Archive for nutrient limitation

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

Exploiting phytoplankton as a biosensor for nutrient limitation

Posted by mmaheigan 
· Wednesday, September 15th, 2021 

In the surface ocean, phytoplankton growth is often limited by a scarcity of key nutrients such as nitrogen, phosphorus, and iron. While this is important, there are methodological and conceptual difficulties in characterizing these nutrient limitations.

A recent paper published in Science Magazine leveraged a global metagenomic dataset from Bio-GO-SHIP to address these challenges. The authors characterized the abundance of genes that confer adaptations to nutrient limitation within the picocyanobacteria Prochlorococcus. Using the relative abundance of these genes as an indicator of nutrient limitation allowed the authors to capture expected regions of nutrient limitation, and novel regions that had not previously been studied. This gene-derived indicator of nutrient limitation matched previous methods of assessing nutrient limitation, such as bottle incubation experiments.

These findings have important implications for the global ocean. Characterizing the impact of nutrient limitation on primary production is especially critical in light of future stratification driven by climate change. In addition, this novel methodological approach allows scientists to use microbial communities as an eco-genomic biosensor of adaptation to changing nutrient regimes. For instance, future studies of coastal microbes or other ecosystems may help communities and environmental managers better understand how local microbial populations are adapting to climate change.

 

Watch an illustrated video overview of this research

Authors:
Lucas J. Ustick, Alyse A. Larkin, Catherine A. Garcia, Nathan S. Garcia, Melissa L. Brock, Jenna A. Lee, Nicola A. Wiseman, J. Keith Moore, Adam C. Martiny
(all University of California, Irvine)

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)

 

Nutrient and carbon limitation drive broad-scale patterns of mixotrophy in the ocean

Posted by mmaheigan 
· Tuesday, May 14th, 2019 

In the ocean, unicellular eukaryotes are often mixotrophic, which means they photosynthesize and also consume prey. In recent decades, it has become clear that mixotrophs are ubiquitous in sunlit ocean habitats. Additionally, models predict that mixotrophs have important impacts on productivity, nutrient cycling, carbon export, and food web structure. However, there is little understanding of the environmental conditions that select for a mixotrophic lifestyle, and it is unclear how mixotrophs succeed in competition with autotrophic and heterotrophic specialists. A recent study in PNAS that synthesized measurements of mixotrophic nanoflagellates showed that mixotrophs are more abundant in stratified, well-lit, low latitude environments (Figure 1A). They are also more abundant, relative to pure heterotrophs, in productive coastal environments (Figure 1B). A trait-based model analysis revealed that the success of mixotrophs depends on the fact that they are less nutrient-limited than autotrophs (due to prey-derived nutrients) and less carbon-limited than heterotrophs (due to photosynthesis). This synergy requires sufficient light, leading to success in low latitude environments. Similarly, a greater supply of dissolved nutrients relative to prey, as commonly observed in coastal environments, favors mixotrophs relative to heterotrophs. One implication of these results is that carbon fixation at lower latitudes may be enhanced by mixotrophy, while limiting nutrients may be more efficiently transferred to higher trophic levels.

Figure 1. Estimated abundance of autotrophic, mixotrophic, and heterotrophic nanoflagellates across environmental gradients in the ocean.

 

Author:
Kyle Edwards (Univ. Hawaii at Manoa)

Widespread nutrient co-limitation discovered in the South Atlantic

Posted by mmaheigan 
· Thursday, March 15th, 2018 

Unicellular photosynthetic microbes—phytoplankton—are responsible for virtually all oceanic primary production, which fuels marine food webs and plays a fundamental role in the global carbon cycle. Experiments to date have suggested that the growth of phytoplankton across much of the ocean is limited by either nitrogen or iron. But simultaneously low concentrations of these and other nutrients have been measured over large areas of the open ocean, raising the question: Are phytoplankton communities only limited by a single nutrient?

Authors of a study recently published in Nature tested this by conducting nutrient addition experiments on a GEOTRACES cruise in the nutrient-deficient South Atlantic gyre. Seawater samples were amended with nitrogen, iron, and cobalt both individually and in various combinations. Concurrent nitrogen and iron addition stimulated increased phytoplankton growth, yielding a ~40-fold increase in chlorophyll a. Supplementary addition of cobalt or cobalt-containing vitamin B12 further enhanced phytoplankton growth in several experiments.

Experiments conducted throughout the southeast Atlantic GEOTRACES GA08 cruise transect (left panel) demonstrated that nitrogen and iron had to be added to significantly stimulate phytoplankton growth (right panel). Supplementary addition of cobalt (or cobalt-containing vitamin B12) stimulated significant additional growth.

In addition to co-limited sites, the study identified ‘singly’ and ‘serially’ limited sites. These limitation regimes could be predicted by the measured ambient seawater nutrient concentrations, demonstrating the potential for using nutrient datasets to make confident predictions about limitation at larger spatial scales, an approach that is being more widely used in programmes like GEOTRACES,.

Finally, a complex, state-of-the-art biogeochemical ocean model suggested a much smaller extent of nutrient co-limitation than the experiments indicated. Authors attributed this to relatively restricted microbial and nutrient diversity in the model. These findings have implications for how such models are constructed if they are to represent nutrient co-limitation in the ocean and accurately project changes in ocean productivity in the future.

 

Authors:
Thomas J. Browning (GEOMAR)
Eric P. Achterberg (GEOMAR)
Insa Rapp (GEOMAR)
Anja Engel (GEOMAR)
Erin M. Bertrand (Dalhousie University)
Alessandro Tagliabue (University of Liverpool)
Mark Moore (University of Southampton)

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