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The arsenic respiratory cycle in pelagic waters of Oxygen Deficient Zones

Posted by mmaheigan

· Wednesday, October 30th, 2019

Oxygen Deficient Zones (ODZs) are naturally occurring functionally anoxic regions of the open ocean which can act as proxies for early Earth’s anoxic ocean. Without free oxygen, microorganisms in these regions use alternative electron acceptors to oxidize organic material. These functionally anoxic regions are also hotspots for chemoautotrophic pathways. Some microorganisms can use arsenic based compounds to oxidize organic material, and others can couple nitrate reduction with arsenic oxidation supporting autotrophic carbon fixation thus linking arsenic respiration with carbon and nitrogen cycling. While arsenic concentrations in modern oceans are relatively low, the Precambrian ocean likely had periods of high arsenic concentrations. Integrating over time and space of anoxic waters, arsenic-based metabolisms may have had significant implications for the biogeochemical cycling of not only arsenic, but also carbon and nitrogen.

Figure 1: Arsenotrophic genes identified in the Eastern Tropical North Pacific Oxygen Deficient Zone. (A) Genomic complement for dissimilatory arsenate reduction assembled from metagenomes which likely supports respiration of organic matter. (B) Genomic complement for putative chemoautotrophic arsenite oxidation pathway assembled from metagenomes which may couple with nitrate reduction to support organic matter production. (C) Relative abundance of genes associated with arsenite oxidase (aioA), dissimilatory arsenate reduction (arrA), and forward dissimilatory sulfite reductase (dsrA) associated with sulfur reduction; abundance shown as a relative contribution to the total microbial community as estimated by abundance of RNA polymerase genes (rpoB). The genes arrA and forward-dsrA are more abundant in the particulate fraction, whereas aioA is more abundant in the free-living fraction. (D) Relative abundance of genes in the microbial community for the more abundant genes aioA-like and reverse form of dsrA associated with sulfur oxidation. aioA-like genes are relatively more abundant within the particulate fraction, with no strong partitioning between fractions identified for the reverse-dsrA genes. Arsenical reduction and chemoautotrophic arsenical oxidation are likely performed by different microbial groups within the ODZ communities.

Recent work in PNAS identified gene sequences for a complete arsenic respiratory cycle from Eastern Tropical North Pacific (ETNP) ODZ metagenomes. The authors identified arsenotrophic genes for dissimilatory arsenate reduction from one group of microorganisms and genes for a putative chemoautotrophic arsenite oxidation pathway from another group within the ETNP ODZ microbial community. Analysis of genomic sequences from a free-living sample and from particulate-associated sample indicate niche differentiation of these pathways—arsenate reduction genes enriched within the particulate fraction and arenite oxidation enriched in the free-living water column. In addition to the presence of these genes in metagenomes, the authors identified the active expression of these arsenotrophic genes in publicly available metatranscriptomes from the ETNP and Eastern Tropical South Pacific ODZs. Theyalso found an abundance of sequences in the ETNP ODZ for the gene aioA-like, which is a closely related enzyme to arsenite oxidase (aioA), but with an unconfirmed function. The identification of these actively expressed genes in modern ODZs enables further investigation of these cycles that were likely important in early oceans. These findings also highlight that there are still yet to be discovered respiratory pathways in ODZs. Arsenotrophy, in conjunction with other niche respiratory pathways – both known and as yet undiscovered – likely sum to a considerable contribution of energy flow and elemental cycling through these anoxic systems.

Authors:
Jaclyn Saunders (University of Washington; present affiliation Woods Hole Oceanographic Institution)
Clara Fuchsman (University of Washington; present affiliation Horn Point Laboratory)
Cedar McKay & Gabrielle Rocap (University of Washington)

See related University of Washington press-release

Air-sea gas exchange estimates biased by multi-day surface trapping

Posted by mmaheigan

· Tuesday, August 20th, 2019

Routine measurements of air-sea gas exchange assume a homogeneous gas concentration across the upper few meters of the ocean. But is this assumption valid? A recent study in Biogeosciences revealed substantial systematic gradients of nitrous oxide (N₂O) in the top few meters of the Peruvian upwelling regime. These gradients lead to a 30% overestimate of integrated N₂O emissions across the entire region, with local emissions overestimated by as much as 800%.

Figure caption: Air-sea gas exchange estimates can be biased by gas concentration gradients within the upper few meters of the ocean; in particular, surface trapping over several days’ duration can generate substantial gradients.

The N₂O gradients off Peru form during multi-day events of surface trapping, in which near-surface stratification dampens turbulent mixing. Until now, surface trapping was assumed to be a diurnal (driven by solar warming) process without memory, whereby only weak gradients would form during the hours of trapping and then dissipate. It is likely that multi-day surface trapping occurs in other ocean regions as well. The total impact on emission estimates of different greenhouse gases is yet to be quantified, but given the findings in the Peruvian upwelling system, could be significant globally.

Authors:
Tim Fischer, Annette Kock, Damian L. Arévalo-Martínez, Marcus Dengler, Peter Brandt, Hermann W. Bange (GEOMAR)

Regional circulation changes and a growing atmospheric CO2 concentration drive accelerated anthropogenic carbon uptake in the South Pacific

Posted by mmaheigan

· Tuesday, August 6th, 2019

About one tenth of human CO₂ emissions are currently being taken up by the Pacific Ocean, which makes the seawater more corrosive to the calcium carbonate shells and skeletons of the plants and animals that live there. Now, thanks to hard work by international teams of scientists from the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP), there are decades of data, enough to test how much this anthropogenic CO₂ accumulation varies throughout the Pacific Ocean and regionally on the timescales of decades.

Figure caption: Map of the concentration of human-emitted CO₂ along the sections where data were available from more than one decade, estimated for the year 2015.

Using a new take on an old technique, along with a wide variety of repeat biogeochemical measurements, a study in Biogeochemical Cycles revealed that Pacific anthropogenic CO₂ accumulation increased from the 1995-2005 decade to the 2005-2015 decade. While the magnitude of the decadal increase was consistent with increases in human CO₂ emissions over this period for most of the Pacific, the rate of change was greater than expected in the South Pacific subtropical gyre. The authors suggest that recent increases in circulation in the gyre region could have delivered an unexpectedly large amount of anthropogenic CO₂-laden seawater from the surface to the ocean interior. Programs like GO-SHIP will continue to be critical for tracking the fate of human CO₂emissions and associated feedbacks on climate and marine ecosystems.

Authors:
B. R. Carter (Univ. Washington and PMEL)
R. A. Feely, G. C. Johnson, J. L. Bullister (PMEL)
R. Wanninkhof (NOAA AOML)
S. Kouketsu, A. Murata (JAMSTEC
R. E. Sonnerup, S. Mecking (Univ. Washington)
P. C. Pardo (Univ. Tasmania)
C. L. Sabine (Univ. Hawai‘i, Mānoa)
B. M. Sloyan, B. Tilbrook (CSIRO, Australia)
K. Speer (Florida State University
L. D. Talley (Scripps Institution of Oceanography)
F. J. Millero (Univ. Miami)
S. E. Wijffels (CSIRO and WHOI)
A. M. Macdonald (WHOI)
N. Gruber (ETH Zurich)

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)

Forecasting air-sea CO2 flux variations several years in advance

Posted by mmaheigan

· Tuesday, July 9th, 2019

Year-to-year changes in the flux of CO₂ between the atmosphere and the ocean impact the global carbon cycle and climate system, and challenge our ability to verify fossil fuel CO₂ emissions. A new study published in Earth System Dynamics suggests that these air-sea CO₂ flux variations are predictable several years in advance.

A novel set of initialized forecasts of past air-sea CO₂ flux from an Earth system model (Figure 1a) confidently predicts year-to-year variations in the globally-integrated flux up to two years in advance. At regional scales, the forecast lead time increases. The predictability of CO₂ flux from the initialized forecast system exceeds that obtained solely from foreknowledge of variations in external forcing (e.g., volcanic eruptions) or a simple persistence forecast (e.g., CO₂ flux this year will be the same as next year). The longest-lasting forecast enhancements are in the subantarctic Southern Ocean and the northern North Atlantic (Figure 1b).

Figure 1: (a) Forecasts of the past evolution of air-sea CO2 flux in the South Pacific using an Earth System model indicate the potential to predict the future evolution of this quantity. (b) In each biome, the maximum forecast lead time in which the initialized forecast of air-sea CO2 flux beats out other forecast methods.

These results are particularly meaningful for those forecasting year-to-year changes in the global carbon budget, especially as these forecasting efforts are blind to the externally-forced variability in advance (i.e., the external forcing of the future is unknown). In this way, forecasts of air-sea CO₂ flux variations can help to inform future predictions of land-air CO₂ flux and atmospheric CO₂ concentration.

Authors:
Nicole Lovenduski (University of Colorado Boulder)
Stephen G. Yeager (National Center for Atmospheric Research)
Keith Lindsay (National Center for Atmospheric Research)
Matthew C. Long (National Center for Atmospheric Research)

See also the OCB Ocean-Atmosphere Interactions: Scoping directions for U.S. research Workshop to be held in October 1-3, 2019

Suddenly shallow: A new aragonite saturation horizon will soon emerge in the Southern Ocean

Posted by mmaheigan

· Monday, May 27th, 2019

Earth System Models (ESMs) project that by the end of this century, the aragonite saturation horizon (the boundary between shallower, saturated waters and deeper, undersaturated waters that are corrosive to aragonitic shells) will shoal all the way to the surface in the Southern Ocean, yet the temporal evolution of the horizon has not been studied in much detail. Rather than a gradual shoaling, a new shallow aragonite saturation horizon emerges suddenly across many locations in the Southern Ocean between now and the end of the century (Figure 1, left), as detailed in a new study published in Nature Climate Change.

Figure 1: Maximum step-change in the depth of the aragonite saturation horizon (left), timing of the step-change (center), and cause of the change (right). Xs on the time axis (center) indicate when the shallow horizon emerges in each ensemble member. (click image to enlarge)

The emergence of the shallow aragonite saturation horizon is apparent in each member of an ensemble of climate projections from an ESM, but the step change occurs during different years (Figure 1, center). The shoaling is driven by the gradual accumulation of anthropogenic CO₂ in the Southern Ocean thermocline, where the carbonate ion concentration exhibits a local minimum and approaches undersaturation (Figure 1, right).

The abrupt shoaling of the Southern Ocean aragonite saturation horizon occurs under both business-as-usual and emission-stabilizing scenarios, indicating an inevitable and sudden decrease in the volume of suitable habitat for aragonitic organisms such as shelled pteropods, foraminifers, cold-water corals, sea urchins, molluscs, and coralline algae. Widespread reductions in these habitats may have far-reaching consequences for fisheries, economies, and livelihoods.

Authors:
Gabriela Negrete-García (Scripps Institution of Oceanography)
Nicole Lovenduski (University of Colorado Boulder)

See also OCB2019 plenary session: Carbon cycle feedbacks from the seafloor (Wednesday, June 26, 2019)

Pteropod populations stable or increasing according to long-term study along the Western Antarctic Peninsula

Posted by mmaheigan

· Thursday, March 21st, 2019

Shelled pteropods (pelagic snails) are abundant planktonic predators and prey, linking grazers and higher trophic levels and contributing to the carbon cycle via consumption and excretion. Pteropods have been heralded as bioindicators of ocean acidification, given their aragonitic shell’s susceptibility to dissolution, which could ultimately lead to declining abundance. However, pteropod population dynamics are understudied, particularly in the Southern Ocean, a region predicted to be highly impacted by both warming and ocean acidification. In a recent publication in Limnology and Oceanography, long-term data sets from the Western Antarctic Peninsula show that while there is considerable interannual variability in pteropod abundance, populations have remained stable over the past 25 years, with some pteropod species (gymnosomes (non-shelled pteropod) overall, L. antarctica and C. pyramidata (shelled pteropods) regionally) even increasing during this period (Figure 1).

Figure 1. Annual pteropod abundance anomalies for the entire Palmer Antarctica Long-Term Ecological Research (LTER) study region along the Western Antarctic Peninsula. (a) Limacina helicina antarctica (shelled pteropod), (b) Gymnosomes – nonshelled pteropods that prey on shelled pteropods (p = 0.007, r² = 0.27), and (c) Clio pyramidata (shelled pteropod). Effect of environment on pteropod abundance. (d) SST vs. L. antarctica abundance, e) Sea ice advance vs. L. antarctica and Gymnosome abundance, (f) Sea ice retreat vs. C. pyramidata abundance. Data plotted are annual anomalies for each year of the time series (1993–2017). Sea ice advance is lagged 2-yr behind pteropod abundance (e.g., 2017 pteropod annual anomaly is plotted against 2015 sea ice advance annual anomaly) SST are lagged 1-yr behind L. antarctica abundance (e.g., 2017 L. antarctica annual anomaly is plotted against 2016 SST). Regression lines for significant linear relationships are shown, regression statistics are as follows: (d) SST vs. L. antarctica (circles): n = 25, p = 0.006, r² = 0.25 (e) sea ice advance vs. L. antarctica (filled-circles) and Gymnosomes (empty-circles): n = 25, p = 0.003, r² = 0.30 (dashed line); (f) sea ice retreat vs. C. pyramidata (squares): n = 14, p = 0.0003, r² = 0.64.

There was no significant influence of carbonate chemistry parameters (e.g., aragonite saturation state) on pteropod abundance, since the Western Antarctic Peninsula has yet to experience prolonged conditions characteristic of ocean acidification. However, other environmental factors such as warming and associated sea ice retreat were more influential. For example, warmer, ice-free waters in one year typically led to higher pteropod abundances the following year, suggesting that pteropods may be better adapted than expected to warming conditions due to climate change. The authors propose that earlier sea ice retreat promotes recruitment and subsequent expansion of pteropods further South, which could explain their increased abundance in this subregion. These results increase our understanding of pteropod responses to environmental variability, which is important for predicting future effects of climate change on regional carbon cycling and plankton trophic interactions in the Southern Ocean.

Authors:
Patricia S. Thibodeau (VIMS)
Deborah K. Steinberg (VIMS)
Sharon E. Stammerjohn (University of Colorado at Boulder)
Claudine Hauri (University of Alaska Fairbanks)

You better repeat it: Serial ocean acidification experiments on fish early life stages

Posted by mmaheigan

· Tuesday, March 5th, 2019

To detect potential effects of acidification on marine organisms, experimenters most commonly use within-experiment replication, but repeating the experiments themselves is rarely done. While the first approach suffices to detect major CO₂ effects, other potentially important responses may get detected and robustly quantified only via serial experimentation. A study by Baumann et al. in Biology Letters comprises a meta-analysis of 20 standard CO₂ exposure experiments conducted over six years on Atlantic silverside (Menidia menidia) offspring.

Figure 1: Robust estimate of silverside CO₂ sensitivity based on serial experimentation. (A, B) Mean CO₂ effect size calculated as the log-transformed response ratio of six early life history traits measured at 20 standard experiments between 2012-2017 (Error: bootstrapped 95% confidence intervals). (C) Seasonal change in CO₂ sensitivity in silverside early life stages. Each symbol represents an individual experiment, using offspring obtained by fertilizing wild spawners throughout their spring/summer spawning season.

Silversides are an abundant and ecologically important forage fish in the North Atlantic. The study revealed that during early life stages, Atlantic silversides tolerate pCO₂levels up to ~2,000 µatm, with seasonal shifts in sensitivity. However, this early exposure to high pCO₂ levels reduces embryo survival by 9% and overall survival by 13% (Figure 1). Future ocean acidification could cause reduced survival of these and other forage fish, and thus impact their diverse marine predators, including seabirds and commercially important fish species. This sustained experimental work resulted in the most robustly constrained estimates of average CO₂ effect sizes for a marine organism to date, demonstrating the utility of serial experimentation as a powerful tool for assessing organism responses to changing CO₂.

Authors:
Hannes Baumann
Emma L. Cross
Chris S. Murray
(all University of Connecticut)

Rapid warming and salinity changes mask acidification in Gulf of Maine waters

Posted by mmaheigan

· Wednesday, February 20th, 2019

Why don’t we see ocean acidification in over a decade of high-frequency observations in the Gulf of Maine? The answer lies in a recent decade of changes that raised sea surface temperature and salinity, and in turn dampened the expected acidification signal and caused the saturation states of calcite minerals to increase. From 2004 to 2014, sea surface temperatures in the Gulf of Maine were higher than any observations recorded in the region over the past 150 years. This greatly impacted both CO₂ solubility and the sea surface carbonate system, as detailed in a recent paper in Biogeochemistry.

Over the 34 years of the time-series, the recent event is extreme, but interannual and decadal salinity and temperature variability also influenced carbonate system parameters, which makes it difficult to isolate and quantify an anthropogenic ocean acidification signal, especially if relying on shorter-term observations (Figure 1).

Figure 1: Modeled Ω_Aragonite (top panel) and pH (bottom panel) anomalies relative to monthly 2004 data. The red lines show trends prior to and after 2004, after which warming accelerated.

For those with a stake in profiting from or managing extractive resources that are susceptible to ocean acidification such as commercially important lobster and bivalves, understanding how ecosystems will be affected is critical. These analyses clearly demonstrate how physical processes can either accelerate or mitigate ocean carbonate system changes, thus confounding the detection of ocean acidification that is expected from increasing atmospheric carbon dioxide. To assess whether an ecosystem or species is at risk or aided by such processes, it is important to observe, understand, and be able to model all sources of carbonate system variability.

Authors:
Joe Salisbury and Bror Jönsson (Both at Ocean Processes Analysis Laboratory, University of New Hampshire)

Biological and physical controls on estuarine nitrous oxide emissions

Posted by mmaheigan

· Tuesday, February 5th, 2019

Nitrous oxide (N₂O) is a potent greenhouse gas with rising atmospheric concentrations. Atmospheric emissions of N₂O are predicted to increase with continued anthropogenic perturbation of the nitrogen cycle, yet the magnitude of these emissions is uncertain, particularly in coastal systems where N₂O fluxes are poorly constrained. How do N₂O emissions from a eutrophic estuary vary in space and time?

Figure 1: Depth profiles of nitrous oxide (N₂O) (circles), salinity (dashed line), and dissolved oxygen (solid line) in the Chesapeake Bay at three stations. Solid circles indicate oversaturation of N₂O with respect to equilibrium with the atmosphere, and open circles indicate undersaturation.

In a recent publication in Estuaries and Coasts, Laperriere et al. (2018) examined how physical and biological processes influence the distribution of N₂O in Chesapeake Bay using dissolved gas measurements (N₂O and N₂/Ar) and stable isotope tracer incubations. During stratified summer conditions, the mesohaline region of the Chesapeake Bay was always a source of N₂O to the atmosphere. The highest N₂O concentrations occurred in the pycnocline at the interface between reducing bottom waters and oxygenated surface waters (Figure 1). Vertical mixing of surface waters across the pycnocline caused elevated rates of ammonia oxidation, a biological source of N₂O, and resulted in the accumulation of nitrite (NO₂^–) below the pycnocline. During periods of weak mixing, ammonia oxidation rates and N₂O concentrations were lower, and low dissolved oxygen concentrations below the pycnocline set the stage for N₂O consumption via denitrification (Figure 1). The interplay between biological and physical processes controlling fluctuations in N₂O concentration was examined using a mass balance approach. Mass balance estimates indicated that both biological processes and physical transport contribute to local changes in N₂O concentration. The authors suggest that the fate of N₂O during stratified summer conditions is governed by vertical mixing across the pycnocline, controlling whether N₂O is released to the atmosphere or consumed at depth.

Authors:
Sarah M. Laperriere (University of California, Santa Barbara)
Nicholas J. Nidzieko (University of California, Santa Barbara)
Rebecca J. Fox (Washington College)
Alexander W. Fisher (University of California, Santa Barbara)
Alyson E. Santoro (University of California, Santa Barbara)

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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