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A close-up view of biomass controls in Southern Ocean eddies

Posted by mmaheigan

· Thursday, August 20th, 2020

Southern Ocean biological productivity is instrumental in regulating the global carbon cycle. Previous correlative studies associated widespread mesoscale activity with anomalous chlorophyll levels. However, eddies simultaneously modify both the physical and biogeochemical environments via several competing pathways, making it difficult to discern which mechanisms are responsible for the observed biological anomalies within them. Two recently published papers track Southern Ocean eddies in a global, eddy-resolving, 3-D ocean simulation. By closely examining eddy-induced perturbations to phytoplankton populations, the authors are able to explicitly link eddies to co-located biological anomalies through an underlying mechanistic framework.

Figure caption: Simulated Southern Ocean eddies modify phytoplankton division rates in different directions of depending on the polarity of the eddy and background seasonal conditions. During summer anticyclones (top right panel) deliver extra iron from depth via eddy-induced Ekman pumping and fuel faster phytoplankton division rates. During winter (bottom right panel) the extra iron supply is eclipsed by deeper mixed layer depths and elevated light limitation resulting in slower division rates. The opposite occurs in cyclones.

In the first paper, the authors observe that eddies primarily affect phytoplankton division rates by modifying the supply of iron via eddy-induced Ekman pumping. This results in elevated iron and faster phytoplankton division rates in anticyclones throughout most of the year. However, during deep mixing winter periods, exacerbated light stress driven by anomalously deep mixing in anticyclones can dominate elevated iron and drive division rates down. The opposite response occurs in cyclones.

The second paper tracks how eddy-modified division rates combine with eddy-modified loss rates and physical transport to produce anomalous biomass accumulation. The biomass anomaly is highly variable, but can exhibit an intense seasonal cycle, in which cyclones and anticyclones consistently modify biomass in different directions. This cycle is most apparent in the South Pacific sector of the Antarctic Circumpolar Current, a deep mixing region where the largest biomass anomalies are driven by biological mechanisms rather than lateral transport mechanisms such as eddy stirring or propagation.

It is important to remember that the correlation between chlorophyll and eddy activity observable from space can result from a variety of physical and biological mechanisms. Understanding the nuances of how these mechanisms change regionally and seasonally is integral in both scaling up local observations and parameterizing coarser, non-eddy resolving general circulation models with embedded biogeochemistry.

Authors:
Tyler Rohr (Australian Antarctic Partnership Program, previously at MIT/WHOI)
Cheryl Harrison (University of Texas Rio Grande Valley)
Matthew Long (National Center for Atmospheric Research)
Peter Gaube (University of Washington)
Scott Doney (University of Virginia)

Unexpected patterns of carbon export in the Southern Ocean

Posted by mmaheigan

· Tuesday, July 7th, 2020

The Southern Ocean is a major player in driving global distributions of heat, carbon dioxide, and nutrients, making it key to ocean chemistry and the earth’s climate system. In the ocean, biological production and export of organic carbon are commonly linked to places with high nutrient availability. A recent paper, published in Global Biogeochemical Cycles, highlighting new observations from robotic profiling floats demonstrates that areas of high carbon export in the Southern Ocean are actually associated with very low concentrations of iron, an important micronutrient for supporting phytoplankton growth. This suggests a decoupling between the production and export of organic carbon in this region.

Figure caption: (A) Meridional pattern of Annual Net Community Production (ANCP) (equivalent to carbon export) (± standard deviation) in the Southern Ocean (blue line with circles and shaded area), carbon export estimates from previous satellite-based analyses (blue dashed line), and silicate to nitrate (Si:NO₃) ratio of the surface water (black continuous line). Grey dotted line shows a Si:NO₃ = 1 mol mol⁻¹, characteristic of nutrient-replete diatoms. (B) Meridional pattern of Southern Ocean nutrient concentrations, including dissolved iron (Fe) concentration (black line), nitrate (red line), and silicate (blue line). (C) Mean 2014–2015 annual zonally averaged air-sea flux of CO₂ computed using neural network interpolation method. STF = Subtropical Front, PF = Antarctic Polar Front, SIF = Seasonal Ice Front.

Using observations of nutrient and oxygen drawdown from a regional network of profiling Biogeochemical-Argo floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM), the authors calculated estimates of Southern Ocean carbon export. A meridional pattern in biological carbon export emerged, showing peak export near the Antarctic Polar Front (PF) associated with minima in surface iron concentrations and dissolved silicate to nitrate ratios. Previous studies have shown that under iron-limiting conditions, diatoms increase their uptake ratio of silicate with respect to other nutrients (e.g., nitrogen), resulting in silicification. Here, the authors hypothesize that iron limitation promotes silicification in Southern Ocean diatoms, as evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic Polar Front. High diatom silicification increases ballasting of particulate organic carbon and hence overall carbon export in this region. The resulting meridional pattern of organic carbon export is similar to that of the air-sea flux of carbon dioxide in the Southern Ocean, underscoring the importance of the biological carbon pump in controlling the spatial pattern of oceanic carbon uptake in this region.

Authors:
Lionel A. Arteaga (Princeton University)
Markus Pahlow (Helmholtz Centre for Ocean Research Kiel, GEOMAR)
Seth M. Bushinsky (University of Hawaii)
Jorge L. Sarmiento (Princeton University)

Physics vs. biology in Southern Ocean nutrient gradients

Posted by mmaheigan

· Tuesday, June 16th, 2020

In the Southern Ocean, surface water silicate (SiO₄) concentrations decline very quickly relative to nitrate concentrations along a northward gradient toward mode water formation regions on the northern edge (Figure 1a, b). These mode waters play a critical role in driving global nutrient concentrations, setting the biogeochemistry of low- and mid-latitude regions around the globe after they upwell further north. To explain this latitudinal surface gradient, most hypotheses have implicated diatoms, which take up and export silicon as well as nitrogen: (1) Diatoms, including highly-silicified species such as Fragilariopsis kerguelensis, are more abundant in the Southern Ocean than elsewhere; (2) Iron limitation, which is prevalent in the Southern Ocean, elevates the Si:N ratio of diatoms; (3) Mass export of empty diatom frustules pumps silicate but not nitrate to deeper waters.

Figure 1: (a) and (b) nitrate and silicate concentrations in surface waters of the Southern Ocean (GLODAPv2_2019 data). (c) Model results of a standard run (black diamonds), a run without biology (red diamonds) and a run without mixing (blue diamonds).

In a recent paper published in Biogeosciences, the authors use an idealized model to explore the relative roles of biological vs. physical processes in driving the observed latitudinal surface nutrient gradients. Over timescales of a few years, removing the effects of biology (no SiO₄uptake or export) from the model elevates silicate concentrations slightly over the entire latitudinal range, but does not remove the strong latitudinal gradient (Figure 1c). However, if the effects of vertical mixing processes such as upwelling and entrainment are removed from the model by eliminating the observed deep [SiO₄] gradient, the observed surface nutrient gradient is greatly altered (Figure 1c). These model results suggest that, over short timescales, physics is more important than biology in driving the observed surface water gradient in SiO₄:NO₃ ratios and forcing silicate depletion of mode waters leaving the Southern Ocean. These findings add to our understanding of Southern Ocean dynamics and the downstream effects on other oceans.

Authors:
P. Demuynck (University of Southampton)
T. Tyrrell (University of Southampton)
A.C. Naveira Garabato (University of Southampton)
C.M. Moore (University of Southampton)
A.P. Martin (National Oceanography Centre)

Can phytoplankton help us determine ocean iron bioavailability?

Posted by mmaheigan

· Wednesday, March 11th, 2020

Iron (Fe) is a key element to sustaining life, but it is present at extremely low concentrations in seawater. This scarcity limits phytoplankton growth in large swaths of the global ocean, with implications for marine food webs and carbon cycling. The acquisition of Fe by phytoplankton is an important process that mediates the movement of carbon to the deep ocean and across trophic levels. It is a challenge to evaluate the ability of marine phytoplankton to obtain Fe from seawater since it is bound by a variety of poorly defined organic complexes.

Figure 1: Schematic representation of the reactions governing dissolved Fe (dFe) bioavailability to phytoplankton (a) Bioavailability of dFe in seawater collected from various basins and depth and probed with different iron-limited phytoplankton species under dim laboratory light and sunlight (b) (See paper for further details on samples and species)

A recent study in The ISME Journal proposes a new approach for evaluating seawater dissolved Fe (dFe) bioavailability based on its uptake rate constant by Fe-limited cultured phytoplankton. The authors collected samples from distinct regions across the global ocean, measured the properties of organic complexation, loaded these complexes with a radioactive Fe isotope, and then tracked the internalization rates from these forms to a diverse set of Fe-limited phytoplankton species. Regardless of origin, all of the phytoplankton acquired natural organic complexes at similar rates (accounting for cell surface area). This confirms that multiple Fe-limited phytoplankton species can be used to probe dFe bioavailability in seawater. Among water types, dFe bioavailability varied by ~4-fold and did not clearly correlate with Fe concentrations or any of the measured Fe speciation parameters. This new approach provides a novel way to determine Fe bioavailability in samples from across the oceans and enables modeling of in situ Fe uptake rates by phytoplankton based simply on measured Fe concentrations.

Authors:
Yeala Shaked (Hebrew University of Jerusalem)
Kristen N. Buck (University of South Florida)
Travis Mellett (University of South Florida)
Maria. T. Maldonado (University of British Columbia)

Diatoms commit iron piracy with stolen bacterial gene

Posted by mmaheigan

· Tuesday, February 4th, 2020

Since diatoms carry out much of the primary production in iron-limited marine environments, constraining the details of how these phytoplankton acquire the iron they need is paramount to our understanding of biogeochemical cycles of iron-depleted high-nutrient low-chlorophyll (HNLC) regions. The proteins involved in this process are largely unknown, but McQuaid et al. (2018) scientists described a carbonate-dependent uptake protein that enables diatoms to access inorganic iron dissolved in seawater. As increasing atmospheric CO₂ results in decreased seawater carbonate ion concentrations, this iron uptake strategy may have an uncertain future. In a recent study published in PNAS, authors used CRISPR technology to characterize a parallel uptake system that requires no carbonate and is therefore not impacted by ocean acidification.

This system targets an organically complexed form of iron (siderophores, molecules that bind and transport iron in microorganisms) that is only produced by co-occurring microbes. Two genes are required to convert siderophores from a potent toxicant to an essential nutrient. One of these (FBP1) is a receptor that was horizontally acquired from siderophore-producing bacteria. The other (FRE2) is a eukaryotic reductase that facilitates the dissociation of Fe-siderophore complexes.

Figure caption: (A) Growth curves of diatom cultures ( • = WT, ◇ = ΔFBP1, ☐ = ΔFRE2) in low iron media. (B) Growth in same media with siderophores added. (C) Diatoms under 1000x magnification, brightfield. (D) mCherry-FBP1. (E) Plastid autofluorescence. (F) YFP-FRE2. (G) Phylogenetic tree of FBP1 and related homologs.

Are diatoms really stealing siderophores from hapless bacteria? The true nature of this interaction is unknown and may at times be mutualistic. For example, when iron availability limits the carbon supply to a microbial community, heterotrophic bacteria may benefit from using siderophores to divert iron to diatom companions. Further work is needed to understand the true ecological basis for this interaction, but these results suggest that as long as diatoms and bacteria co-occur, iron limitation in marine ecosystems will not be exacerbated by ocean acidification.

Authors:
Tyler Coale (Scripps Institution of Oceanography, J.Craig Venter Institute)
Mark Moosburner (Scripps Institution of Oceanography, J.Craig Venter Institute)
Aleš Horák (Biology Centre CAS, Institute of Parasitology, University of South Bohemia)
Miroslav Oborník (Biology Centre CAS, Institute of Parasitology, University of South Bohemia)
Katherine Barbeau (Scripps Institution of Oceanography)
Andrew Allen (Scripps Institution of Oceanography, J.Craig Venter Institute)

Also see joint post on the GEOTRACES website

Biogeochemical controls of surface ocean phosphate

Posted by mmaheigan

· Tuesday, November 12th, 2019

Phosphorus availability is important for phytoplankton growth and more broadly ocean biogeochemical cycles. However, phosphate concentration is often below the analytical detection limit of the standard auto-analyzer technique. Thus, we know little about geographic phosphate variation across most low latitude regions. To address this issue, a global collaboration of scientists conducted a study published in Science Advances on combined phosphate measurements using high-sensitivity methods that yielded a detailed map of surface phosphate (Figure 1).

Figure 1: Fine-scale global variation of surface phosphate. Surface phosphate measured using high-sensitivity techniques revealed previously unrecognized low latitude differences in phosphate drawdown.

The study’s new globally expansive phosphate data set revealed previously unrecognized low-phosphate areas, including large regions of the Pacific Ocean—really low phosphate in the western North Pacific and to a lesser extent in the South Pacific. Although atmospheric iron input and nitrogen fixation are commonly described as regulators of surface phosphate, this study shows that shifts in the elemental stoichiometry (N:P:Fe) of the vertical nutrient supply play an additional role. Previous studies and climate models have suggested that the availability of phosphate is a first-order driver of ocean biogeochemical changes. Interestingly, this study suggests that marine ecosystems are more resilient to phosphate stress than previously thought. These findings underscore the importance of accurately quantifying nutrients at low concentrations for understanding the regulation of ocean ecosystem processes and biogeochemistry now and under future climate conditions.

And the data are of course available in BCO-DMO!

Authors:
Adam C. Martiny (University of California, Irvine)
Michael W. Lomas (Bigelow Laboratory for Ocean Sciences)
Weiwei Fu (University of California, Irvine)
Philip W. Boyd (University of Tasmania)
Yuh-ling L. Chen (National Sun Yat-sen University)
Gregory A. Cutter (Old Dominion University)
Michael J. Ellwood (Australian National University)
Ken Furuya (The University of Tokyo)
Fuminori Hashihama (Tokyo University of Marine Science and Technology)
Jota Kanda (Tokyo University of Marine Science and Technology)
David M. Karl (University of Hawaii)
Taketoshi Kodama (Japan Fisheries Research and Education Agency)
Qian P. Li (Chinese Academy of Sciences)
Jian Ma (Xiamen University)
Thierry Moutin (Université de Toulon)
E. Malcolm S. Woodward (Plymouth Marine Laboratory)
J. Keith Moore (University of California, Irvine)

Pumped up by the cold: Increased elemental density in polar diatoms

Posted by mmaheigan

· Monday, October 28th, 2019

Large diatoms are common in polar phytoplankton blooms, contributing significantly to food webs and carbon export, but relatively little is known about their elemental biogeochemistry. A recent study in Frontiers in Marine Science showed that the size-dependent increase in cell nutrient content for polar diatoms was similar to published values for temperate diatoms, whereas the elemental density (mass per unit volume) of polar diatoms was substantially greater for all elements measured (carbon, nitrogen, silicon and phosphorus). Furthermore, at near freezing culture temperatures, there was a positive relationship between diatom size and realized growth rates near their theoretical maximum (Figure 1). Because of the differences in elemental density between carbon and silica, these diatoms exhibited particulate C:Si ratios that are commonly interpreted as a sign of iron limitation; yet these cultures were trace metal-replete. The observed elemental composition differences suggest that it may be important for polar biogeochemical models to include different representations of diatom biogeochemistry by accounting for the functions of size and near freezing temperature.

Figure 1. Left: Cellular carbon content for polar diatoms across four orders of magnitude in biovolume compared to the same relationship for a wide range of non-polar diatoms (MD&L = Menden-Deuer & Lessard, 2000). The y-intercept is the estimate of the baseline carbon density in these polar diatoms, and is significantly higher than the literature values reviewed in MD&L (2000). Right: Growth rate of the same polar diatoms expressed as a percent of their calculated maximum growth rate at 2°C. Error bars represent the range of values observed in the experiments. Maximum growth rate was estimated by 1) applying the growth rate/biovolume relationships published by Chisholm (1992) and Edwards et al. (2012) to the observed biovolume for each culture, and 2) scaling this growth rate to 2°C growth temperature using the relationship of Eppley (1972).

Authors:
Michael Lomas (Bigelow Laboratory for Ocean Sciences)
Steven Baer (Maine Maritime Academy)
Sydney Acton (Dauphin Island Sea Lab)
Jeffrey Krause (Dauphin Island Sea Lab and University of South Alabama)

The causes of the 90-ppm glacial atmospheric CO2 drawdown still strongly debated

Posted by mmaheigan

· Tuesday, July 9th, 2019

Joint feature with GEOTRACES

Figure: Illustration of the two main mechanisms identified by this study to explain lower atmospheric CO₂ during glacial periods. Left: present-day conditions; right: conditions around 19,000 years ago during the Last Glacial Maximum. The obvious explanation for lower CO₂ during glacial periods – cooler ocean temperatures (darker blue shade) making CO₂ more soluble, much as a glass of sparkling wine will remain fizzier for longer when it is colder – has long been dismissed as not being a significant factor. However, previous calculations assumed that the ocean cooled uniformly and was saturated in dissolved CO₂. The model, consistent with reconstructions of sea surface temperature, predicts more cooling at mid latitudes compared with polar regions and also accounts for undersaturation. This nearly doubles the effect of temperature change and accounts for almost half the 90 ppm glacial-interglacial atmospheric CO₂ difference. Another quarter is explained in this model by increased growth of marine algae (green blobs and inset) in the waters off Antarctica. Algae absorb CO₂ from the atmosphere during photosynthesis and “pump” it into the deep ocean when they die and sink. But their growth in the present-day ocean, especially the waters off Antarctica, is limited by the availability of iron, an essential micronutrient primarily supplied by wind-borne dust. In our model an increased supply of iron to the Southern Ocean, likely originating from Patagonia, Australia and New Zealand, enhances their growth and sucks CO₂ out of the atmosphere. This “fertilization” effect was greatly underestimated by previous studies. The study also finds that, contrary to the current consensus, a large expansion of sea ice off Antarctica and reconfiguration of ocean circulation may have played only a minor role in glacial-interglacial CO₂ changes. Credit: Illustration by Andrew Orkney, University of Oxford.

Using an observationally constrained earth system model, S. Khatiwala and co-workers compare different processes that could lead to the 90-ppm glacial atmospheric CO₂drawdown, with an important improvement on the deep carbon storage quantification (i.e. Biological Carbon Pump efficiency). They demonstrate that circulation and sea ice changes had only a modest net effect on glacial ocean carbon storage and atmospheric CO₂, whereas temperature and iron input effects were more important than previously thought due to their effects on disequilibrium carbon storage.

Authors:
Samar Khatiwala (University of Oxford, UK)
Andreas Schmittner and Juan Muglia (Oregon State University)

Upwelled hydrothermal Fe stimulates massive phytoplankton blooms in the Southern Ocean

Posted by mmaheigan

· Tuesday, July 9th, 2019

Joint feature with GEOTRACES

Figure 1a: Southern Ocean phytoplankton blooms showing distribution, biomass (circle size) and type (color key).

In a recent study, Ardyna et al combined observations of profiling floats with historical trace element data and satellite altimetry and ocean color data from the Southern Ocean to reveal that dissolved iron of hydrothermal origin can be upwelled to the surface. Furthermore, the activity of deep hydrothermal sources can influence upper ocean biogeochemical cycles of the Southern Ocean, and in particular stimulate the biological carbon pump.

Authors:
Mathieu Ardyna
Léo Lacour
Sara Sergi
Francesco d’Ovidio
Jean-Baptiste Sallée
Mathieu Rembauville
Stéphane Blain
Alessandro Tagliabue
Reiner Schlitzer
Catherine Jeandel
Kevin Robert Arrigo
Hervé Claustre

Dust-borne iron in the Southern Ocean was more bioavailable during glacial periods

Posted by mmaheigan

· Wednesday, January 23rd, 2019

The Southern Ocean is iron (Fe)-limited, and increased fluxes of dust-borne Fe to the Southern Ocean during the Last Glacial Maximum (LGM) have been associated with phytoplankton growth and CO₂ drawdown. Dust contains different mixes of Fe-bearing minerals, depending on the source region. Fe(II) silicate minerals from physical weathering are more bioavailable than Fe(III) oxyhydroxide minerals from chemical weathering. The Fe(II) silicates are dominant in dust sources that have been weathered from bedrock by glaciers in Patagonia, but the impact of glacial activity on dust-borne Fe speciation (Fe oxidation state and mineral composition) and bioavailability over the last glacial cycle has not previously been quantified.

Figure 1. The fraction of Fe(II) in dust (Fe(II)/Fetotal, dominated by Fe(II) silicates, shown as blue dots connected with dotted lines on blue axes) in marine sediment cores from (A) the South Atlantic and (B) the South Pacific plotted with the total dust flux (grey lines on grey axes).

A recent study in PNAS reconstructs the speciation of dust-borne Fe over the last glacial cycle in South Atlantic and South Pacific marine sediment cores using Fe K-edge X-ray absorption spectroscopy. The authors observed that the highly bioavailable Fe(II) silicate content of dust-borne Fe is higher in both regions during cold glacial periods, suggesting that a given flux of Fe is more bioavailable in glacial versus interglacial periods (Figure 1). Therefore, all Fe cannot be considered equal in biogeochemical models working on glacial-interglacial timescales. The bioavailability of a given flux of Fe at the LGM was likely a dominant driver of phytoplankton growth, with more bioavailable Fe driving increased phytoplankton activity and associated atmospheric CO₂ drawdown and subsequent cooling. The observed association between glacial periods and increased Fe bioavailability in the Southern Ocean may indicate an important positive feedback mechanism between glacial activity and cold glacial temperatures through Fe speciation and the efficiency of the biological pump.

Paper link: https://doi.org/10.1073/pnas.1809755115

Authors:
Elizabeth M. Shoenfelt (Lamont-Doherty Earth Observatory, Columbia University)
Gisela Winckler (Lamont-Doherty Earth Observatory, Columbia University)
Frank Lamy (Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research)
Robert F. Anderson (Lamont-Doherty Earth Observatory, Columbia University)
Benjamin C. Bostick (Lamont-Doherty Earth Observatory, Columbia University)

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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