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Coastal Antarctic marine ecosystems are significant in carbon cycling because of their intense seasonal phytoplankton blooms. Southern Ocean algae are primarily limited by light and iron (Fe) and can be co-limited by cobalamin (vitamin B12). Micronutrient limitation controls productivity and shapes the composition of blooms which are typically dominated by either diatoms or the haptophytePhaeocystis antarctica. However, the vitamin requirements and ecophysiology of the keystone speciesP. antarcticaremain poorly characterized. Using cultures, physiological analysis, and comparative omics, we examined the response ofP. antarcticato a matrix of Fe-B12conditions. We show thatP. antarcticais not auxotrophic for B12, as previously suggested, and identify mechanisms underlying its B12response in cultures of predominantly solitary and colonial cells. A combination of proteomics and proteogenomics reveals a B12-independent methionine synthase fusion protein (MetE-fusion) that is expressed under vitamin limitation and interreplaced with the B12-dependent isoform under replete conditions. Database searches return homologues of the MetE-fusion protein in multiplePhaeocystisspecies and in a wide range of marine microbes, including other photosynthetic eukaryotes with polymorphic life cycles as well as bacterioplankton. Furthermore, we find MetE-fusion homologues expressed in metaproteomic and metatranscriptomic field samples in polar and more geographically widespread regions. As climate change impacts micronutrient availability in the coastal Southern Ocean, our finding thatP. antarcticahas a flexible B12metabolism has implications for its relative fitness compared to B12-auxotrophic diatoms and for the detection of B12-stress in a more diverse set of marine microbes.
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This content will become publicly available on October 1, 2026
Exploring the Phaeosphere: Characterizing the microbiomes of Phaeocystis antarctica colonies from the coastal Southern Ocean and laboratory culture
Abstract Interactions between phytoplankton and bacteria play critical roles in shaping marine ecosystems. However, the intricate relationships within these communities—particularly in rapidly changing polar environments—remain poorly understood. We use targeted methods to directly characterize the microbiomes of individual colonies ofPhaeocystis antarctica, a keystone phytoplankton species in the Southern Ocean, and showed that colony microbiomes were consistent across individual colonies collected 108 nautical miles apart. These results suggest that hosting specific colony microbiomes is a shared trait across colony‐formingPhaeocystisspecies, with different species hosting colony microbiomes suited to their respective environments. The bacterial orders Alteromonadales, Oceanospirillales, and Sphingomonadales dominated the microbiomes of all field‐collectedP. antarcticacolonies. The relative abundances of bacterial taxa comprising the majority of field‐collected colony microbiomes—for example,Paraglaciecolasp. (Alteromonadales) and Nitrincolaceae (Oceanospirillales)—correlated withPhaeocystisabundance in surface waters, highlighting their potential roles in bloom dynamics and carbon cycling. After a year of laboratory culture, we observed a reduction in colony microbiome diversity, and Caulobacterales, Cellvibrionales, and Rhodobacterales dominated the cultured colony microbiomes. Notably, abundant genera in field‐collected colony microbiomes that were lost in culture were psychrophiles. The shift in microbiome structure emphasizes the importance of field‐based studies to capture the complexity of microbial interactions, especially for species from polar environments that are difficult to replicate in laboratory conditions. This research provides valuable insights into the ecological significance of prokaryotic interactions with a key phytoplankton species and underscores the necessity of considering these dynamics in the context of climate‐driven shifts in marine ecosystems.
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- Award ID(s):
- 2023192
- PAR ID:
- 10650446
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Journal of Phycology
- Volume:
- 61
- Issue:
- 5
- ISSN:
- 0022-3646
- Page Range / eLocation ID:
- 1124 to 1139
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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