Biogeochemical Cycling of Trace Metals in Oceanic Systems

The biogeochemical cycling of trace metals in the ocean encompasses the sources, transformations and sinks of elements such as iron, copper, zinc and manganese that are present at nanomolar to picomolar concentrations yet exert outsized control on marine productivity and global climate. These metals enter the ocean via aeolian dust, riverine discharge, hydrothermal vents, continental margin sediments and glacial runoff. In seawater they undergo complexation with organic ligands, adsorption onto particles (scavenging), uptake by phytoplankton and bacteria, incorporation into food-webs, release during remineralisation of sinking biomass and eventual burial in sediments. Spatial and temporal variability in metal speciation, driven by physical mixing, biological demand and redox conditions, regulates their bioavailability and hence primary production, nitrogen fixation and the efficiency of the biological carbon pump. A comprehensive understanding of these cycles is critical for improving biogeochemical models, predicting ecosystem responses to climate change and evaluating geoengineering approaches such as iron fertilisation. Recent advances in high-resolution sampling, mass spectrometry and molecular profiling have revealed unexpected reservoirs of reactive metals in ice, the role of megafauna as nutrient vectors and the genomic responses of microbial communities to trace-metal limitation, highlighting the intricate links between physical forcing, chemical speciation and biological function in the modern ocean.

Research from Nature Portfolio

Recent studies have quantified the global contribution of large marine mammals to the cycling of multiple trace nutrients. Analyses of defecation by whale and dolphin communities demonstrate that cetaceans release suites of metals and major nutrients in proportions that vary with water-column productivity and community composition, thereby influencing phytoplankton blooms across temperate and tropical regions. This work extends the concept of animals as active vectors in elemental cycles beyond carbon and nitrogen to include essential metals. Another line of investigation has revealed ice sheets and their meltwaters as previously under-appreciated sources of highly reactive, nanoparticulate iron. Measurements of subglacial and iceberg-derived particulates show that glacial runoff delivers iron to the near-coastal euphotic zone in quantities comparable to atmospheric dust deposition. The bioavailability of this iron fraction has implications for coastal fertilisation, polar ecosystem productivity and future changes under accelerated ice-sheet melting.

Biogeochemical Cycling of Trace Metals in Oceanic Systems publication trend

The graph below shows the total number of articles in biogeochemical cycling of trace metals in oceanic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Euphotic zone: The upper layer of the ocean penetrated by sunlight, where photosynthesis can occur.

Scavenging: The removal of dissolved metals from seawater by adsorption onto sinking particles.

Organic ligand: An organic molecule that binds trace metals, affecting their solubility and bioavailability.

Remineralisation: The microbial breakdown of organic matter back into inorganic nutrients and trace metals.

Biological carbon pump: The process by which marine organisms fix CO₂ into organic matter that is exported to the deep ocean.

References

1. Composition of cetacean communities worldwide shapes their contribution to ocean nutrient cycling. Nature Communications (2023).
2. Iron limitation of heterotrophic bacteria in the California Current System tracks relative availability of organic carbon and iron. The ISME Journal: Multidisciplinary Journal of Microbial Ecology (2024).
3. The GEOTRACES Intermediate Data Product 2017. Chemical Geology (2018).
4. The oceanic mass balance of copper and zinc isotopes, investigated by analysis of their inputs, and outputs to ferromanganese oxide sediments. Geochimica et Cosmochimica Acta (2014).
5. Ice sheets as a significant source of highly reactive nanoparticulate iron to the oceans. Nature Communications (2014).