SciHigh Slider-Home :: Ocean Carbon & Biogeochemistry

Studying marine ecosystems and biogeochemical cycles in the face of environmental change

Archive for SciHigh Slider-Home

Microbial Iron limitation in the ocean’s twilight zone

Posted by mmaheigan

· Monday, March 31st, 2025

How deep in the ocean do microbes feel the effects of nutrient limitation? Microbial production in one third of the surface ocean is limited by the essential micronutrient iron (Fe). This limitation extends to at least the bottom of the euphotic zone, but what happens below that?

In a study that recently published in Nature we investigated the abundance and distribution of siderophores, small metabolites synthesized by bacteria to promote Fe uptake. When environmental Fe concentrations become limiting and microbes become Fe deficient, some bacteria release siderophores into the environment to bind iron and facilitate its uptake. Siderophores are therefore a window into how microbes “see” environmental Fe. We found that siderophore concentrations were high in low Fe surface waters, but surprisingly we also found siderophores to be abundant in the twilight zone (200-500 m) underlying the North and South Pacific subtropical gyres, two key ecosystems for the marine carbon cycle. In shipboard experiments with siderophores labeled with the rare ⁵⁷Fe isotope, we found rapid uptake of the label in twilight zone samples. After removing ⁵⁷Fe from the ⁵⁷Fe-siderophores complex, bacteria released the now unlabeled siderophores back into seawater to complex additional Fe (Figure. 1).

Figure 1: Iron-siderophore cycling in the twilight zone. When the seawater becomes Fe-deficient, some bacteria are able to synthesize siderophores and release them into the environment (middle left). These metabolites bind Fe (middle right) and the Fe-siderophore complex is taken up by bacteria using specialized TonB dependent transporters (TBDT; bottom right). Inside the cell, Fe is recovered from the Fe-siderophore complex (bottom left) and the siderophore excreted back into the environment to start the cycle anew.

Our results show that in large parts of the ocean microbes feel the effects of nutrient limitation deep in the water column, to at least 500 m. This greatly expands the region of the ocean where nutrients limit microbial metabolism. The effects of limitation this deep in the water column are unexplored, but twilight zone Fe deficiency could have unanticipated consequences for the efficiency of the ocean’s biological carbon pump.

Authors
Jingxuan Li, Lydia Babcock-Adams and Daniel Repeta
(all at Woods Hole Oceanographic Institution)

How do ocean microbes share the job of denitrification?

Posted by mmaheigan

· Monday, March 31st, 2025

Denitrification is a crucial multi-step process for ecosystem productivity and sustainability because some of its steps can result in the loss of the essential nutrient nitrogen or the production of greenhouse gas nitrous oxide. We do not understand why microbial functional groups conducting different steps of denitrification can coexist in the ocean and why certain groups are more abundant than others.

In a recent study published in PNAS, we uncover ecological mechanisms that govern the coexistence of these microbes. For the microbial groups utilizing different nitrogen substrates, the “stronger” groups rely on the “weaker” groups to feed them nitrogen (with respect to the organic substrates that they compete for), enabling them to coexist. For the groups competing for the same nitrogen substrates, microbes that invest more to build longer denitrification steps win the competition when nitrogen is limiting, but lose the game when nitrogen is repleted and organic carbon is limiting. The spatial and temporal variability of nutrients in the ocean allows these microbes to be observed in the same water mass.

Figure caption: Temporal and spatial heterogeneity in nutrients promotes the coexistence of functionally diverse denitrifiers in the ocean.

These hypothesized coexistence patterns help us predict where and when nitrogen loss and nitrous oxide production may occur. As human activities continue to alter marine nutrient balances, these predictions help us better anticipate ocean responses and design better strategies for mitigating negative anthropogenic impacts on the ocean.

Authors
Xin Sun (Carnegie Institution for Science) @xinsun-putiger.bsky.social
Emily Zakem (Carnegie Institution for Science) @carnegiescience.bsky.social

The ocean is shifting toward phosphorus limitation

Posted by mmaheigan

· Friday, February 28th, 2025

Biogeochemical models predict that ocean warming is weakening the vertical transport of nutrients to the upper ocean, with severe implications for marine productivity. However, nutrient concentrations across the ocean surface often fall below detection limits, making it difficult to observe long-term changes.

In a recent study in PNAS, we analyzed over 30,000 nitrate and phosphate depth profiles observed between 1972 and 2022 to quantify nutricline depths, where nutrient concentrations are reliably detected. These depths accurately represent nutrient supplies in a global model, allowing us to assess long-term trends. Over the past five decades, upper ocean phosphate has mostly declined worldwide, while nitrate has remained mostly stable. Model simulations support that this difference is likely due to nitrogen fixation replenishing upper ocean nitrate, whereas phosphate has no equivalent biological source.

Figure caption: Five decades of global and regional nutricline depth data reveal declining phosphate-to-nitrate trends. Nutricline depths were defined based on threshold concentrations of 3 μmol kg−1 nitrate (TNO3) and 3/16 μmol kg−1 phosphate (TPO4). Site-specific trends were quantified for each unique pair of geographic coordinates where sufficient data was available (TNO3, n = 1,859 sites; TPO4, n = 1,641 sites). Shown are 95% confidence intervals (CI95%) calculated for each median trend by generating 10,000 bootstrap samples. The curves over the histograms depict the kernel densities. The sets of error bars from top to bottom are the interquartile ranges of TNO3 and TPO4 from a monthly climatology, the total observations, and the total observations with added measurement error.

These findings suggest that the ocean is becoming more limited in phosphorus. This decline could make phytoplankton less nutritious for marine animals. Fish larvae growth rates correlate with phosphorus availability in the ecosystem, so intensifying phosphorus limitation may greatly impact fisheries worldwide.

Authors
Skylar Gerace (University of California, Irvine)
Jun Yu (University of California, Irvine)
Keith Moore (University of California, Irvine)
Adam Martiny (University of California, Irvine)

@UCI_OCEANS

Persistent bottom trawling impairs seafloor carbon sequestration

Posted by mmaheigan

· Friday, February 28th, 2025

Bottom trawling, a fishing method that uses heavy nets to catch animals that live on and in the seafloor, could release a large amount of organic carbon from seafloor into the water, that metabolizes to CO₂ then outgasses to the atmosphere. The magnitude of this indirect emission has been heavily debated, with estimates spanning from negligibly small to global climate relevant. Thus, a lack of reliable data and insufficient understanding of the process hinders management of bottom trawling for climate protection.

We set out to solve this problem in two steps. First, we analyzed a large field dataset containing more than 2000 sediment samples from one of the most intensely trawled regions globally, the North Sea. We identified a trawling-induced carbon reduction trend in the data, but only in samples taken in persistently intensively trawled areas with multi-year averaged swept area ratio larger than 1 yr^-1. In less intensely trawled areas, there was no clear effect. In a second step, we applied numerical modelling to understand the processes behind the observed change (Fig. 1). Our model results suggest that bottom trawling annually releases one million tonnes of CO₂ in the North Sea and 30 million tonnes globally. Along with sediment resuspension in the wake of the trawls, the main cause for altered sedimentary carbon storage is the depletion of macrofauna, whose locomotion and burrowing effectively buries freshly deposited carbon into deeper sediment layers. By contrast, macrofauna respiration is reduced owing to trawling-caused mortality, partly offsetting the organic carbon loss. Following a cessation of trawling, the simulated benthic biomass can recover in a few years, but the sediment carbon stock would take several decades to be restored to its natural state.

Figure 1. (a) Benthic–pelagic coupling in a natural system. (b) Processes involved in bottom trawling. (c) Model-estimated source and sink terms of organic carbon in surface sediments in the No-trawling (solid fill, n = 67 annual values for 1950–2016) and trawling (pattern fill, n = 67 ensemble-averaged values for 1950–2016) scenarios of the North Sea. © 2024, Zhang, W. et al., CC BY 4.0.

Marine conservation strategies traditionally favor hard bottoms, such as reefs, that are biologically diverse but accumulate limited amounts of organic carbon. Our results indicate that carbon in muddy sediments is more susceptible to trawling impacts than carbon in sand and point out a need to safeguard muddy habitats for climate protection. Our methods and results might be used in the context of marine spatial planning policies to gauge the potential benefits of limiting or ending bottom trawling within protected areas.

Zhang, W., Porz, L., Yilmaz, R. et al. Long-term carbon storage in shelf sea sediments reduced by intensive bottom trawling. Nat. Geosci. 17, 1268–1276 (2024). https://doi.org/10.1038/s41561-024-01581-4

Authors
Wenyan Zhang (Hereon)
Lucas Porz (Hereon)
Rümeysa Yilmaz (Hereon)
Klaus Wallmann (GEOMAR)
Timo Spiegel (GEOMAR)
Andreas Neumann (Hereon)
Moritz Holtappels (AWI)
Sabine Kasten (AWI)
Jannis Kuhlmann (BUND)
Nadja Ziebarth (BUND)
Bettina Taylor (BUND)
Ha Thi Minh Ho-Hagemann (Hereon)
Frank-Detlef Bockelmann (Hereon)
Ute Daewel (Hereon)
Lea Bernhardt (HWWI)
Corinna Schrum (Hereon)

Rain increases the global carbon sink

Posted by mmaheigan

· Tuesday, January 28th, 2025

The global ocean dampens the anthropic CO₂ increase in the atmosphere by absorbing around 25% of the carbon emitted each year. Of the processes involved in exchanges of energy and mass between ocean and atmosphere that may impact this carbon sink, rainfall has never been systematically and comprehensively quantified. A study recently published in Nature Geosciences suggests that about 6% of the global ocean CO₂ sink is mediated by rainfall.

Figure 1. Histograms of 2008-2018 global ocean (60°S-60°N) CO2 sink increase due to rain-induced turbulence only, rain-induced dilution only, the resultant of turbulence and dilution (named the interfacial effect), the wet deposition of CO2 absorbed during the raindrops fall and the total (interfacial plus wet deposition) using 1-h rain rates from IMERG (blue) and ERA5 (red). The rain-induced dilution is diagnosed from a satellite-derived empirical relationship (full) or a 1D physical model (stripes). Figure based on Parc et al. (2024) Table 1.

The exchange of CO₂ at the ocean interface is controlled by chemical, physical, and biological properties and processes. Rainfall, one of these processes, can alter the properties of the ocean surface and perturb the carbon exchange in three ways:

(i) Turbulence: Raindrops increase the momentum transfer to the ocean and generate turbulence enhancing the renewal of interfacial water (first column in Fig. 1). This tends to increase both in- and out-gassing. The impact of this effect alone is weak because wind dominates the generation of turbulence in the ocean;

(ii) Dilution + Interfacial: Rain dilutes and cools the near surface waters, which perturbs chemical equilibria and leads the ocean to absorb more CO₂ (second column in Fig. 1). The result of dilution and turbulence effects of rain, which is named “Interfacial”, is a clear increase in global CO₂ sink (third column in Fig. 1);

(iii) Wet deposition: Finally, raindrops directly inject CO₂ molecules into the ocean that they absorbed during their fall through the atmosphere (fourth column in Fig. 1).

Using two rainfall datasets (the satellite-derived product IMERG and the ERA5 reanalysis) and two ways to quantify the rain-induced dilution, the authors show that rain increases the ocean carbon sink by 140 to 190 million tonnes of carbon per year, equivalent to 5% to 7% of the 2.66 billion tonnes of carbon absorbed annually by the oceans. Because rainfall amounts and patterns will change in the future, impacting the ocean carbon sink, these results call for explicitly including rain effects in the annual global carbon budget estimates.

Authors
Laëtitia Parc (Laboratoire de Météorologie Dynamique)
Hugo Bellenger (Laboratoire de Météorologie Dynamique)
Laurent Bopp (Laboratoire de Météorologie Dynamique) @bopplaurent.bsky.social
Xavier Perrot (Laboratoire de Météorologie Dynamique)
David T. Ho (University of Hawaii at Manoa; [C]Worthy) @davidho.bsky.social

Parc, L., Bellenger, H., Bopp, L., Perrot, X., and Ho, D. T. Global ocean carbon uptake enhanced by rainfall.Nat. Geosci. 17, 851–857 (2024). https://doi.org/10.1038/s41561-024-01517-y

Deep dive into carbon transport: How bacteria feast and compete on lipids in sinking particles

Posted by mmaheigan

· Tuesday, January 28th, 2025

What drives carbon from the atmosphere to the deep ocean? The journey of phytoplankton-derived carbon is critical in the global carbon cycle, yet the influence of interacting bacteria in degrading lipid-rich particles during their descent has remained a mystery—until now.

Using an innovative combination of nano-scale lipidomics and microscopy, researchers investigated how bacteria target and degrade diverse lipid molecules in sinking oceanic particles. The study, published in Science, revealed that bacteria exhibit distinct dietary preferences, governed by their lipid-degrading genes rather than taxonomic affiliation. Interactions among bacteria influenced both degradation rates and timing, together reshaping our view on the efficiency of lipid transport to the ocean depths. These findings were incorporated into a mathematical model, revealing how microbial communities could regulate the carbon transfer efficiency.

This research enhances our understanding of the ocean’s carbon pump, highlighting the pivotal role of bacterial communities in carbon sequestration. By uncovering how microbial interactions affect carbon transfer, these findings improve climate models and support the development of strategies to mitigate atmospheric CO₂.

Authors
Lars Behrendt (Uppsala University, Sweden)
Benjamin van Mooy (Woods Hole Oceanographic Institution)

Twitter: LarsBehrendt4
Bluesky: @belab1.bsky.social

OA could boost carbon export by appendicularia

Posted by mmaheigan

· Wednesday, December 4th, 2024

Gelatinous zooplankton comprise a widespread group of animals that are increasingly recognized as important components of pelagic ecosystems. Historically understudied, we have little knowledge of how much key taxa contribute to carbon fluxes. Likewise, there’s a critical knowledge gap of the impact of ocean change on these taxa.

Appendicularia are the most abundant gelatinous zooplankton in the world oceans. Their population dynamics display typical boom-and-bust characteristics, i.e. high grazing rates in combination with a short generation time and life cycle, results in intense blooms. The most prominent feature of appendicularians is their mucous feeding-structure (“house”), which is produced and discarded several times per day. These sinking houses can contribute substantially to carbon export.

Figure 1: Influence of ocean acidification on the Appendicularia Oikopleura dioica and carbon export. Appendicularian populations display typical boom-and-bust characteristics, resulting in intense blooms. The sinking of appendicularians’ discarded mucous feeding-structure several times per day can contribute substantially to carbon export. Low pH conditions (as expected for future ocean acidification extreme events) enhanced its population growth and contribution to carbon fluxes shown above (red lines/diamonds) vs ambient (blue lines/diamonds).
(Figure sources: Picture by Jean-Marie Bouquet, data plots from Taucher et al. (2024): The appendicularian Oikopleura dioica can enhance carbon export in a high CO2 ocean. Global Change Biology, doi:10.1111/gcb.17020)

A recent study in Global Change Biology quantified how much appendicularia can contribute to carbon export via the biological pump, and how this carbon flux could markedly increase under future ocean acidification and associated extreme pH events.

The findings are based on a large-volume in situ experimental approach that allowed observing natural plankton populations and carbon export under close-to-natural conditions for almost two months. Thereby, O. dioica population dynamics could be directly linked to sediment trap data to quantify the influence of this key species on carbon fluxes at unprecedented detail. During the appendicularia bloom up to 39% of total carbon export was attributed to them.

The most striking finding was that high CO₂ conditions elevated carbon export by appendicularia increased by roughly 50%. Appendicularians physiologically benefit from low pH conditions, giving them a competitive advantage over other zooplankton, allowing them to contribute to a disproportionally large role in carbon export from the ecosystem.

Authors
Jan Taucher (GEOMAR)
Anna Katharina Lechtenbörger (GEOMAR)
Jean-Marie Bouquet (University of Bergen)
Carsten Spisla (GEOMAR)
Tim Boxhammer (GEOMAR)
Fabrizio Minutolo (GEOMAR)
Lennart Thomas Bach (University of Tasmania)
Kai T. Lohbeck (University of Konstanz)
Michael Sswat (GEOMAR)
Isabel Dörner (GEOMAR)
Stefanie M. H. Ismar-Rebitz (GEOMAR)
Eric M. Thompson (University of Bergen)
Ulf Riebesell (GEOMAR)

Quantifying uncertainties in future projections of Chesapeake Bay Hypoxia

Posted by mmaheigan

· Wednesday, December 4th, 2024

Climate change is expected to especially impact coastal zones, worsening deoxygenation in the Chesapeake Bay by reducing oxygen solubility and increasing remineralization rates of organic matter. However, simulated responses of this often fail to account for uncertainties embedded within the application of future climate scenarios.

Recent research published in Biogeosciences and in Scientific Reports sought to tackle multiple sources of uncertainty in future impacts to dissolved oxygen levels by simulating multiple climate scenarios within the Chesapeake Bay region using a coupled hydrodynamic-biogeochemical model. In Hinson et al. (2023), researchers showed that a multitude of climate scenarios projected a slight increase in hypoxia levels due solely to watershed impacts, although the choice of global earth system model, downscaling methodology, and watershed model equally contributed to the relative uncertainty in future hypoxia estimates. In Hinson et al. (2024), researchers also found that the application of climate change scenario forcings itself can have an outsized impact on Chesapeake Bay hypoxia projections. Despite using the same inputs for a set of three experiments (continuous, time slice, and delta), the more commonly applied delta method projected an increase in levels of hypoxia nearly double that of the other experiments. The findings demonstrate the importance of ecosystem model memory, and fundamental limitations of the delta approach in capturing long-term changes to both the watershed and estuary. Together these multiple sources of uncertainty interact in unanticipated ways to alter estimates of future discharge and nutrient loadings to the coastal environment.

Figure 1: Chesapeake Bay hypoxia is sensitive to multiple sources of uncertainty related to the type of climate projection applied and the effect of management actions. Percent contribution to uncertainty from Earth System Model (ESM), downscaling methodology (DSC), and watershed model (WSM) for estimates of (a) freshwater streamflow, (b) organic nitrogen loading, (c) nitrate loading, and (d) change in annual hypoxic volume (ΔAHV). (e) Summary of all experiment results for ΔAHV, expressed as a cumulative distribution function. The Multi-Factor experiment (blue line) used a combination of multiple ESMs, DSCs, and WSMs, the All ESMs experiment (pink line) simulated 20 ESMs while holding the DSC and WSM constant, and the Management experiment (green line) only simulated 5 ESMs with a single DSC and WSM but incorporated reductions in nutrient inputs to the watershed. The vertical dashed black line marks no change in AHV.

Understanding the relative sources of uncertainty and impacts of environmental management actions can improve our confidence in mitigating negative climate impacts on coastal ecosystems. Better quantifying contributions of model uncertainty, that is often unaccounted for in projections, can constrain the range of outcomes and improve confidence in future simulations for environmental managers.

Figure 2: A schematic of differences between the Continuous and Delta experiments. In the Delta experiment a combination of altered distributions in future precipitation and changes to long-term soil nitrogen stores eventually result in increased levels of hypoxia (right panel).

Authors
Kyle E. Hinson (Virginia Institute of Marine Science, William & Mary)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science, William & Mary)
Raymond G. Najjar (The Pennsylvania State University)
Maria Herrmann (The Pennsylvania State University)
Zihao Bian (Auburn University)
Gopal Bhatt (The Pennsylvania State University, USEPA Chesapeake Bay Program Office)
Pierre St-Laurent (Virginia Institute of Marine Science, William & Mary)
Hanqin Tian (Boston College)
Gary Shenk (USGS Virginia/West Virginia Water Science Center)

Swirling Currents: How Ocean Mesoscale Affects Air-Sea CO2 Exchange

Posted by mmaheigan

· Friday, October 25th, 2024

Due to a sparsity of in‐situ observations and the computational burden of eddy‐resolving global simulations, there has been little analysis on how mesoscale processes (e.g., eddies, meanders—lateral scales of 10s to 100s km) influence air‐sea CO₂ fluxes from a global perspective. Recently, it became computationally feasible to implement global eddy‐resolving [O (10) km] ocean biogeochemical models. Many questions related to the influence of mesoscale motions on CO₂ fluxes remain open, including whether ocean eddies serve as hotspots for CO₂ sink or source in specific dynamic regions.

A recent study in Geophysical Research Letters investigated the contribution of ocean mesoscale variability to air-sea CO₂ fluxes by analyzing the CO₂ flux anomaly within the mesoscale band using a coarse-graining approach in a global eddy-resolving biogeochemical simulation. We found that in eddy-rich mid-latitude regions, ocean mesoscale variability can contribute to over 30% of the total CO₂ flux variability. The cumulative net CO₂ flux associated with mesoscale motions is on the order of 10⁵ tC per year. The global pattern of cumulative mesoscale-related CO₂ flux exhibits significant spatial heterogeneity, with the highest values in western boundary currents, the Antarctic Circumpolar Current, and the equatorial Pacific. The local distribution of cumulative mesoscale-related CO₂ flux displays zonal bands alternate between positive (a net source) and negative (a net sink) due to the meandering nature of ocean mesoscale currents, which is related to local relative vorticity and the background cross-stream pCO₂ gradient.

Figure caption. Mesoscale (<nominal 2 degree) contribution to air‐sea CO₂ flux (F^<2°_CO2)in the model. (a)–(d) Monthly time series of F^<2°_CO2 (black lines) and cumulative F^<2°_CO2 (green/red solid lines) in four locations marked in (e). Dashed lines are the least squares regression of cumulative flux for the period 1982–2000; slopes are indicated in the bottom left; (e) Blue colors imply a CO₂ sink, and red colors represent a source. The figure shows the global distribution of the regressed slopes of cumulative F^<2°_CO2. Units are converted from mol m^-2 per year to kg of CO₂ per year using the atomic mass of CO₂. This figure shows significant spatial heterogeneity of mesoscale-modulated CO₂ flux, showing contributions to both CO₂ sources and sinks across different regions of the ocean, with a magnitude on the order of 10⁵ tC per year.

Authors
Yiming Guo (Yale University; now at Woods Hole Oceanographic Institution)
Mary-Louise Timmermans (Yale University)

How tiny teeth and their prey shape ocean ecosystems

Posted by mmaheigan

· Friday, October 25th, 2024

It has long been suggested that diatoms, microscopic algae enclosed in silica-shells, developed these structures to defend against predators like copepods, small crustaceans that graze diatoms. Copepods evolved silica-lined teeth presumably to counteract this. But actual evidence for how this predator-prey relationship may drive natural selection and evolutionary change has been lacking.

Figure caption: Left: Copepod teeth may suffer damage when feeding on thick-shelled diatoms. The red arrows indicate damage to the copepod tooth, cracks or missing setae. When fed a large diatom, the row of spinose cusps was damaged in all analyzed teeth. Scale bar = 10 µm. Right: A Temora longicornis (ca. 750 µm) copepod tethered to a human hair using super glue, allowing for the capture of high-speed videography to quantify the fraction of cells that eaten or discarded by the copepod. The hair was kindly provided by the first author’s wife.

A recent publication in Proceedings of the National Academy of Sciences U.S.A. revealed a fascinating dynamic: Copepods that feed on diatoms may suffer significant damage to their teeth, causing them to become more selective eaters. The wear and tear on the copepod teeth were particularly pronounced when copepods consumed thick-shelled diatoms compared to “softer” prey like a dinoflagellate. By glueing copepods to human hair and filming them with a high-speed video camera, the authors found that copepods with damaged teeth were more likely to reject diatoms with thick shells than those with thin shells as prey. Shell thickness varies among and within diatom species and some can respond to copepod presence by increasing shell thickness. A thicker shell, however, may come at a cost to the cell in terms of reduced growth rate or increased sinking speed. This suggests that the evolutionary “arms race” between diatoms and copepods plays a crucial role in shaping and sustaining the diversity of these species.

Diatoms and copepods are important organisms in global biogeochemical cycles and hence understanding this microscopic interaction can help predict shifts in marine ecosystems, potentially affecting nutrient cycles and food webs that support fisheries.

Authors
Fredrik Ryderheim (Technical University of Denmark/University of Copenhagen)
Jørgen Olesen (University of Copenhagen)
Thomas Kiørboe (Technical University of Denmark)

Twitter
@fryderheim (Fredrik Ryderheim)
@OlesenCrust (Jørgen Olesen)
@Thomaskiorboe (Thomas Kiørboe)
@OceanLifeCentre (FR, TK group at DTU)
@NHM_Denmark (Natural History Museum of Denmark, JO employer)

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.

Archive for SciHigh Slider-Home

Filter by Keyword