Published in Nature Communications on August 20, the groundbreaking research utilized state-of-the-art ultrahigh-resolution mass spectrometry to analyze the complex molecular composition of dissolved organic carbon (DOC) produced by six major phytoplankton taxa. This advanced analytical technique allowed the research team to peer into the intricate chemical fingerprints of the carbon compounds secreted by these organisms. Astonishingly, more than 10% of the carbon released by phytoplankton was found in the form of recalcitrant dissolved organic carbon (RDOC), a chemically robust pool of organic molecules resistant to microbial decomposition and environmental breakdown. This discovery defies the longstanding paradigm that algal carbon is mainly ephemeral and rapidly cycled, suggesting instead that phytoplankton contribute directly to a long-lived oceanic carbon reservoir.

The study delineates two distinct pathways through which phytoplankton-derived DOC infiltrates the marine carbon pool. The first pathway involves the excretion of bioavailable dissolved organic carbon (BDOC), a labile fraction readily consumed by heterotrophic microorganisms that rapidly metabolize and recycle this carbon. Some of that BDOC is subsequently transformed into RDOC via intricate microbial processing and biogeochemical reactions. Crucially, the researchers confirmed a second, direct secretion of RDOC by phytoplankton themselves, bypassing microbial mediation. This direct release mechanism was previously undetected and suggests a more immediate and substantial contribution of persistent organic molecules from phytoplankton to the ocean.

The implications of identifying phytoplankton as significant, direct sources of RDOC are profound for the marine carbon cycle and global climate regulation. RDOC serves as a molecular reservoir sequestering carbon for centuries to millennia, playing a vital role in the ocean’s capacity to buffer atmospheric carbon dioxide. By better understanding the molecular composition and fluxes of RDOC, scientists gain critical insight into the longevity and stability of organic carbon in ocean waters. This newfound pathway directly links microscopic algal activity to long-term carbon storage, expanding the conceptual framework of how carbon is cycled and preserved in the marine environment.

Intriguingly, the research underscores considerable variability in DOC production linked to the taxonomic diversity of phytoplankton and their successional growth phases. Phytoplankton communities are composed of myriad species with heterogeneous metabolic pathways, and their population dynamics fluctuate rapidly, often in response to environmental conditions such as nutrient availability and temperature. This ecological complexity results in significant temporal and spatial differences in the quantity and quality of DOC released. For example, the team’s analysis demonstrated that phytoplankton blooms in their exponential growth phase secrete markedly higher amounts of RDOC compared to those in decline stages, indicating that bloom timing and composition critically influence the global carbon reservoir.

Historically, capturing the molecular diversity and global distribution of algal-derived DOC has been hampered by technical and observational limitations. To overcome these challenges, the researchers innovatively combined ultrahigh-resolution mass spectrometry with satellite remote sensing and sophisticated machine learning algorithms. This interdisciplinary approach enabled them to generate a global-scale dataset of marine DOC, mapping its variations across different phytoplankton groups and oceanic regions. By integrating molecular-level data with vast satellite observations, their models effectively link microscopic biochemical processes to large-scale biogeochemical patterns in the ocean, a leap forward in marine carbon cycle modeling.

This integrative methodology also offers powerful predictive capabilities. By synthesizing chemical signatures with environmental and biological parameters, the team developed optimized oceanic DOC assessment models capable of forecasting how alterations in phytoplankton bloom duration or shifts in community composition driven by climate change will impact the ocean’s DOC pool. These models hold promise for predicting carbon cycle feedbacks under future climate scenarios, providing critical tools for assessing ocean health and its role in global climate regulation.

Co-first author Dr. LU Zhe emphasized the practical value of these insights, stating that the global dataset and enhanced predictive models allow for rapid and accurate assessments of how changes in marine ecosystems affect carbon sequestration processes. This research, therefore, not only advances fundamental science but also delivers tangible benefits for environmental monitoring and climate mitigation strategies. The ability to track and anticipate shifts in oceanic carbon reservoirs strengthens the scientific basis for policy decisions addressing climate change and ocean stewardship.

Beyond its immediate scientific impact, this study redefines the ecological importance of phytoplankton beyond their recognized role as primary producers. By acting as active agents in long-term carbon sequestration, phytoplankton emerge as pivotal contributors to the Earth’s carbon budget. Their ability to transform and export stable organic carbon compounds underscores the interconnectedness of marine microbial ecology and global climate processes, highlighting the ocean’s complexity as an integrated biosphere-carbon system.

The discovery also invites renewed investigation into the biochemical pathways enabling phytoplankton to produce RDOC. Understanding the enzymatic and metabolic mechanisms behind the synthesis and release of recalcitrant organic molecules could unlock further clues about the resilience and adaptability of phytoplankton under changing oceanic conditions. Such knowledge may inform biotechnological applications aimed at enhancing natural carbon sequestration or developing biomimetic materials for carbon capture.

Furthermore, this research highlights the critical role of advanced analytical technologies such as ultrahigh-resolution mass spectrometry in oceanography. The ability to resolve thousands of individual molecular entities within complex DOC mixtures marks a significant leap over traditional bulk measurements, enabling unprecedented detail in carbon cycle studies. Its application in marine sciences opens new frontiers for tracking organic matter fluxes and understanding the chemical ecology of marine microorganisms.

Collectively, these findings redefine how we view the marine carbon cycle and emphasize the ocean’s role as a dynamic and long-term regulator of atmospheric carbon dioxide. By unveiling the direct contribution of phytoplankton to recalcitrant carbon pools, the study reshapes climate models and calls for integrating molecular-level insights into Earth system science. In an era of rapid environmental change, such knowledge is vital for predicting and managing the ocean’s function as a global carbon sink.

Subject of Research: Marine carbon cycling and phytoplankton contributions to recalcitrant dissolved organic carbon

Article Title: Unveiling Phytoplankton’s Direct Role in Long-Term Oceanic Carbon Sequestration

News Publication Date: August 20, 2025

Web References:
https://doi.org/10.1038/s41467-025-63105-x

Image Credits: Imaged by LU Zhe et al.

Keywords: Marine biology, Ecological processes, Ocean chemistry

Radiocarbon analyses of dissolved inorganic carbon (DIC), carbon dioxide (CO₂), and methane (CH₄) in rivers have historically been fragmented, focused on local or regional scales with inconsistent variables, making comparison across studies difficult. This new work synthesizes over a thousand observations, harmonizing data across continents and ecosystems, enabling an unparalleled global comparison of the radiocarbon content of riverine carbon emissions.

Central to the study’s innovation is the normalization of the radiocarbon content (expressed as F14C) of riverine carbon to atmospheric radiocarbon levels at the time of sample collection. This approach controls for temporal fluctuations in atmospheric carbon isotopic composition, especially those caused by anthropogenic influences such as nuclear testing, allowing for consistent comparison of carbon ages downriver and across regions.

The findings indicate that river CO₂ emissions are often a complex mixture of modern carbon recently cycled through ecosystems and significantly older carbon derived from soils and petrogenic sources, such as weathered rock organic matter and carbonate minerals. This "old" carbon component, which can be thousands of years in age, challenges traditional assumptions that rivers primarily release newly fixed carbon and highlights their role as conduits of fossil and stored terrestrial carbon to the atmosphere.

To unravel the contributions of these diverse carbon sources, the researchers implemented sophisticated isotope mixing models coupled with Monte Carlo simulations, anchored by global estimates of weathering fluxes. This modelling revealed that petrogenic carbon accounts for roughly 7% of the total riverine carbon flux, but the remainder includes a substantial millennial-aged carbon fraction—that is, carbon with residence times ranging from centuries to thousands of years within soils before entering the river system.

This legacy carbon, released through riverine transport and evasion to the atmosphere, underscores a major pathway by which ancient carbon stores are mobilized in the modern environment. Such insights have profound implications for carbon budgeting, as they suggest that terrestrial carbon reservoirs hold a reservoir of old carbon that is actively connected to atmospheric CO₂ levels through fluvial systems.

The study also leveraged global hydrological and environmental data extracted from HydroATLAS, a comprehensive spatial database offering consistent catchment and reach-scale attributes such as size, lithology, and biome classification. This allowed the team to link chemical signatures of carbon age with landscape characteristics, revealing that catchment size and lithology are significant controls on the age and source of river carbon emissions.

Notably, rivers draining small catchments (≤10 km²) and large catchments (>10 km²) exhibit distinct carbon age profiles, a finding supported and validated through random forest machine learning models. These models, trained on a suite of catchment and climate variables, identify the key environmental factors that correlate with variations in the radiocarbon content of riverine carbon, highlighting the non-linear interactions of geography, geology, and ecosystem processes.

New radiocarbon data included in the analysis originate from diverse locations including heavily urbanized rivers in London, pristine mountain rivers on the Qinghai–Tibet Plateau, and rivers sampled in Taiwan, Cambodia, and China. These additions offer fresh insights into how human activities and natural settings influence the transport and emission of aged carbon via rivers.

Sample collection and processing spanned advanced techniques, including super headspace equilibrations and membrane-based gas extraction methods, followed by accelerator mass spectrometry (AMS) for precise radiocarbon quantification. These rigorous methods ensure high-quality isotopic data essential for the interpretation of carbon provenance and cycling timescales.

The study also made significant progress evaluating the isotopic equilibrium between dissolved inorganic carbon and dissolved CO₂, lending confidence that DIC radiocarbon measurements can reliably represent the radiocarbon signature of riverine CO₂ emissions, a crucial step for integrating published and new datasets.

By integrating data from multiple continents and across varying catchment scales and biomes, the authors provide a global-scale perspective of how river systems transport and release carbon with a broad age spectrum. This integrative view challenges the previously held perception of rivers as vectors of primarily contemporary carbon and positions them as key participants in the mobilization of deep, stored carbon.

These findings demand a reevaluation of global carbon budgets and climate models, as the flux of aged carbon via rivers represents a significant and previously underappreciated source of atmospheric CO₂. Understanding the dynamics of this carbon pool could improve predictions of future atmospheric carbon trajectories, particularly in the context of land use change and climate-driven alterations in hydrology and soil carbon dynamics.

In sum, this comprehensive analysis of riverine carbon isotopes highlights the complex interplay between terrestrial carbon reservoirs and atmospheric emissions, revealing rivers as vital arteries through which ancient carbon is continually released back to the atmosphere. As climate change progresses, appreciating the role of these aged stores becomes critical for crafting effective carbon management and mitigation strategies.

Subject of Research: Global-scale patterns and sources of river carbon emissions, with emphasis on radiocarbon content and the contribution of old carbon to atmospheric CO₂ fluxes.

Article Title: Old carbon routed from land to the atmosphere by global river systems.

Article References:
Dean, J.F., Coxon, G., Zheng, Y. et al. Old carbon routed from land to the atmosphere by global river systems. Nature 642, 105–111 (2025). https://doi.org/10.1038/s41586-025-09023-w

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41586-025-09023-w