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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)

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)

The fate of the 21st century marine carbon cycle could hinge on zooplankton’s appetite

Posted by mmaheigan

· Wednesday, September 11th, 2024

Both climate change and the efforts to abate have the potential to reshape phytoplankton community composition, globally. Shallower mixed layers in a warming ocean and many marine CO₂ removal (CDR) technologies will shift the balance of light, nutrients, and carbonate chemistry, benefiting certain species over others. We must understand how such shifts could ripple through the marine carbon cycle and modify the ocean carbon reservoir. Two new publications in Geophysical Research Letters and Global Biogeochemical Cycles highlight an often over looked pathway in this response: The appetite of zooplankton.

We have long known that the appetite of zooplankton—i.e. the half-saturation concertation for grazing—varies dramatically. This variability is largely based on laboratory incubations of specific species. An open-ocean perspective has been much more elusive. Using two independent inverse modelling approaches, both studies reached the same conclusion: Even at the community level, the appetite of zooplankton in the open-ocean is incredibly diverse.

Moreover, variability in zooplankton appetites maps well onto the biogeography of phytoplankton species. As these phytoplankton niches evolve, the composition of the zooplankton will likely follow. To help understand the impact of this response on the biological pump, we compared two models, one with only two types of zooplankton, and another with an unlimited amount, each with different appetites, all individually tuned to their unique environment. Including more realistic diversity reduced the strength of the biological pump by 1 PgC yr^-1.

Figure Caption. A) Variability in the abundance and characteristic composition of phytoplankton drives B) large differences in the associated appetite and characteristic composition of zooplankton in two independent inverse modelling studies. C) When more realistic diversity in the appetite of zooplankton is simulated in models, the strength of biological pump is dramatically reduced.

That is the same order as the most optimistic scenarios for ocean iron fertilization. This means that when simulating the efficacy of many CDR scenarios, the bias introduced by insufficiently resolved zooplankton diversity could be just as large as the signal. Moving forward, it is imperative to improve the representation of zooplankton in Earth System Models to understand how the marine carbon sink will respond to inadvertent and deliberate perturbations.

Authors (GRL):
Tyler Rohr (The University of Tasmania; Australian Antarctic Program Partnership)
Anthony Richardson (The University of Queensland; CSIRO)
Andrew Lenton (CSIRO)
Matthew Chamberlain (CSIRO)
Elizabeth Shadwick (Australian Antarctic Program Partnership; CSIRO)

Authors (GBC):
Sophie Meyjes (Cambridge)
Colleen Petrick (Scripps Institute of Oceanography)
Tyler Rohr (The University of Tasmania; Australian Antarctic Program Partnership)
B.B. Cael (NOC)
Ali Mashayek (Cambridge)

Carbon sequestration by the biological pump is not exclusive to the deep ocean

Posted by mmaheigan

· Tuesday, April 16th, 2024

The biological carbon pump plays a key role in ocean carbon sequestration by transporting organic carbon from the upper ocean to deeper waters via three broad processes: the sinking of organic particles, vertical migration of organisms, and physical mixing. Most studies assume that century-scale carbon sequestration occurs only in the deep ocean, thus have missed sequestration that happens in the water column above 1,000m.

A recent publication reassessed the biological pump’s century-scale (≥100 years) carbon sequestration fluxes throughout the water column, by implementing the concept of ‘continuous vertical sequestration’ (CONVERSE). The resulting CONVERSE estimates were up to three times higher than those estimated at 1,000 m. This method shows that not only are these fluxes higher than previously thought, but also that vertical migration and physical mixing, which are generally neglected, make a significant contribution (20-30%) to carbon sequestration.

The CONVERSE method provides a new metric for calculations of the biological pump’s century-scale carbon sequestration flux that can be used to diagnose future changes in carbon sequestration fluxes in prognostic models of ocean biogeochemistry.

Interested in learning more? View more results and figures here.

Authors
Florian Ricour (Institute of Natural Sciences, Belgium)
Lionel Guidi (CNRS and Sorbonne University, France)
Marion Gehlen (CEA, CNRS and Paris-Saclay University, France)
Timothy Devries (University of California at Santa Barbara, USA)
Louis Legendre (Sorbonne University, France)

@LionelGuidi
@ComplexLov
@CNRS_INSU

Ocean iron fertilization may amplify pressures on marine biomass with only a limited climate benefit

Posted by hbenway

· Friday, January 26th, 2024

Amidst a heightened focus on the need for both drastic and immediate emissions reductions and carbon dioxide removal to limit warming to 1.5°C (IPCC, 2022), attention is returning to ocean iron fertilization (OIF) as a means of marine carbon dioxide removal (mCDR). First discussed in the early 1990s by John Martin, the concept posits that fertilization of iron-limited marine phytoplankton would lead to enhanced ocean carbon storage via a stimulation of the ocean’s biological carbon pump. However, we lack knowledge about how OIF might operate in concert with an ocean responding to climate change and what the consequences of altered nutrient consumption patterns might be for marine ecosystems, particularly for fisheries in national exclusive economic zones (EEZs). Tagliabue et al. (2023) addressed this in a recent study using state-of-the-art climate, ocean biogeochemical, and ecosystem models under a high-emissions scenario.

The study’s findings suggested that OIF can contribute at most a few 10s of Pg of mCDR under a high-emissions climate change scenario. This is equivalent to fewer than five years of current emissions and is consistent with earlier modeling assessments. This estimate is based on the modeled representation of carbon and iron cycling and a highly efficient OIF strategy that may be difficult to achieve in practice. Enhanced surface uptake of major nutrients due to OIF also led to a drop in global net primary production, in addition to that due to climate change alone. By then coupling a complex model of upper trophic levels, the projected declines in animal biomass due to climate change were amplified by around a third due to OIF, with the most negative impacts projected to occur in the low latitude EEZs, which are already facing increasing pressures due to climate change.

This work highlights feedbacks within the ocean’s biogeochemical and ecological systems in response to OIF that emerged over large spatial and temporal scales. Associated pressures on marine ecosystems pose major challenges for proposed management and monitoring. Restricting OIF to the highest latitudes of the Southern Ocean might mitigate some of these negative effects, but this only further reduces the minor mCDR benefit, suggesting that OIF may not make a significant contribution.

Authors
A. Tagliabue (Univ. Liverpool)
B. S. Twining (Bigelow Laboratory)
N. Barrier & O. Maury (MARBEC, IRD, IFREMER, CNRS, Université de Montpellier, France)
M. Berger & Laurent Bopp (ENS-LMD, Paris, France)

IPCC. Summary for Policymakers. in Climate Change, 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (eds. Shukla, P. R. et al.) (Cambridge University Press, 2022).

New evidence suggests that tiny zooplankton might be the biggest problem with carbon cycling in IPCC climate models

Posted by mmaheigan

· Friday, December 1st, 2023

The ocean is the most important sink of anthropogenic emissions and is being considered as a medium to manipulate to draw down even more. Essential in the ocean’s role as a natural carbon-sponge is the net production of organic matter by phytoplankton, some of which sinks and is stored for 100s-1000s of years. Successfully simulating this biological carbon pump is essential for projecting any climate scenario, but it appears that massive uncertainties in the way zooplankton consume phytoplankton are compromising predictions of future climate and our assessment of some strategies to deliberately engineer it.

Figure caption. Grazing pressure is largest source of uncertainty for marine carbon cycling in CMIP6 models a) The global and zonal median winter grazing pressure is shown for all models. b) the coefficient of variation across models (std/mean) is largest for grazing pressure compared 14 major terms in the marine carbon cycle.

A new publication in Communications Earth and Environment explains how our poor understanding of zooplankton biases our best projections of marine carbon sequestration. We compared 11 IPCC climate models and found zooplankton grazing is largest source uncertainty in marine carbon cycling. This uncertainty is over three times larger than that of net primary production and is driven by large differences in different models assumptions about the rate at which zooplankton can consume phytoplankton. Yet, very small changes in zooplankton grazing dynamics (roughly only 5% of the full range used across IPCC models) can increase carbon sequestrations by 2 PgC/yr, which is double the maximum theoretical potential of Southern Ocean Iron Fertilization! Moving forward, to move beyond merely treating zooplankton as a closure term, modelers must look towards novel observational constraints on grazing pressure.

Authors
Tyler Rohr, Anthony J. Richardson, Andrew Lenton, Matthew A. Chamberlain, and Elizabeth H. Shadwick

Size does matter: larger krill leads to more POC export in the West Antarctic Peninsula

Posted by mmaheigan

· Friday, December 1st, 2023

Despite the importance of particulate organic carbon (POC) export on carbon sequestration and marine ecology, there have been few multi-decade studies in the world’s oceans. A new analysis published in Nature analyzed two decades of POC export data in the West Antarctic Peninsula and found that export oscillates on a 5-year cycle.

Figure caption: A) Particulate organic carbon (POC) export oscillates on a 5-year timescale in sync with the oscillation in the body size of the krill Euphausia superba on the West Antarctic Peninsula. B) POC export is significantly correlated with krill body size (p = 0.01).

Using a unique combination of krill data from penguin diet samples and net tows over two decades, Trinh et al. found that the cycle of POC export is intimately tied to the Antarctic krill (Euphausia superba) life cycle, as the bulk of the POC in their sediment traps was krill fecal pellets. Surprisingly, more krill did not lead to more POC export. Instead, when the krill population size was smaller but dominated by larger, older adults, POC export increased.

E. superba is the longest-lived (5-6 years) and largest krill species. They exhibit continuous annual growth throughout their life cycle. After about five years a krill population reaches its end stage and the population size is at a minimum. This end-stage population is composed of large, 50-60 mm long individuals that produce large, fast-sinking fecal pellets, leading to increased POC export. Increasing temperatures and deterioration of sea ice cover during the winter season due to climate change will likely impact the recruitment of new cohorts of krill and their success in replenishing aging populations. It is unclear how changes in the krill population and life cycle will impact long-term carbon sequestration on the West Antarctic Peninsula and nutrients exported to the benthic ecosystem

Authors:
Rebecca Trinh (Lamont Doherty Earth Observatory, Columbia University)
Hugh Ducklow (Lamont Doherty Earth Observatory, Columbia University)
Deborah Steinberg (Virginia Institute of Marine Science, College of William and Mary)
William Fraser (Polar Oceans Research Group)

Hydrostatic pressure substantially reduces deep-sea microbial activity

Posted by mmaheigan

· Thursday, May 11th, 2023

Deep sea microbial communities are experiencing increasing hydrostatic pressure with depth. It is known that some deep sea microbes require high hydrostatic pressure for growth, but most measurements of deep-sea microbial activity have been performed under atmospheric pressure conditions.

In a recent paper published in Nature Geoscience, the authors used a new device coined ‘In Situ Microbial Incubator’ (ISMI) to determine prokaryotic heterotrophic activity under in situ conditions. They compared microbial activity in situ with activity under atmospheric pressure at 27 stations from 175 to 4000 m depths in the Atlantic, Pacific, and the Southern Ocean. The bulk of heterotrophic activity under in situ pressure is always lower than under atmospheric pressure conditions and is increasingly inhibited with increasing hydrostatic pressure. Single-cell analysis revealed that deep sea prokaryotic communities consist of a small fraction of pressure-loving (piezophilic) microbes while the vast majority is pressure-insensitive (piezotolerant). Surprisingly, the piezosensitive fraction (~10% of the total community) responds with a more than 100-fold increase of activity upon depressurization. In the microbe proteomes, the authors uncovered taxonomically characteristic survival strategies in meso- and bathypelagic waters. These findings indicate that the overall heterotrophic microbial activity in the deep sea is substantially lower than previously assumed, which implies major impacts on the carbon budget of the ocean’s interior.

Figure caption: Deep sea microbial activity under varying pressure. (a) In situ bulk leucine incorporation rates normalized to rates obtained at atmospheric pressure conditions. (b) A microscopic view of a 2000 m sample collected in the Atlantic and incubated under atmospheric pressure conditions. The black halos around the cells are silver grains corresponding to their activities. The highly active cells (indicated by arrows) were rarely found in in situ pressure incubations. (c) Depth-related changes in the metaproteome of three abundant deep sea bacterial taxa (Alteromonas, Bacteroidetes, and SAR202). The number indicates shared and unique up- and down-regulated proteins in different depth zones.

Authors
Chie Amano (University of Vienna, Austria)
Zihao Zhao (University of Vienna, Austria)
Eva Sintes (University of Vienna, IEO-CSIC, Spain)
Thomas Reinthaler (University of Vienna, Austria)
Julia Stefanschitz (University of Vienna, Austria)
Murat Kisadur (University of Vienna, Austria)
Motoo Utsumi (University of Tsukuba, Japan)
Gerhard J. Herndl (University of Vienna, Netherlands Institute for Sea Research)

Twitter @microbialoceanW

Severe warming = 15% increase in bacterial respiration: Southern Ocean most impacted

Posted by mmaheigan

· Thursday, March 30th, 2023

The utilization, respiration, and remineralization of organic matter exported from the ocean surface to its depths are key processes in the ocean carbon cycle. Marine heterotrophic Bacteria play a critical role in these activities. However, most three-dimensional (3-D) coupled physical-biogeochemical models do not explicitly include Bacteria as a state variable. Instead, they rely on parameterization to account for the bacteria’s impact on particle flux attenuation.

A recent study examined how bacteria respond to climate change by employing a 3-D coupled ocean biogeochemical model that incorporates explicit bacterial dynamics. The model (CMCC-ESM2) is a part of the Coupled Model Intercomparison Project Phase 6. The authors first evaluated the reliability of century-scale forecasts (2015-2099) for bacterial stocks and rates in the upper 100 m layer against the compiled measurements from the contemporary period (1988-2011). Next the authors analyzed the predicted trends in bacterial stocks and rates under diverse climate scenarios and explored their association with regional differences in temperature and organic carbon stocks. Three crucial findings were revealed. There is a global-scale decrease in bacterial biomass of 5-10%, with a 3-5% increase in the Southern Ocean (Figure 1). In the Southern Ocean, the rise in semi-labile dissolved organic carbon (DOC) leads to an increase in DOC uptake rates of free-living bacteria; in the northern high and low latitudes, the increase in temperature drives the increase in their DOC uptake rates. Importantly, extreme warming could result in a global increase (up to 15%) and even higher in the Southern Ocean (21% increase) in bacterial respiration (Figure 1), potentially leading to a decline in the biological carbon pump.

This analysis is an unprecedented and early effort to understand the climate-induced changes in bacterial dynamics on a global scale in a systematic manner. This study takes us one step closer to comprehending how bacteria influence the functioning of the biological carbon pump and the distribution of organic carbon pools between surface and deep layers, especially their response to climate change.

Figure 1. Global projections of bacterial carbon stocks and rates during the baseline period (1990-2013) and their changes as anomalies under the most-severe climate change scenario (i.e., SSP5-8.5) relative to the baseline period (2076-2099). The stocks and rates during the baseline period (a, b, c, g, h, i) and their changes as anomalies under the most-severe climate change scenario (d, e, f, j, k, l). All variables are depth-integrated in the upper 100 m. Solid-line contours as standard deviation from averaging over 1990-2013. Anomalies are 2076-2099 average values relative to 1990-2013 average values. Global bacterial biomass has decreased by 5-10%, with a 3-5% increase in the Southern Ocean. However, extreme warming may increase bacterial respiration worldwide, thereby reducing the efficiency of the biological carbon pump. This study provides an early attempt to understand the response of bacteria to climate change and their impact on the distribution of organic carbon in the ocean.

Author
Heather Kim, Woods Hole Oceanographic Institution

Small particles contribute significantly to the biological carbon pump in the subpolar North Atlantic

Posted by mmaheigan

· Monday, February 13th, 2023

The ocean’s biological carbon pump (BCP) is a collection of processes that transport organic carbon from the surface to the deep ocean where the carbon is sequestered for decades to millennia. Variations in the strength of the BCP can substantially change atmospheric CO₂ levels and affect the global climate. It is important to accurately estimate this carbon flux, but direct measurement is difficult so this remains a challenge.

Figure 1. (a) A schematic illustrating the downward transport of small and large POC into the deep ocean and the subsequent remineralization and fragmentation which breaks large POC into small POC. (b) Trajectories of BGC-Argo float segments. (c) Relative contributions to the annually averaged vertical carbon flux show the dominant role of gravitational sinking flux of large POC as well as the significant contributions from small POC at 100 m due to different mechanisms and at 600 m due to fragmentation.

A recent paper published in Limnology and Oceanography performed a novel mass budget analysis using observations of dissolved oxygen and particulate organic carbon (POC) from BGC-Argo floats in the subpolar North Atlantic. The authors assessed relative importance of different mechanisms contributing to the BCP and related processes, the sinking velocity and remineralization rate of different particle size classes as well as the rate of fragmentation which breaks large particles into smaller ones. Results suggest that on annual timescales, the gravitational settling of large POC is the dominant mechanism. Small POC supplements the vertical carbon flux at 100 m significantly, through various mechanisms, and contributes to carbon sequestration below 600 m due to fragmentation of large POC. In addition, sensitivity experiments highlight the importance of considering remineralization and fragmentation when estimating the vertical carbon flux of small POC.

This novel method provides additional independent constraints on current estimates and improves our mechanistic understanding of the BCP. In addition, it demonstrates the great potential of BGC-Argo float data for studying the biological carbon pump.

Authors:
Bin Wang (Dalhousie University)
Katja Fennel (Dalhousie 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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