Southern Ocean Diatoms Reveal Hidden Metal Quotas

Proteins inside a dominant Southern Ocean diatom account for a surprising share of the ocean’s scarce metals, reshaping how scientists understand polar food webs. The diatom species Fragilariopsis kerguelensis relies heavily on a specialized enzyme requiring manganese, revealing how microscopic algae manage life in extreme Southern Ocean waters.

Trace metals like iron and manganese control the growth of marine plant life across vast areas of the global ocean. While iron has received intense research focus for its role in driving phytoplankton blooms, manganese is increasingly recognized as a vital limiting nutrient in the Southern Ocean. Exactly how microscopic algae allocate these metals inside their cells has remained a major question in chemical oceanography. Researchers have long struggled to link microscopic molecular measurements directly to large-scale elemental cycles in the sea.

Combining Proteomic Techniques to Track Cellular Manganese

To solve this scaling problem, researchers turned to a dual approach combining untargeted and targeted metaproteomics. The study, published in Science Advances, outlines how scientists sampled the Amundsen Sea during the austral summer of 2018 aboard the icebreaker RV Araon. Water samples collected across 15 different stations underwent size-fractionated filtration to isolate specific microbial communities.

Untargeted mass spectrometry identified thousands of peptides within the samples, establishing a quantitative baseline for the protein composition of different taxonomic groups. The team observed a clear geographic separation between dominant groups. Open waters were dominated by the haptophyte Phaeocystis antarctica, while marginal sea ice zones were dominated by diatoms from the genus Fragilariopsis.

To measure specific proteins with high precision, the team used targeted mass spectrometry aided by isotopically labeled synthetic peptide standards. By focusing on Fragilariopsis kerguelensis, the researchers quantified the exact abundance of antioxidant enzymes including manganese superoxide dismutase, copper-zinc superoxide dismutase, and peroxiredoxin.

Metals in Biomass and Stoichiometric Calculations

The combined metaproteomic measurements revealed that manganese superoxide dismutase represents a substantial cellular pool. The team estimated a median of 94 femtomoles of the enzyme per microgram of total protein, translating to roughly 340,000 molecules per cell. Using a Bayesian statistical framework to account for measurement errors, the researchers calculated that this single protein accounts for a median of 0.7 micromoles of manganese per mole of carbon in cellular biomass.

Comparing these in situ estimates against empirical data on total diatom manganese quotas from across the Southern Ocean indicates that manganese superoxide dismutase accounts for approximately 20 percent of the total cellular manganese quota. The remaining variation is likely tied to photosynthetic proteins and cellular storage pools.

Environmental Drivers and Culture Comparisons

The team also examined whether environmental factors such as dissolved iron, dissolved manganese, light, and temperature directly correlate with antioxidant protein abundances in natural field samples. No strong environmental correlations emerged across the station samples, likely due to the relatively narrow range of in situ conditions and the historical exposure of the cells to shifting waters.

To test how trace metal concentrations influence protein expression under controlled conditions, the researchers analyzed culture data from the related polar diatom Fragilariopsis cylindrus. Surprisingly, experiments across multiple iron concentrations showed that manganese superoxide dismutase abundance did not vary as a function of dissolved iron, defying previous assumptions derived from temperate phytoplankton models. However, under high iron and low manganese conditions, the abundance of the manganese enzyme decreased markedly, while the appearance of a nickel-based superoxide dismutase suggested a cellular compensation mechanism.

These empirical findings provide molecular justification for parameters used in large-scale biogeochemical models that incorporate nonphotosynthetic metal quotas. By bridging the gap between molecular proteomics and global elemental cycles, the work offers a new way to represent microbial physiology in climate and ocean models.

The research was published in Science advances on November 13, 2026.