the Tropical Atlantic Ocean network could focus on, or promote an exchange of ideas:
- theme 1: Carbon fluxes in Amazonia. Impact of climate variability and terestrial CO2 - emission on the carbon budget.
- theme 2: Transport and conversion processes in the atmosphere of the tropical Atlantic Ocean. Dust transport and climate Photochemistry of tropospheric aerosols and the barrier of the ozon layer
- theme 3: Abiotic influences on the ecosystem of the Atlantic Ocean.
Control of nitrogen fixation by external input of trace metals and macro nutrients
Transfer of recently fixed nitrogen within the food web
Pelagic processes and nitrogen cycle in coastal waters off southern central
Vietnam: mesocosm experiments, field work and modelling" , DFG funded (2006
The DFG - project "Pelagic processes and biogeochemical fluxes in the South China Sea (SCS) off southern central Vietnam" is part of a German - Vietnamese Cooperation in Marine Science. The focus of our work is on the pelagic nitrogen cycle in the upwelling area off the Vietnamese coast. Seasonal changes in the relative importance of different N-nutrient sources for primary production (upwelling, input by Mekong-river, and biological fixation of atmospheric N2) and their impact on pelagic system dynamics are investigated. Since 2003 planktological and biogeochemical investigations were carried out jointly with our partner Institute of Oceanography Nha Trang (ION), the work of which is funded by Ministry of Science and Technology (MOST), Hanoi. We also work in close collaboration with the physical-oceanographic projects of University Hamburg (IfM) and of ION which elucidated hydrodynamics as the prime forcing of nutrient supply in our joint investigation area.
With a modified approach the project will be continued until 2008. Whereas field
sampling on expeditions with large research vessels characterized the first
project phase, emphasis is now on land-based mesocosm experiments in which
upwelling and Mekong inflow will be simulated by mixing water from the different
pelagic systems (Mekong plume, upwelling water, oligotrophic surface water). Our
goal is to better understand pelagic production regimes typically encountered
seasonally and regionally, regarding the contributions by different plankton
(nitrogen fixing organisms and other functional groups), their response to
upwelling and admixture of river water, and the impact of this on the overall
nitrogen- and carbon cycles in the SCS. The experiments provide the opportunity
to closely follow trends in primary production, uptake of different nitrogen
species (NO3-, NH4+, N2), and succession as the plankton community responds to
changing nutrient conditions. The mesocosm experiments are combined with
ecosystem modelling of key processes in the N-cycle, results of which will be
tested against field results and can be implemented in circulation models. Field
studies using small ships in the Mekong river plume and the upwelling region
will document in situ conditions of pelagic systems which provide the source
material for the experiments and further link our work to partner
We start the second phase of the project with the following hypotheses:
- The nutrient concentration and N/P ratios in the upwelling water drive the production and the species selection in the upwelling region off Vietnam.
- Mekong and other River water may act as additional driver of the production and of species selection through nutrient, silica, and DOM input
- A nitrogen cycle model can constrain the driving factors of productivity and species selection.