Phytoplankton, the microscopic photosynthetic organisms that form the foundation of marine ecosystems, have traditionally been understood as fleeting contributors to the ocean’s carbon cycle. Their lifecycle, typified by rapid blooms followed by die-offs, was believed to result in the quick recycling of carbon contained within their biomass back into the marine environment. However, a transformative study led by Professor WANG Faming at the South China Botanical Garden (SCBG) of the Chinese Academy of Sciences has revealed a previously underappreciated facet of phytoplankton’s role in global carbon dynamics. This new research demonstrates that these tiny marine algae are capable of directly injecting a form of highly resistant organic carbon into the ocean, where it may remain sequestered for centuries, significantly altering our understanding of carbon persistence in marine systems.
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
