My general interest is to understand what drives marine ecosystem, biogeochemical cycles and climate to interact. I look in particular into the cycling of nitrogen, phosphorus, iron, oxygen and carbon using mathematical and numerical methods in close comparison with observations. I am currently involved with projects related to:- Coccolithophore and foraminifera ecology- Oceanic Anoxic Events of the Mesozoic using an Earth system model (GENIE)- Marine nitrogen cycle (nitrogen fixation, nitrification and denitrification)- Marine ecosystem and plankton diversity using an adaptive ecosystem model (MITgcm) Supervisors: Mick Follows (Ph.D. supervisor) Address: School of Geographical Sciences University Road Bristol, BS8 1SS UK
We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitro... more We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitrogen fixers) in the oceans. In the model, the structure of the marine phytoplankton community self‐assembles from a large number of potentially viable physiologies. Amongst them, analogs of Trichodesmium, unicellular diazotrophs and diatom‐diazotroph associations (DDA) are successful and abundant. The simulated biogeography and nitrogen fixation rates of the modeled diazotrophs compare favorably with a compilation of published observations, which includes both traditional and molecular measurements of abundance and activity of marine diazotrophs. In the model, the diazotroph analogs occupy warm subtropical and tropical waters, with higher concentrations and nitrogen fixation rates in the tropical Atlantic Ocean and the Arabian Sea/Northern Indian Ocean, and lower values in the tropical and subtropical South Pacific Ocean. The three main diazotroph types typically co‐exist in the model, al...
Coccolithophores are globally important marine calcifying phytoplankton. They contribute to the o... more Coccolithophores are globally important marine calcifying phytoplankton. They contribute to the organic carbon pump through the primary production and the ballast of organic matter, and to the carbonate pump through the production of calcium carbonate. Here we compiled all available scanning electron microscopy (SEM) coccolithophore abundance observations. Taxa were standardized following NannoTax3 to a species level where possible. Subspecies (e.a. C. leptoporus subsp. leptoporus and C. leptoporus subsp. quadriperforatus) were grouped as single species. The database contains 2556 abundance observations from 35 different publications. The data span the period of 1993-2017, with observations from all ocean basins and all seasons, and at depths ranging from the surface to 5000 m. We limited our compilation to SEM observations (or observations which further identified samples with SEM) because SEM provides greater detail of coccolithophore diversity than more commonly used polarized li...
Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and ir... more Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and iron cycles have important implications for the control of primary production and carbon storage in the ocean. The biological process of nitrogen fixation is thought to be particularly ...
Calcifying marine phytoplankton-coccolithophores- are some of the most successful yet enigmatic o... more Calcifying marine phytoplankton-coccolithophores- are some of the most successful yet enigmatic organisms in the ocean and are at risk from global change. To better understand how they will be affected, we need to know "why" coccolithophores calcify. We review coccolithophorid evolutionary history and cell biology as well as insights from recent experiments to provide a critical assessment of the costs and benefits of calcification. We conclude that calcification has high energy demands and that coccolithophores might have calcified initially to reduce grazing pressure but that additional benefits such as protection from photodamage and viral/bacterial attack further explain their high diversity and broad spectrum ecology. The cost-benefit aspect of these traits is illustrated by novel ecosystem modeling, although conclusive observations remain limited. In the future ocean, the trade-off between changing ecological and physiological costs of calcification and their benefit...
Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability o... more Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability of iron in surface waters, which is thought to exert a significant control upon atmospheric CO2 concentrations on glacial/interglacial timescales. The zone bordering the Antarctic Ice Sheet exhibits high PP and seasonal plankton blooms in response to light and variations in iron availability. The sources of iron stimulating elevated SO PP are in debate. Established contributors include dust, coastal sediments/upwelling, icebergs and sea ice. Subglacial meltwater exported at the ice margin is a more recent suggestion, arising from intense iron cycling beneath the ice sheet. Icebergs and subglacial meltwater may supply a large amount of bioavailable iron to the SO, estimated in this study at 0.07–0.2 Tg yr−1. Here we apply the MIT global ocean model (Follows et al., 2007) to determine the potential impact of this level of iron export from the ice sheet upon SO PP. The export of iron from the ice sheet raises modelled SO PP by up to 40%, and provides one plausible explanation for seasonally very high in situ measurements of PP in the near-coastal zone. The impact on SO PP is greatest in coastal regions, which are also areas of high measured marine PP. These results suggest that the export of Antarctic runoff and icebergs may have an important impact on SO PP and should be included in future biogeochemical modelling.
Once thought to be devoid of life, the Antarctic Ice Sheet is now known to be a dynamic reservoir... more Once thought to be devoid of life, the Antarctic Ice Sheet is now known to be a dynamic reservoir of organic carbon and metabolically active microbial cells. At the ice-bed interface, subglacial lake and sedimentary environments support low diversity microbial populations, adapted to perennial cold, anoxia and lack of light. The dynamic exchange of water between these shallow environments conveys meltwaters and associated sediments into the coastal ocean. This, together with the release of iceberg-rafted debris to more distal coastal environments, could be important for sustaining primary productivity in the iron-limited Southern Ocean, via the release of associated nutrients and bioavailable iron. We estimate the magnitude and review the wider impacts of the potential export of nutrients (N, P, C, Si and bioavailable Fe) dissolved and associated with suspended sediments in Antarctic runoff and entombed in iceberg rafted debris. Located beneath subglacial lakes and the subglacial till complex are deep sedimentary basins up to 14 km thick, located largely around the Antarctic periphery. These sedimentary basins are largely hydrologically decoupled from shallower lake and till environments by the presence of highly consolidated sediments which limit the penetration of glacial meltwaters to depth. They provide extensive habitats for sustained microbial activity over Ma timescales, and are likely to be a focal point for the anaerobic cycling of organic carbon and other elements in the deep sub-surface. Organic carbon buried in these basins during ice sheet formation is thought to be microbially cycled to methane gas, and the methane largely stored as hydrate within sediments, stabilised by the high pressure/low temperature conditions. We conclude that the export of nutrients and biogenic gases from deep and shallow subglacial Antarctic environments designates Antarctica as a potentially important component of the Earth's carbon cycle, and highlight the importance of evaluating these potential impacts further via global and regional-scale biogeochemical modelling.
The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by large perturbations in th... more The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by large perturbations in the oxygen and sulfur cycles of the ocean, potentially resulting from changes in oxygen supply (via oxygen solubility and ocean circulation) and in marine productivity. We assess the relative impact of these mechanisms, comparing model experiments with a new compilation of observations for seafloor dysoxia/anoxia and photic-zone euxinia. The model employed is an intermediate-complexity Earth system model which accounts for the main ocean dynamics and biogeochemistry of the Cretaceous climate. The impact of higher temperature and marine productivity is evaluated in the model as a result of higher atmospheric carbon dioxide and oceanic nutrient concentrations. The model shows that temperature is not alone able to reproduce the observed patterns of oceanic redox changes associated with OAE2. Observations are reproduced in the model mainly via enhanced marine productivity due to higher nutrient content (responsible for 85% of the change). Higher phosphate content could have been sustained by increased chemical weathering and phosphorus regeneration from anoxic sediments, which in turn induced an enhanced nitrogen nutrient content of the ocean via nitrogen fixation. The model also shows that the presence of seafloor anoxia, as suggested by black-shale deposition in the proto-North Atlantic Ocean before the event, might be the result of the silled shape and lack of deep-water formation of this basin at the Late Cretaceous. Overall our model-data comparison shows that OAE2 anoxia was quasi-global spreading from 5% of the ocean volume before the event to at least 50% during OAE2.
Regional and global nitrogen fixation rates are often estimated from geochemical tracers related ... more Regional and global nitrogen fixation rates are often estimated from geochemical tracers related to N* (= NO3 − 16PO4). However the patterns of this tracer reflect the influence of numerous processes including nitrogen fixation, denitrification, remineralization of organic matter, variable stoichiometry, atmospheric deposition and physical transport. Here we have used idealized models to illustrate how preferential remineralization of organic phosphorous may explain observed features of N* distribution in the North Atlantic Ocean, including a subsurface maximum and an increased temporal variability in the mid-thermocline. If preferential remineralization of phosphorus is key in shaping the oceanic distribution of N*, published estimates of nitrogen fixation may be underestimating the marine nitrogen fixation rate by as much as a factor of three.
We examine the interplay between iron supply, iron concentrations and phytoplankton communities i... more We examine the interplay between iron supply, iron concentrations and phytoplankton communities in the Pacific Ocean. We present a theoretical framework which considers the competition for iron and nitrogen resources between phytoplankton to explain where nitrogen fixing autotrophs (diazotrophs, which require higher iron quotas, and have slower maximum growth) can co-exist with other phytoplankton. The framework also indicates that iron and fixed nitrogen concentrations can be strongly controlled by the local phytoplankton community. Together with results from a three-dimensional numerical model, we characterize three distinct biogeochemical provinces: 1) where iron supply is very low diazotrophs are excluded, and iron-limited nondiazotrophic phytoplankton control the iron concentrations; 2) a transition region where nondiazotrophic phytoplankton are nitrogen limited and control the nitrogen concentrations, but the iron supply is still too low relative to nitrate to support diazotrophy; 3) where iron supplies increase further relative to the nitrogen source, diazotrophs and other phytoplankton coexist; nitrogen concentrations are controlled by nondiazotrophs and iron concentrations are controlled by diazotrophs. The boundaries of these three provinces are defined by the rate of supply of iron relative to the supply of fixed nitrogen. The numerical model and theory provide a useful tool to understand the state of, links between, and response to changes in iron supply and phytoplankton community structure that have been suggested by observations.
We interpret the environmental controls on the global ocean diazotroph biogeography in the contex... more We interpret the environmental controls on the global ocean diazotroph biogeography in the context of a three-dimensional global model with a self-organizing phytoplankton community. As is observed, the model’s total diazotroph population is distributed over most of the oligotrophic warm sub-and-tropical waters, with the exception of the South eastern Pacific Ocean. This biogeography broadly follows temperature and light constraints which are often used in both field-based and model studies to explain the distribution of diazotrophs. However the model suggests that diazotroph habitat is not directly controlled by temperature and light, but is restricted to the ocean regions with low fixed nitrogen and sufficient dissolved iron and phosphate concentrations. We interpret this regulation by iron and phosphate using resource competition theory which provides an excellent qualitative and quantitative framework.
The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open oc... more The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open ocean ecosystems and are predicted to be limited by iron in most marine environments. Here we use global and targeted proteomic analyses on a key unicellular marine diazotroph Crocosphaera watsonii to reveal large scale diel changes in its proteome, including substantial variations in concentrations of iron metalloproteins involved in nitrogen fixation and photosynthesis, as well as nocturnal flavodoxin production. The daily synthesis and degradation of enzymes in coordination with their utilization results in a lowered cellular metalloenzyme inventory that requires ∼40% less iron than if these enzymes were maintained throughout the diel cycle. This strategy is energetically expensive, but appears to serve as an important adaptation for confronting the iron scarcity of the open oceans. A global numerical model of ocean circulation, biogeochemistry and ecosystems suggests that Crocosphaera’s ability to reduce its iron-metalloenzyme inventory provides two advantages: It allows Crocosphaera to inhabit regions lower in iron and allows the same iron supply to support higher Crocosphaera biomass and nitrogen fixation than if they did not have this reduced iron requirement.
We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitro... more We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitrogen fixers) in the oceans. In the model, the structure of the marine phytoplankton community self‐assembles from a large number of potentially viable physiologies. Amongst them, analogs of Trichodesmium, unicellular diazotrophs and diatom‐diazotroph associations (DDA) are successful and abundant. The simulated biogeography and nitrogen fixation rates of the modeled diazotrophs compare favorably with a compilation of published observations, which includes both traditional and molecular measurements of abundance and activity of marine diazotrophs. In the model, the diazotroph analogs occupy warm subtropical and tropical waters, with higher concentrations and nitrogen fixation rates in the tropical Atlantic Ocean and the Arabian Sea/Northern Indian Ocean, and lower values in the tropical and subtropical South Pacific Ocean. The three main diazotroph types typically co‐exist in the model, although Trichodesmium analogs dominate the diazotroph population in much of the North and tropical Atlantic Ocean and the Arabian Sea, while unicellular‐diazotroph analogs dominate in the South Atlantic, Pacific and Indian oceans. This pattern reflects the relative degree of nutrient limitation by iron or phosphorus. The model suggests in addition that unicellular diazotrophs could add as much new nitrogen to the global ocean as Trichodesmium.
Time-series observations of geochemical tracers and diazotroph abundances in the northern subtrop... more Time-series observations of geochemical tracers and diazotroph abundances in the northern subtropical gyres suggest variability in nitrogen fixation on interannual and longer timescales. Using a highly idealized model of the biogeochemistry and ecology of a subtropical gyre, we explore the previously proposed hypothesis that such variability is regulated by an internal “biogeochemical oscillator.” We find, in certain parameter regimes, self-sustained oscillations in nitrogen fixation, community structure and biogeochemical cycles even with perfectly steady physical forcing. During the oscillations of nitrogen fixation, “blooms” of diazotrophs occur at intervals between a year and several decades, consistent with the observed variability. The period of the oscillations is strongly regulated by the exchange rate between the thermocline and mixed-layer waters. The oscillatory solutions occur in a relatively small region of parameter space, but one in which the relative fitness of diazotrophs and non-diazotrophs are closely matched and the time-averaged biomass of each class of phytoplankton is maximized.
Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and ir... more Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and iron cycles have important implications for the control of primary production and carbon storage in the ocean. The biological process of nitrogen fixation is thought to be particularly important where the ocean is nitrogen limited and oligotrophic. This thesis examines some of the mechanisms responsible for the distribu- tion, rates and temporal variability of nitrogen fixation and its geochemical signature in the modern ocean. I employ simple analytical theories and numerical models of ecosystems and biogeochemical cycles, and closely refer to direct observations of the phytoplanktonic community and geochemical tracers of the marine nitrogen cycle. Time-series observations of geochemical tracers and abundances of nitrogen fixers (or diazotrophs) in the northern subtropical gyres suggest variability in nitrogen fixation on interannual and longer timescales. I use a highly idealized, two-layer model of the nitrogen and phosphorus biogeochemistry and ecology of a subtropical gyre to explore the previously proposed hypothesis that such variability is regulated by an internal biogeochemical oscillator. I find, in certain parameter regimes, self- sustained oscillations in nitrogen fixation, community structure and biogeochemical cycles even with perfectly steady physical forcing. The period of the oscillations is strongly regulated by the exchange rate between the thermocline and mixed-layer waters, suggesting a period of several years to several decades for the North Pacific subtropical gyre regime, but would likely be shorter (only a year or so) for the North Atlantic Ocean. Geochemical tracers such as DINxs (=NO3−16PO3) measure the oceanic departure from the Redfield ratio. DINxs is often used to estimate the rate of nitrogen fixation in the ocean, by quantifying the tracer accumulation along isopycnals. However this tracer reflects an interwoven set of processes including nitrogen fixation, but also denitrification, atmospheric and riverine sources, differential remineralization and complex transport pathways. I examine analytical solutions of the prognostic equation of DINxs and an idealized three-dimensional model of the basin-scale circulation, biogeochemical cycles and ecology of the North Atlantic Ocean. The two approaches demonstrate that the observations of a subsurface maximum in the North Atlantic Ocean and the temporal variability at the station BATS of DINxs can be explained simply by preferential remineralization of organic phosphorus relative to nitrogen. A further analysis reveals that the current geochemical estimates based on inorganic forms of phosphorus and nitrogen underestimate integrated nitrogen fixation rates by a factor of two to six, by neglecting the preferential remineralization effect. Most current understanding of oceanic nitrogen fixation is based on the Trichodesmium, though unicellular cyanobacteria, diatom-diazotroph associations (DDA) and heterotrophic bacteria might be as important in adding nitrogen into the ocean. I employ a self-assembling global ocean ecosystem model to simulate diverse phytoplanktonic diazotrophs in the global ocean and examine how temperature, oligotrophy, iron and phosphate limitations influence the global marine diazotroph distribution. Analogs of Trichodesmium, unicellular diazotrophs and DDA are successful in the model, showing very similar distributions with observations. The total diazotrophic population is distributed over most of the oligotrophic warm (sub)tropical waters in the model. The model demonstrates that temperature is not the primary control, but suggests instead that diazotroph biogeography is restricted to the low fixed nitrogen oceanic regions which have sufficient dissolved iron and phosphate. The theory of resource competition is used to map out regions of iron and phosphate regulation of diazotroph distribution. The theory suggests that diazotrophs are largely regulated by iron availability, in particular in the Pacific and Indian Oceans. The iron cycle is currently not well enough constrained to confidently predict the diazotroph distribution in global ocean models.
We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitro... more We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitrogen fixers) in the oceans. In the model, the structure of the marine phytoplankton community self‐assembles from a large number of potentially viable physiologies. Amongst them, analogs of Trichodesmium, unicellular diazotrophs and diatom‐diazotroph associations (DDA) are successful and abundant. The simulated biogeography and nitrogen fixation rates of the modeled diazotrophs compare favorably with a compilation of published observations, which includes both traditional and molecular measurements of abundance and activity of marine diazotrophs. In the model, the diazotroph analogs occupy warm subtropical and tropical waters, with higher concentrations and nitrogen fixation rates in the tropical Atlantic Ocean and the Arabian Sea/Northern Indian Ocean, and lower values in the tropical and subtropical South Pacific Ocean. The three main diazotroph types typically co‐exist in the model, al...
Coccolithophores are globally important marine calcifying phytoplankton. They contribute to the o... more Coccolithophores are globally important marine calcifying phytoplankton. They contribute to the organic carbon pump through the primary production and the ballast of organic matter, and to the carbonate pump through the production of calcium carbonate. Here we compiled all available scanning electron microscopy (SEM) coccolithophore abundance observations. Taxa were standardized following NannoTax3 to a species level where possible. Subspecies (e.a. C. leptoporus subsp. leptoporus and C. leptoporus subsp. quadriperforatus) were grouped as single species. The database contains 2556 abundance observations from 35 different publications. The data span the period of 1993-2017, with observations from all ocean basins and all seasons, and at depths ranging from the surface to 5000 m. We limited our compilation to SEM observations (or observations which further identified samples with SEM) because SEM provides greater detail of coccolithophore diversity than more commonly used polarized li...
Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and ir... more Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and iron cycles have important implications for the control of primary production and carbon storage in the ocean. The biological process of nitrogen fixation is thought to be particularly ...
Calcifying marine phytoplankton-coccolithophores- are some of the most successful yet enigmatic o... more Calcifying marine phytoplankton-coccolithophores- are some of the most successful yet enigmatic organisms in the ocean and are at risk from global change. To better understand how they will be affected, we need to know "why" coccolithophores calcify. We review coccolithophorid evolutionary history and cell biology as well as insights from recent experiments to provide a critical assessment of the costs and benefits of calcification. We conclude that calcification has high energy demands and that coccolithophores might have calcified initially to reduce grazing pressure but that additional benefits such as protection from photodamage and viral/bacterial attack further explain their high diversity and broad spectrum ecology. The cost-benefit aspect of these traits is illustrated by novel ecosystem modeling, although conclusive observations remain limited. In the future ocean, the trade-off between changing ecological and physiological costs of calcification and their benefit...
Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability o... more Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability of iron in surface waters, which is thought to exert a significant control upon atmospheric CO2 concentrations on glacial/interglacial timescales. The zone bordering the Antarctic Ice Sheet exhibits high PP and seasonal plankton blooms in response to light and variations in iron availability. The sources of iron stimulating elevated SO PP are in debate. Established contributors include dust, coastal sediments/upwelling, icebergs and sea ice. Subglacial meltwater exported at the ice margin is a more recent suggestion, arising from intense iron cycling beneath the ice sheet. Icebergs and subglacial meltwater may supply a large amount of bioavailable iron to the SO, estimated in this study at 0.07–0.2 Tg yr−1. Here we apply the MIT global ocean model (Follows et al., 2007) to determine the potential impact of this level of iron export from the ice sheet upon SO PP. The export of iron from the ice sheet raises modelled SO PP by up to 40%, and provides one plausible explanation for seasonally very high in situ measurements of PP in the near-coastal zone. The impact on SO PP is greatest in coastal regions, which are also areas of high measured marine PP. These results suggest that the export of Antarctic runoff and icebergs may have an important impact on SO PP and should be included in future biogeochemical modelling.
Once thought to be devoid of life, the Antarctic Ice Sheet is now known to be a dynamic reservoir... more Once thought to be devoid of life, the Antarctic Ice Sheet is now known to be a dynamic reservoir of organic carbon and metabolically active microbial cells. At the ice-bed interface, subglacial lake and sedimentary environments support low diversity microbial populations, adapted to perennial cold, anoxia and lack of light. The dynamic exchange of water between these shallow environments conveys meltwaters and associated sediments into the coastal ocean. This, together with the release of iceberg-rafted debris to more distal coastal environments, could be important for sustaining primary productivity in the iron-limited Southern Ocean, via the release of associated nutrients and bioavailable iron. We estimate the magnitude and review the wider impacts of the potential export of nutrients (N, P, C, Si and bioavailable Fe) dissolved and associated with suspended sediments in Antarctic runoff and entombed in iceberg rafted debris. Located beneath subglacial lakes and the subglacial till complex are deep sedimentary basins up to 14 km thick, located largely around the Antarctic periphery. These sedimentary basins are largely hydrologically decoupled from shallower lake and till environments by the presence of highly consolidated sediments which limit the penetration of glacial meltwaters to depth. They provide extensive habitats for sustained microbial activity over Ma timescales, and are likely to be a focal point for the anaerobic cycling of organic carbon and other elements in the deep sub-surface. Organic carbon buried in these basins during ice sheet formation is thought to be microbially cycled to methane gas, and the methane largely stored as hydrate within sediments, stabilised by the high pressure/low temperature conditions. We conclude that the export of nutrients and biogenic gases from deep and shallow subglacial Antarctic environments designates Antarctica as a potentially important component of the Earth's carbon cycle, and highlight the importance of evaluating these potential impacts further via global and regional-scale biogeochemical modelling.
The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by large perturbations in th... more The Cenomanian-Turonian oceanic anoxic event (OAE2) is characterized by large perturbations in the oxygen and sulfur cycles of the ocean, potentially resulting from changes in oxygen supply (via oxygen solubility and ocean circulation) and in marine productivity. We assess the relative impact of these mechanisms, comparing model experiments with a new compilation of observations for seafloor dysoxia/anoxia and photic-zone euxinia. The model employed is an intermediate-complexity Earth system model which accounts for the main ocean dynamics and biogeochemistry of the Cretaceous climate. The impact of higher temperature and marine productivity is evaluated in the model as a result of higher atmospheric carbon dioxide and oceanic nutrient concentrations. The model shows that temperature is not alone able to reproduce the observed patterns of oceanic redox changes associated with OAE2. Observations are reproduced in the model mainly via enhanced marine productivity due to higher nutrient content (responsible for 85% of the change). Higher phosphate content could have been sustained by increased chemical weathering and phosphorus regeneration from anoxic sediments, which in turn induced an enhanced nitrogen nutrient content of the ocean via nitrogen fixation. The model also shows that the presence of seafloor anoxia, as suggested by black-shale deposition in the proto-North Atlantic Ocean before the event, might be the result of the silled shape and lack of deep-water formation of this basin at the Late Cretaceous. Overall our model-data comparison shows that OAE2 anoxia was quasi-global spreading from 5% of the ocean volume before the event to at least 50% during OAE2.
Regional and global nitrogen fixation rates are often estimated from geochemical tracers related ... more Regional and global nitrogen fixation rates are often estimated from geochemical tracers related to N* (= NO3 − 16PO4). However the patterns of this tracer reflect the influence of numerous processes including nitrogen fixation, denitrification, remineralization of organic matter, variable stoichiometry, atmospheric deposition and physical transport. Here we have used idealized models to illustrate how preferential remineralization of organic phosphorous may explain observed features of N* distribution in the North Atlantic Ocean, including a subsurface maximum and an increased temporal variability in the mid-thermocline. If preferential remineralization of phosphorus is key in shaping the oceanic distribution of N*, published estimates of nitrogen fixation may be underestimating the marine nitrogen fixation rate by as much as a factor of three.
We examine the interplay between iron supply, iron concentrations and phytoplankton communities i... more We examine the interplay between iron supply, iron concentrations and phytoplankton communities in the Pacific Ocean. We present a theoretical framework which considers the competition for iron and nitrogen resources between phytoplankton to explain where nitrogen fixing autotrophs (diazotrophs, which require higher iron quotas, and have slower maximum growth) can co-exist with other phytoplankton. The framework also indicates that iron and fixed nitrogen concentrations can be strongly controlled by the local phytoplankton community. Together with results from a three-dimensional numerical model, we characterize three distinct biogeochemical provinces: 1) where iron supply is very low diazotrophs are excluded, and iron-limited nondiazotrophic phytoplankton control the iron concentrations; 2) a transition region where nondiazotrophic phytoplankton are nitrogen limited and control the nitrogen concentrations, but the iron supply is still too low relative to nitrate to support diazotrophy; 3) where iron supplies increase further relative to the nitrogen source, diazotrophs and other phytoplankton coexist; nitrogen concentrations are controlled by nondiazotrophs and iron concentrations are controlled by diazotrophs. The boundaries of these three provinces are defined by the rate of supply of iron relative to the supply of fixed nitrogen. The numerical model and theory provide a useful tool to understand the state of, links between, and response to changes in iron supply and phytoplankton community structure that have been suggested by observations.
We interpret the environmental controls on the global ocean diazotroph biogeography in the contex... more We interpret the environmental controls on the global ocean diazotroph biogeography in the context of a three-dimensional global model with a self-organizing phytoplankton community. As is observed, the model’s total diazotroph population is distributed over most of the oligotrophic warm sub-and-tropical waters, with the exception of the South eastern Pacific Ocean. This biogeography broadly follows temperature and light constraints which are often used in both field-based and model studies to explain the distribution of diazotrophs. However the model suggests that diazotroph habitat is not directly controlled by temperature and light, but is restricted to the ocean regions with low fixed nitrogen and sufficient dissolved iron and phosphate concentrations. We interpret this regulation by iron and phosphate using resource competition theory which provides an excellent qualitative and quantitative framework.
The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open oc... more The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open ocean ecosystems and are predicted to be limited by iron in most marine environments. Here we use global and targeted proteomic analyses on a key unicellular marine diazotroph Crocosphaera watsonii to reveal large scale diel changes in its proteome, including substantial variations in concentrations of iron metalloproteins involved in nitrogen fixation and photosynthesis, as well as nocturnal flavodoxin production. The daily synthesis and degradation of enzymes in coordination with their utilization results in a lowered cellular metalloenzyme inventory that requires ∼40% less iron than if these enzymes were maintained throughout the diel cycle. This strategy is energetically expensive, but appears to serve as an important adaptation for confronting the iron scarcity of the open oceans. A global numerical model of ocean circulation, biogeochemistry and ecosystems suggests that Crocosphaera’s ability to reduce its iron-metalloenzyme inventory provides two advantages: It allows Crocosphaera to inhabit regions lower in iron and allows the same iron supply to support higher Crocosphaera biomass and nitrogen fixation than if they did not have this reduced iron requirement.
We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitro... more We employ a global three‐dimensional model to simulate diverse phytoplanktonic diazotrophs (nitrogen fixers) in the oceans. In the model, the structure of the marine phytoplankton community self‐assembles from a large number of potentially viable physiologies. Amongst them, analogs of Trichodesmium, unicellular diazotrophs and diatom‐diazotroph associations (DDA) are successful and abundant. The simulated biogeography and nitrogen fixation rates of the modeled diazotrophs compare favorably with a compilation of published observations, which includes both traditional and molecular measurements of abundance and activity of marine diazotrophs. In the model, the diazotroph analogs occupy warm subtropical and tropical waters, with higher concentrations and nitrogen fixation rates in the tropical Atlantic Ocean and the Arabian Sea/Northern Indian Ocean, and lower values in the tropical and subtropical South Pacific Ocean. The three main diazotroph types typically co‐exist in the model, although Trichodesmium analogs dominate the diazotroph population in much of the North and tropical Atlantic Ocean and the Arabian Sea, while unicellular‐diazotroph analogs dominate in the South Atlantic, Pacific and Indian oceans. This pattern reflects the relative degree of nutrient limitation by iron or phosphorus. The model suggests in addition that unicellular diazotrophs could add as much new nitrogen to the global ocean as Trichodesmium.
Time-series observations of geochemical tracers and diazotroph abundances in the northern subtrop... more Time-series observations of geochemical tracers and diazotroph abundances in the northern subtropical gyres suggest variability in nitrogen fixation on interannual and longer timescales. Using a highly idealized model of the biogeochemistry and ecology of a subtropical gyre, we explore the previously proposed hypothesis that such variability is regulated by an internal “biogeochemical oscillator.” We find, in certain parameter regimes, self-sustained oscillations in nitrogen fixation, community structure and biogeochemical cycles even with perfectly steady physical forcing. During the oscillations of nitrogen fixation, “blooms” of diazotrophs occur at intervals between a year and several decades, consistent with the observed variability. The period of the oscillations is strongly regulated by the exchange rate between the thermocline and mixed-layer waters. The oscillatory solutions occur in a relatively small region of parameter space, but one in which the relative fitness of diazotrophs and non-diazotrophs are closely matched and the time-averaged biomass of each class of phytoplankton is maximized.
Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and ir... more Oceanic nitrogen fixation and biogeochemical interactions between the nitrogen, phosphorus and iron cycles have important implications for the control of primary production and carbon storage in the ocean. The biological process of nitrogen fixation is thought to be particularly important where the ocean is nitrogen limited and oligotrophic. This thesis examines some of the mechanisms responsible for the distribu- tion, rates and temporal variability of nitrogen fixation and its geochemical signature in the modern ocean. I employ simple analytical theories and numerical models of ecosystems and biogeochemical cycles, and closely refer to direct observations of the phytoplanktonic community and geochemical tracers of the marine nitrogen cycle. Time-series observations of geochemical tracers and abundances of nitrogen fixers (or diazotrophs) in the northern subtropical gyres suggest variability in nitrogen fixation on interannual and longer timescales. I use a highly idealized, two-layer model of the nitrogen and phosphorus biogeochemistry and ecology of a subtropical gyre to explore the previously proposed hypothesis that such variability is regulated by an internal biogeochemical oscillator. I find, in certain parameter regimes, self- sustained oscillations in nitrogen fixation, community structure and biogeochemical cycles even with perfectly steady physical forcing. The period of the oscillations is strongly regulated by the exchange rate between the thermocline and mixed-layer waters, suggesting a period of several years to several decades for the North Pacific subtropical gyre regime, but would likely be shorter (only a year or so) for the North Atlantic Ocean. Geochemical tracers such as DINxs (=NO3−16PO3) measure the oceanic departure from the Redfield ratio. DINxs is often used to estimate the rate of nitrogen fixation in the ocean, by quantifying the tracer accumulation along isopycnals. However this tracer reflects an interwoven set of processes including nitrogen fixation, but also denitrification, atmospheric and riverine sources, differential remineralization and complex transport pathways. I examine analytical solutions of the prognostic equation of DINxs and an idealized three-dimensional model of the basin-scale circulation, biogeochemical cycles and ecology of the North Atlantic Ocean. The two approaches demonstrate that the observations of a subsurface maximum in the North Atlantic Ocean and the temporal variability at the station BATS of DINxs can be explained simply by preferential remineralization of organic phosphorus relative to nitrogen. A further analysis reveals that the current geochemical estimates based on inorganic forms of phosphorus and nitrogen underestimate integrated nitrogen fixation rates by a factor of two to six, by neglecting the preferential remineralization effect. Most current understanding of oceanic nitrogen fixation is based on the Trichodesmium, though unicellular cyanobacteria, diatom-diazotroph associations (DDA) and heterotrophic bacteria might be as important in adding nitrogen into the ocean. I employ a self-assembling global ocean ecosystem model to simulate diverse phytoplanktonic diazotrophs in the global ocean and examine how temperature, oligotrophy, iron and phosphate limitations influence the global marine diazotroph distribution. Analogs of Trichodesmium, unicellular diazotrophs and DDA are successful in the model, showing very similar distributions with observations. The total diazotrophic population is distributed over most of the oligotrophic warm (sub)tropical waters in the model. The model demonstrates that temperature is not the primary control, but suggests instead that diazotroph biogeography is restricted to the low fixed nitrogen oceanic regions which have sufficient dissolved iron and phosphate. The theory of resource competition is used to map out regions of iron and phosphate regulation of diazotroph distribution. The theory suggests that diazotrophs are largely regulated by iron availability, in particular in the Pacific and Indian Oceans. The iron cycle is currently not well enough constrained to confidently predict the diazotroph distribution in global ocean models.
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Papers by Fanny Monteiro
Most current understanding of oceanic nitrogen fixation is based on the Trichodesmium, though unicellular cyanobacteria, diatom-diazotroph associations (DDA) and heterotrophic bacteria might be as important in adding nitrogen into the ocean. I employ a self-assembling global ocean ecosystem model to simulate diverse phytoplanktonic diazotrophs in the global ocean and examine how temperature, oligotrophy, iron and phosphate limitations influence the global marine diazotroph distribution. Analogs of Trichodesmium, unicellular diazotrophs and DDA are successful in the model, showing very similar distributions with observations. The total diazotrophic population is distributed over most of the oligotrophic warm (sub)tropical waters in the model. The model demonstrates that temperature is not the primary control, but suggests instead that diazotroph biogeography is restricted to the low fixed nitrogen oceanic regions which have sufficient dissolved iron and phosphate. The theory of resource competition is used to map out regions of iron and phosphate regulation of diazotroph distribution. The theory suggests that diazotrophs are largely regulated by iron availability, in particular in the Pacific and Indian Oceans. The iron cycle is currently not well enough constrained to confidently predict the diazotroph distribution in global ocean models.
Most current understanding of oceanic nitrogen fixation is based on the Trichodesmium, though unicellular cyanobacteria, diatom-diazotroph associations (DDA) and heterotrophic bacteria might be as important in adding nitrogen into the ocean. I employ a self-assembling global ocean ecosystem model to simulate diverse phytoplanktonic diazotrophs in the global ocean and examine how temperature, oligotrophy, iron and phosphate limitations influence the global marine diazotroph distribution. Analogs of Trichodesmium, unicellular diazotrophs and DDA are successful in the model, showing very similar distributions with observations. The total diazotrophic population is distributed over most of the oligotrophic warm (sub)tropical waters in the model. The model demonstrates that temperature is not the primary control, but suggests instead that diazotroph biogeography is restricted to the low fixed nitrogen oceanic regions which have sufficient dissolved iron and phosphate. The theory of resource competition is used to map out regions of iron and phosphate regulation of diazotroph distribution. The theory suggests that diazotrophs are largely regulated by iron availability, in particular in the Pacific and Indian Oceans. The iron cycle is currently not well enough constrained to confidently predict the diazotroph distribution in global ocean models.