Primary vitamin B12 limitation and widespread stress in Antarctic phytoplankton

R. H. Lampe; D. Rao; R. M. Kell; R. J. Chmiel; N. L. Schanke; S. Y. Kim; L. E. Lees; F. Bolinesi; O. Mangoni; R. Casotti; C. Balestra; H. Zheng; D. M. Moran; A. E. Allen; M. R. McIlvin; G. R. DiTullio; M. A. Saito

doi:10.64898/2026.11.05.764676

AI summary · DOI: 10.64898/2026.11.05.764676 · bioRxiv: Ecology

Primary vitamin B12 limitation and widespread stress in Antarctic phytoplankton

R. H. Lampe, D. Rao, R. M. Kell, R. J. Chmiel, N. L. Schanke, S. Y. Kim, L. E. Lees, F. Bolinesi,

and 9 more authors

O. Mangoni, R. Casotti, C. Balestra, H. Zheng, D. M. Moran, A. E. Allen, M. R. McIlvin, G. R. DiTullio, M. A. Saito

Original paper Published on 06 Nov. 2026 bioRxiv preprint License: CC BY-NC 4.0 Also in: fr

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The essentials

An Amundsen Sea experiment found primary cobalamin limitation of phytoplankton growth, while Southern Ocean protein data indicate widespread cobalamin stress in diatoms and Phaeocystis.

Question

The study asks whether vitamin B12 (cobalamin) limits Antarctic phytoplankton growth, and how nutrient stress varies among taxa and across the Southern Ocean. It also examines whether cobalamin stress is evident beyond the experimental site.

Method

Researchers incubated a natural Amundsen Sea community for six days in triplicate, comparing control, iron (1 nmol L-1), cobalamin (200 pmol L-1), combined iron and cobalamin, and iron-chelator treatments. They measured growth and nutrient responses, community composition, uptake, and proteins; metaproteomic indicators were also assessed across an approximately 10,000 km transect with 80 samples.

Findings

Result Cobalamin alone increased chlorophyll a relative to treatments without cobalamin, and adding iron with cobalamin raised chlorophyll a a further 24%, supporting primary cobalamin and secondary iron limitation in the experimental community.
See the passage in the paper
Following six days of incubation, chlorophyll a (Chla) concentrations were significantly higher in the cobalamin-only (+B12) treatment compared to all treatments without cobalamin (P < 0.02, Fig. 1d). With the addition of both iron and cobalamin (+FeB12), Chla concentrations were 24% higher than with cobalamin alone (P = 0.01, Fig. 1d).
Result Cobalamin addition stimulated diatom and prasinophyte indicators, whereas cryptophytes increased in the iron-only treatment.
See the passage in the paper
At the final timepoint, fuco and biogenic silica concentrations, another indicator of diatom biomass39, were significantly higher in the +B12 and +FeB12 treatments compared to the control and +Fe treatments, demonstrating a strong response to cobalamin addition (Fig. 2d and Fig. S1b, P < 0.006).
Result Diatom and prasinophyte protein markers showed lower cobalamin-stress signals with cobalamin addition, while iron addition reduced iron-stress protein abundances.
See the passage in the paper
In the +B12 and +FeB12 treatments, indicators of cobalamin stress were low compared to all treatments. These include cobalamin acquisition protein 1 (CBA1), a shift from MetE to MetH, methionine adenosyltransferase (MetK), and serine hydroxymethyltransferase (SHMT)53.
Authors’ interpretation Across the transect, MetE exceeded MetH in most samples from diatoms and Phaeocystis, which the authors interpret as evidence of widespread cobalamin stress.
See the passage in the paper
In both taxa, MetE exceeded MetH abundances in 79% of samples.
Authors’ interpretation The authors argue that increased iron supply associated with ice melt may heighten the importance of cobalamin limitation and affect phytoplankton community composition.
See the passage in the paper
With decreased iron stress, demand for cobalamin increases; however, cobalamin availability may not be rising accordingly, with low bioavailable cobalt that is largely consumed by eukaryotes and a microbial community that is largely not capable of cobalamin synthesis21.

Limitations

The direct nutrient-enrichment experiment tested one natural community at one Amundsen Sea station, so its treatment responses do not establish the same limitation state at all locations or times.
The transect evidence uses protein-abundance ratios as indicators of cobalamin stress, rather than direct cobalamin-addition experiments at every sampled location.
See the passage in the paper
Within these samples, the MetH:MetE ratio in diatoms and Phaeocystis was used to evaluate cobalamin stress (Fig. 4A,B).

Why it matters

The authors argue that iron should not be assumed to be the primary nutrient limiting Antarctic phytoplankton: cobalamin can be primary, and nutrient responses differ among taxa. Accounting for cobalamin availability and taxon-specific requirements may improve projections of community structure and productivity as coastal conditions change.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: Vitamin B12; Antarctic phytoplankton; nutrient limitation; iron; Southern Ocean

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Comprehensive Summary
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Original abstract

The paper asks whether vitamin B12 (cobalamin), usually considered a secondary or co-limiting nutrient in the Southern Ocean, can instead be the primary nutrient limiting Antarctic phytoplankton, and whether responses differ among taxa. The authors also assess whether signs of cobalamin stress occur across a much broader region. This study is a bioRxiv preprint that has not been certified by peer review. The central experiment used a natural surface-water community collected in December 2017 in the Amundsen Sea Polynya, over the Dotson–Getz Trough. Triplicate bottles were incubated for six days with no nutrient addition, iron, cobalamin, both iron and cobalamin, or the iron-binding chelator desferrioxamine B. Iron and cobalamin were added at concentrations of 1 nanomole per liter and 200 picomoles per liter, respectively. The researchers tracked chlorophyll, nutrient drawdown, pigments, metal and cobalamin uptake, and community composition using rRNA sequencing and protein measurements. They also measured dissolved trace metals and cobalamin uptake at stations in the Amundsen Sea. To assess the wider region, they analyzed phytoplankton proteins in underway surface samples collected along a roughly 10,000-kilometer transect and in vertical profiles. After six days, chlorophyll a was significantly higher in the cobalamin-only treatment than in treatments without cobalamin. Chlorophyll a concentrations in the combined iron-and-cobalamin treatment were 24% higher than in the cobalamin-only treatment. Cobalamin addition also increased nitrate and phosphate uptake, but not silicic-acid uptake. Together, these results suggest that cobalamin was the primary growth-limiting nutrient in this community, with iron as a secondary limitation. Cobalamin uptake was greatest near the surface at the experimental site and was largely attributable to organisms in the larger size fraction. Organisms at least 3 micrometers in size accounted for 56–95% of uptake across the sampled stations. In the incubations, uptake rates were three to six times higher when cobalamin was added. Dissolved cobalt declined by 19% in the cobalamin treatment and by 44% in the combined treatment, indicating substantial uptake of cobalt supplied as part of the cobalamin addition. Diatoms and prasinophytes were the main groups that responded to cobalamin. Pigments associated with diatoms and biogenic silica increased in the cobalamin and combined-addition treatments. Prasinophyte-associated pigments also increased with cobalamin addition. Prasinophytes accounted for 29–38% of 18S rRNA reads in the smaller size fraction, most of which were assigned to the mixotrophic genus *Pyramimonas*. In contrast, cryptophytes increased only with iron addition, consistent with iron limitation and potentially lower cobalamin requirements or alternative acquisition strategies. Protein indicators supported these contrasts: cobalamin-stress markers were generally lower after cobalamin addition, while iron-stress proteins declined with iron addition. The authors also found that the bacterial groups present had limited genomic capacity for cobalamin synthesis, potentially restricting the community’s supply. Across the broader transect, the ratio of the cobalamin-independent enzyme MetE to the cobalamin-dependent enzyme MetH indicated cobalamin stress in both diatoms and *Phaeocystis*: MetE exceeded MetH in 79% of samples. The authors conclude that cobalamin limitation may be widespread and that taxon-specific nutrient requirements could shape future Antarctic plankton communities as ice-shelf melt changes iron supply. Important limitations include the fact that the direct nutrient-addition experiment was conducted at one site, over six days, and in bottles.

Could Vitamin B12 Be the Main Brake on Antarctic Phytoplankton?

In the Southern Ocean, iron is often treated as the leading nutrient constraint on phytoplankton growth. Vitamin B12, also known as cobalamin, is more often viewed as a secondary limitation. But what if that assumption misses an important part of the story?

In the preprint Primary vitamin B12 limitation and widespread stress in Antarctic phytoplankton, R. H. Lampe, D. Rao, R. M. Kell, R. J. Chmiel, N. L. Schanke, S. Y. Kim et al. investigate whether B12 can be the primary growth-limiting nutrient for Antarctic plankton—and whether different groups respond in different ways. Their findings suggest that B12 may matter more than expected, while iron remains an additional constraint.

A nutrient test in the Amundsen Sea

The researchers collected natural surface-water plankton in December 2017 from the Amundsen Sea Polynya, over the Dotson–Getz Trough. They divided the community among bottles and, for six days, compared several conditions: no added nutrients, iron alone, B12 alone, iron and B12 together, and the iron-binding chelator desferrioxamine B. The additions were 1 nanomole per liter of iron and 200 picomoles per liter of B12.

They tracked chlorophyll, nutrient use, pigments, metal and B12 uptake, and changes in community composition. They also examined proteins associated with nutrient stress. These measurements helped reveal not just whether the community grew, but which groups responded and what the cells appeared to be short of.

B12 came first, with iron also in the picture

After six days, chlorophyll a—a measure of phytoplankton biomass—was significantly higher in bottles receiving B12 alone than in treatments without B12. Adding both nutrients produced chlorophyll a concentrations 24% higher than B12 alone. This pattern points to B12 as the primary limitation in the tested community, with iron acting as a secondary limitation.

B12 addition also increased nitrate and phosphate uptake, but not silicic-acid uptake. That difference matters because it suggests the response was not simply a uniform increase in every kind of nutrient consumption. It reflected changes in how the community grew and which organisms benefited.

The uptake measurements offered further evidence. In the incubations, B12 uptake rates were three to six times higher when B12 was added. Dissolved cobalt—a component of the vitamin—fell by 19% in the B12 treatment and 44% when iron and B12 were added together. Across sampled Amundsen Sea stations, organisms at least 3 micrometers in size accounted for 56–95% of B12 uptake, and uptake at the experimental site was greatest near the surface.

Different plankton, different nutrient needs

The response was not shared equally across the community. Diatoms and prasinophytes were the main groups that increased with B12. Pigments associated with diatoms and biogenic silica rose in B12 and combined-addition treatments. Prasinophyte-associated pigments also increased after B12 was added.

Prasinophytes made up 29–38% of the 18S rRNA reads in the smaller size fraction. Most were assigned to Pyramimonas, a mixotrophic genus that can obtain resources in more than one way. In contrast, cryptophytes increased only when iron was added. This fits the idea that cryptophytes were responding to iron limitation, and may have lower B12 requirements or alternative ways to acquire it.

Protein measurements supported these differences: indicators of B12 stress generally declined after B12 addition, while iron-stress proteins declined when iron was supplied. The researchers also found that the bacterial groups present had limited genomic capacity to make B12, potentially restricting the vitamin available to the wider community.

Signs of B12 stress beyond the experiment

To explore how widespread B12 stress might be, the team examined phytoplankton proteins in surface samples collected along an approximately 10,000-kilometer transect, as well as in vertical profiles. They compared two forms of an enzyme involved in cell metabolism: MetE, which does not require B12, and MetH, which does.

MetE exceeded MetH in 79% of samples, an indication of B12 stress in both diatoms and Phaeocystis. These observations do not prove that B12 limits growth everywhere along the transect. They do, however, suggest that signs of B12 stress are geographically widespread.

Why the findings matter—and what remains uncertain

If B12 availability helps shape Antarctic plankton communities, changes in nutrient supply could shift which groups thrive. The authors suggest that future changes in iron delivery associated with ice-shelf melt may interact with taxon-specific nutrient needs, influencing the composition of plankton communities.

The evidence should be interpreted with care. The direct nutrient-addition experiment was conducted at one site, lasted six days, and used bottles, which cannot fully reproduce conditions in the open ocean. The wider protein survey indicates a broad pattern of stress, but is not the same as testing nutrient limitation experimentally at every location. The paper is a bioRxiv preprint and has not been certified by peer review.

Even with those limits, the study challenges the idea that B12 is only a minor player in Southern Ocean productivity. At least in the tested community, it was the strongest growth constraint—and its effects differed markedly among plankton groups.

Created on 09 Nov. 2026

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Primary vitamin B12 limitation and widespread stress in Antarctic phytoplankton

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Could Vitamin B12 Be the Main Brake on Antarctic Phytoplankton?

A nutrient test in the Amundsen Sea

B12 came first, with iron also in the picture

Different plankton, different nutrient needs

Signs of B12 stress beyond the experiment

Why the findings matter—and what remains uncertain

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