Marine Protists and Biological Processes in Ocean Ecosystems

Marine protists constitute a highly diverse assemblage of unicellular eukaryotes that drive critical biological processes in the world’s oceans. As primary producers, many phytoplanktonic protists fix carbon through photosynthesis and form the base of marine food webs. Heterotrophic and mixotrophic protists graze on bacteria and smaller plankton, regulating microbial populations and recycling nutrients. In deeper waters, skeletal‐bearing Rhizaria and large radiolarians contribute to the vertical export of organic matter and biogenic silica, promoting carbon sequestration in the mesopelagic zone. Symbiotic associations between protist hosts and microalgae further enhance productivity in oligotrophic regions, while DNA‐based methods reveal shifts in community composition over geological timescales. Advances in imaging, genomics and machine‐learning are providing quantitative insights into protist biomass, biogeochemical fluxes and their responses to changing environmental conditions, underscoring their global significance in climate regulation and marine ecosystem health.

Research from Nature Portfolio

Recent studies have applied machine-learning to extensive in situ imaging datasets to quantify the biomass and biogeochemical roles of giant mesopelagic Rhizaria. These protists, especially flux-feeding Phaeodaria, account for a substantial fraction of carbon biomass in the 200–1 000 m layer and represent a unique source of biogenic silica where diatoms are absent. Their carbon demand contributes several hundred teragrams of carbon export annually, highlighting an under-recognised component of the biological pump. In parallel, whole‐community DNA metabarcoding of sediment cores has demonstrated the effectiveness of planktonic protist eDNA in reconstructing past marine–freshwater transitions. Shifts in plankton assemblages correlate with salinity and redox changes, providing high-resolution records of environmental perturbations over the last two millennia and validating protist metabarcoding as a tool for palaeo-oceanography.

Marine Protists and Biological Processes in Ocean Ecosystems publication trend

The graph below shows the total number of articles in marine protists and biological processes in ocean ecosystems across all publications each year (not limited to Nature Index journals).

Technical terms

Mesopelagic zone: Oceanic layer between 200 m and 1 000 m depth where light diminishes and many biogeochemical transformations occur.

Mixotrophy: Nutritional strategy combining photosynthesis and heterotrophic feeding within a single organism.

Photosymbiosis: Mutualistic association in which a host protist harbours photosynthetic algae that supply organic carbon in exchange for nutrients and protection.

Biomineralization: Biological process by which organisms produce minerals, such as silica or celestite, to form skeletal elements.

Holobiont: Integrated unit comprising a host organism and its associated microbial or algal symbionts, acting as a functional ecological entity.

References

1. Global census of the significance of giant mesopelagic protists to the marine carbon and silicon cycles. Nature Communications (2024).
2. Metabolic interdependence and rewiring in radiolaria-microalgae photosymbioses. The ISME Journal: Multidisciplinary Journal of Microbial Ecology (2025).
3. DNA metabarcoding focusing on the plankton community: an effective approach to reconstruct the paleo-environment. Scientific Reports (2023).
4. Phaeodaria: An Important Carrier of Particulate Organic Carbon in the Mesopelagic Twilight Zone of the North Pacific Ocean. Global Biogeochemical Cycles (2019).