Abstract Aims In recent years, gut microbiota have gained a growing interest as an environmental factor that may affect the predisposition toward adiposity. In this review, we describe and discuss ...the research that has focused on the involvement of gut microbiota in human obesity. We also summarize the current knowledge concerning the health effects of the composition of gut microbiota, acquired using the most recent methodological approaches, and the potential influence of gut microbiota on adiposity, as revealed by animal studies. Data synthesis Original research studies that were published in English or French until December 2011 were selected through a computer-assisted literature search. The studies conducted to date show that there are differences in the gut microbiota between obese and normal-weight experimental animals. There is also evidence that a high-fat diet may induce changes in gut microbiota in animal models regardless of the presence of obesity. In humans, obesity has been associated with reduced bacterial diversity and an altered representation of bacterial species, but the identified differences are not homogeneous among the studies. Conclusions The question remains as to whether changes in the intestinal microbial community are one of the environmental causes of overweight and obesity or if they are a consequence of obesity, specifically of the unbalanced diet that often accompanies the development of excess weight gain. In the future, larger studies on the potential role of intestinal microbiota in human obesity should be conducted at the species level using standardized analytical techniques and taking all of the possible confounding variables into account.
Ongoing and future changes in wind and temperature are predicted to alter upper ocean vertical mixing across the Southern Ocean. How these changes will affect primary production (PP) remains unclear ...as mixing influences the two controlling factors: light and iron. We used a large ensemble of 1‐D‐biogeochemical model simulations to explore the impacts of changes in mixed‐layer depths on PP in the Southern Ocean. In summer, shoaling mixed‐layer depth always reduced depth‐integrated PP, despite increasing production rates. In winter, shoaling mixed layers had a two‐staged impact: for moderate shoaling PP increased as light conditions improved, but more pronounced shoaling decreased iron supply, which reduced PP. The fraction of PP exported below 100 m also presented a nonmonotonic behavior. This suggests a potential future shift from a situation where reduced winter mixing increases PP and export, to a situation where PP and export may collapse if the ML shoals above a threshold depth.
Plain Language Summary
In the Southern Ocean, atmospheric warming associated to climate change is altering the depth at which surface waters are stirred, the so‐called mixed‐layer depth. A change in the mixed‐layer depth impacts the phytoplankton cells that inhabit it by altering their two main limiting factors: iron and light. However, the sign and magnitude of this impact are still not clear. In this work we used mathematical simulations to explain how changes in the seasonal mixed‐layer depth modify the supply of iron and the amount of light, and how these changes impact phytoplankton activity. Our results show that mixed‐layer depth changes in summer and in winter have different impacts. Reducing summer mixed‐layer depth did not change the iron supply, but it reduced the volume of water where phytoplankton thrived. In winter, shallower mixed‐layer depth altered iron and light but in opposed ways. At first, phytoplankton increased its activity as more light became available. However, a continued shallowing of the mixed‐layer depth eventually reduced the iron supply and the phytoplankton activity. Our study proposes a new interpretation on how ongoing changes in the Southern Ocean impact phytoplankton activity and alerts of the presence of threshold depths for the winter mixed layer above which phytoplankton may struggle to survive.
Key Points
Moderate shoaling of winter mixed layer causes increase in primary production, but shoaling below a threshold depth can lead to a collapse
Reversal in this response is due to balancing effects of iron and light limitation and associated with a change in community structure and export efficiency
Interestingly, shoaling summer mixed layer always reduces PP and export because of a reduction of the productive volume
Distinctively‐light isotopic signatures associated with Fe released from anthropogenic activity have been used to trace basin‐scale impacts. However, this approach is complicated by the way Fe cycle ...processes modulate oceanic dissolved Fe (dFe) signatures (δ56Fediss) post deposition. Here we include dust, wildfire, and anthropogenic aerosol Fe deposition in a global ocean biogeochemical model with active Fe isotope cycling, to quantify how anthropogenic Fe impacts surface ocean dFe and δ56Fediss. Using the North Pacific as a natural laboratory, the response of dFe, δ56Fediss, and primary productivity are spatially and seasonally variable and do not simply follow the footprint of atmospheric deposition. Instead, the effect of anthropogenic Fe is regulated by the biogeochemical regime, specifically the degree of Fe limitation and rates of primary production. Overall, we find that while δ56Fediss does trace anthropogenic input, the response is muted by fractionation during phytoplankton uptake, but amplified by other isotopically‐light Fe sources.
Plain Language Summary
Iron released into the atmosphere by anthropogenic activities (e.g., combustion, metal industry) can get transported to open ocean areas, where it can fertilize biological production upon deposition. The distinctively‐light isotopic signatures of such anthropogenic iron have been used to trace its oceanic impact, and disentangle its contribution from that of other external iron sources. However, this approach is complicated by fractionation during surface ocean processing, which can affect the dissolved iron isotopic signature. To quantify the impact of anthropogenic iron on surface ocean iron and its isotopes, we added iron deposition from anthropogenic and other (dust, wildfire) sources to a global ocean model which incorporates iron isotopes. Focusing on the North Pacific, we find the impact of anthropogenic iron varies in time and space, whereby changes in iron concentration and isotopic signatures are distinct and also differ from the footprint of atmospheric deposition. These discrepancies relate to differences in biology, specifically the productivity of a surface ocean system, and whether this productivity is limited by the availability of iron. We find dissolved iron isotopic signatures to be useful to trace anthropogenic iron, provided that fractionating (biological) processes and the impact of other external iron sources are accounted for.
Key Points
Anthropogenic Fe deposition has a variable impact on surface ocean biogeochemistry which can differ from the pattern of deposition
Anthropogenic Fe drives distinct responses in primary production, dFe and δ56Fediss, dependent on the surface ocean biogeochemical regime
δ56Fediss traces anthropogenic Fe impact, but requires careful evaluation of other Fe sources and isotope fractionation during Fe uptake
While phytoplankton play a key role in ocean biogeochemical cycles, the availability and supply pathways of resources that support their growth remain poorly constrained. Here, we show that the ...availability of various resources varies over several orders of magnitude throughout the Atlantic Ocean, causing regional contrasts in resource deficiency. Regional variations in the relative availability of nitrogen, phosphorous, silicon, iron, zinc, manganese, cobalt, and cadmium are important and result from the contrasts between winter mixing depths and differences in vertical profiles of the different resources. The winter‐time thickening of the mixed layer may replenish or deplete resources via entrainment, depending on the vertical nutrient profile. For nutrients like nitrate, phosphate, and cadmium, entrainment is a consistent source term. While for others, such as manganese and iron, entrainment can reduce ocean resource availability, particularly in subtropical regions. Any future change to the depth of winter‐time mixing will cause region‐specific changes in relative availability of different resources that may have important ecological consequences.
Key Points
There are large scale regimes present in mixed‐layer nutrient availability throughout the Atlantic Ocean
Entrainment decreases the mixed layer availability of resources where the concentration decreases with depth
Variations in nutrient availability and entrainment are linked to gradients in resource profiles combined with the depth of mixing
Due to its importance as a limiting nutrient for phytoplankton growth in large regions of the world's oceans, ocean water column observations of concentration of the trace-metal iron (Fe) have ...increased markedly over recent decades. Here we compile >13 000 global measurements of dissolved Fe (dFe) and make this available to the community. We then conduct a synthesis study focussed on the Southern Ocean, where dFe plays a fundamental role in governing the carbon cycle, using four regions, six basins and five depth intervals as a framework. Our analysis highlights depth-dependent trends in the properties of dFe between different regions and basins. In general, surface dFe is highest in the Atlantic basin and the Antarctic region. While attributing drivers to these patterns is uncertain, inter-basin patterns in surface dFe might be linked to differing degrees of dFe inputs, while variability in biological consumption between regions covaries with the associated surface dFe differences. Opposite to the surface, dFe concentrations at depth are typically higher in the Indian basin and the Subantarctic region. The inter-region trends can be reconciled with similar ligand variability (although only from one cruise), and the inter-basin difference might be explained by differences in hydrothermal inputs suggested by modelling studies (Tagliabue et al., 2010) that await observational confirmation. We find that even in regions where many dFe measurements exist, the processes governing the seasonal evolution of dFe remain enigmatic, suggesting that, aside from broad Subantarctic - Antarctic trends, biological consumption might not be the major driver of dFe variability. This highlights the apparent importance of other processes such as exogenous inputs, physical transport/mixing or dFe recycling processes. Nevertheless, missing measurements during key seasonal transitions make it difficult to better quantify and understand surface water replenishment processes and the seasonal Fe cycle. Finally, we detail the degree of seasonal coverage by region, basin and depth. By synthesising prior measurements, we suggest a role for different processes and highlight key gaps in understanding, which we hope can help structure future research efforts in the Southern Ocean.
High adiposity in middle age is associated with higher dementia risk. The association between weight loss and cognitive function in older adults is still controversial. A meta‐analysis was undertaken ...to estimate the effectiveness of intentional weight loss on cognitive function in overweight and obese adults. A structured strategy was used to search randomized and non‐randomized studies reporting the effect of intentional and significant weight loss on cognitive function in overweight and obese subjects. Information on study design, age, nutritional status, weight‐loss strategy, weight lost and cognitive testing was extracted. A random‐effect meta‐analysis was conducted to obtain summary effect estimates for memory and attention–executive domains. Twelve studies met inclusion criteria. Seven were randomized trials and the remaining five included a control group. A low‐order significant effect was found for an improvement in cognitive performance with weight loss in memory (effect size 0.13, 95% CI 0.00–0.26, P = 0.04) and attention/executive functioning (effect size 0.14, 95% CI 0.01–0.27, P < 0.001). Studies were heterogeneous in study design, sample selection, weight‐loss intervention and assessment of cognitive function. Weight loss appears to be associated with low‐order improvements in executive/attention functioning and memory in obese but not in overweight individuals.
The ocean is a substantial source of nitrous oxide (N2O) to the atmosphere, but little is known about how this flux might change in the future. Here, we investigate the potential evolution of marine ...N2O emissions in the 21st century in response to anthropogenic climate change using the global ocean biogeochemical model NEMO-PISCES. Assuming nitrification as the dominant N2O formation pathway, we implemented two different parameterizations of N2O production which differ primarily under low-oxygen (O2) conditions. When forced with output from a climate model simulation run under the business-as-usual high-CO2 concentration scenario (RCP8.5), our simulations suggest a decrease of 4 to 12 % in N2O emissions from 2005 to 2100, i.e., a reduction from 4.03/3.71 to3.54/3.56 TgN yr-1 depending on the parameterization. The emissions decrease strongly in the western basins of the Pacific and Atlantic oceans, while they tend to increase above the oxygen minimum zones (OMZs), i.e., in the eastern tropical Pacific and in the northern Indian Ocean. The reduction in N2O emissions is caused on the one hand by weakened nitrification as a consequence of reduced primary and export production, and on the other hand by stronger vertical stratification, which reduces the transport of N2O from the ocean interior to the ocean surface. The higher emissions over the OMZ are linked to an expansion of these zones under global warming, which leads to increased N2O production, associated primarily with denitrification. While there are many uncertainties in the relative contribution and changes in the N2O production pathways, the increasing storage seems unequivocal and determines largely the decrease in N2O emissions in the future. From the perspective of a global climate system, the averaged feedback strength associated with the projected decrease in oceanic N2O emissions amounts to around -0.009 W m-2 K-1, which is comparable to the potential increase from terrestrial N2O sources. However, the assessment for a potential balance between the terrestrial and marine feedbacks calls for an improved representation of N2O production terms in fully coupled next-generation Earth system models.
Constraining the role of dust deposition in regulating the concentration of the essential micronutrient iron in surface ocean waters requires knowledge of the flux of seawater‐soluble iron in ...aerosols and the replacement time of dissolved iron (DFe) in the euphotic zone. Here we estimate these quantities using seasonally resolved DFe data from the Bermuda Atlantic Time‐series Study region and weekly‐scale measurements of iron in aerosols and rain from Bermuda during 2019. In response to seasonal changes in vertical mixing, primary production and dust deposition, surface DFe concentrations vary from ∼0.2 nM in early spring to >1 nM in late summer, with DFe inventories ranging from ∼30 to ∼80 μmol/m2, respectively, over the upper 200 m. Assuming the upper ocean approximates steady state for DFe on an annual basis, our aerosol and rainwater data require a mean euphotic‐zone residence time of ∼0.8–1.9 years for DFe with respect to aeolian input.
Plain Language Summary
Primary production by phytoplankton in ocean surface waters is the foundation of the marine ecosystem, and plays a key role in maintaining the ocean‐atmosphere balance of carbon dioxide, which regulates global climate. Iron is an essential micronutrient that is required by phytoplankton, and the availability of dissolved iron (DFe) is thought to limit phytoplankton growth over large areas of the ocean. In this context, it is important to constrain the sources and persistence of DFe in surface ocean waters, which control the amount of DFe that is available to support phytoplankton growth. This study focuses on the Bermuda region of the North Atlantic Ocean, where deposition of airborne soil dust is the major source of DFe to surface waters. By combining measurements of the atmospheric loading and solubility of iron in soil dust over Bermuda with measurements of DFe in adjacent ocean waters over a full year, we are able to estimate the rate of supply of DFe from dust deposition in this region, as well as the average time that this DFe persists in the surface ocean. The latter, termed the DFe replacement time, is around 1 year, which agrees well with recent estimates from comparable ocean regions.
Key Points
An imbalance between input and removal produces an ∼3‐fold seasonal increase in the euphotic‐zone inventory of dissolved iron (DFe) near Bermuda
Analyses of iron in seasonal‐scale aerosol, rain and water‐column samples allow direct estimates of the replacement time of DFe
We derive a mean residence time of ∼0.8–1.9 years for DFe in the euphotic zone (<200 m) of the Sargasso Sea near Bermuda
The trace metal iron (Fe) is now routinely included in state-of-the-art ocean general circulation and biogeochemistry models (OGCBMs) because of its key role as a limiting nutrient in regions of the ...world ocean important for carbon cycling and air-sea CO2 exchange. However, the complexities of the seawater Fe cycle, which impact its speciation and bioavailability, are simplified in such OGCBMs due to gaps in understanding and to avoid high computational costs. In a similar fashion to inorganic carbon speciation, we outline a means by which the complex speciation of Fe can be included in global OGCBMs in a reasonably cost-effective manner. We construct an Fe speciation model based on hypothesised relationships between rate constants and environmental variables (temperature, light, oxygen, pH, salinity) and assumptions regarding the binding strengths of Fe complexing organic ligands and test hypotheses regarding their distributions. As a result, we find that the global distribution of different Fe species is tightly controlled by spatio-temporal environmental variability and the distribution of Fe binding ligands. Impacts on bioavailable Fe are highly sensitive to assumptions regarding which Fe species are bioavailable and how those species vary in space and time. When forced by representations of future ocean circulation and climate we find large changes to the speciation of Fe governed by pH mediated changes to redox kinetics. We speculate that these changes may exert selective pressure on phytoplankton Fe uptake strategies in the future ocean. In future work, more information on the sources and sinks of ocean Fe ligands, their bioavailability, the cycling of colloidal Fe species and kinetics of Fe-surface coordination reactions would be invaluable. We hope our modeling approach can provide a means by which new observations of Fe speciation can be tested against hypotheses of the processes present in governing the ocean Fe cycle in an integrated sense
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