CBIOMES researchers have developed a new parameterization of photoinhibition that could significantly improve predictions of marine primary productivity under changing climate conditions.
Reporting by Helen Hill for CBIOMES
The study, led by M. Amirian and colleagues from Dalhousie University and published in Communications Earth & Environment, tackles a longstanding challenge in oceanography: how to accurately represent the effects of high light on phytoplankton, the microscopic algae that form the base of the marine food web and drive global carbon cycling.
“Photoinhibition is a paradoxical phenomenon,” said Amirian. “Light is essential for photosynthesis, but too much of it can damage the photosynthetic machinery. Our work provides a robust way to quantify this effect across diverse ocean conditions.”
Phytoplankton are responsible for nearly half of Earth’s photosynthetic carbon fixation, yet their productivity is not simply a function of light availability. At high irradiance levels — such as near the ocean surface or in clear tropical waters — photosystem II (PSII), a key protein complex in the photosynthetic apparatus, becomes vulnerable to light-induced damage. This damage, primarily to the D1 protein encoded by the psbA gene, leads to a decline in photosynthetic efficiency and growth.
Previous models of photoinhibition have relied on empirical fitting of photosynthesis-irradiance (P-I) curves, often using an exponential decay term in the models, overlapping the concept of photodamage and photoinhibition, and consequently overestimating the photosynthesis parameters. Amirian and the team sought to overcome these limitations by developing a novel model that not only quantifies the photoinhibition region of the P–I curve, but also provides biologically meaningful parameters that can be directly estimated from empirical data. Unlike previous models, this approach enables direct estimation of both maximum photosynthetic capacity and the photoinhibition rate from P–I measurements. The model was validated against global datasets, outperforming all the existing models in the literature.
The teams approach builds on the widely used Jassby & Platt (1976) model, which describes photosynthesis rate as a hyperbolic tangent function of light intensity, incorporating both light-limited and light-saturated phases. The innovation lies in how the authors redefined the photoinhibition term — traditionally a curvature parameter — to explicitly represent the rate of PSII damage relative to repair. This reformulation yields the first practical model capable of capturing all three phases of the P–I curve (low light, moderate light, and high light) and allows the model to distinguish between reversible downregulation and irreversible photodamage, a key distinction for predicting long-term productivity.
To calibrate/ validate the model, the researchers drew on 50 years of in situ open-ocean measurements of phytoplankton photosynthetic parameters collected across diverse regions. They found that the new model significantly improved fits to observed data, especially in high-light environments where traditional models tend to overestimate productivity.
“By linking photoinhibition to physiological stress rather than just curve shape, we can better capture how phytoplankton respond to real-world light conditions,” said study co-author Andrew Irwin.
The model also accounts for variability among phytoplankton groups. For example, picophytoplankton like Prochlorococcus are more susceptible to photoinhibition due to their small size and limited photoprotective capacity, while larger diatoms and dinoflagellates often exhibit greater resilience. This taxon-specific sensitivity is crucial for ecosystem models that simulate community composition and biogeochemical fluxes.
“Photoinhibition is as common as light,” said Irwin. “It’s not just a high-light phenomenon — it’s a dynamic process that depends on the balance between damage and repair, which is influenced by many environmental variables.”
The new parameterization is designed for integration into global ocean models, including Earth system models used to project climate-carbon feedback. By improving the representation of phytoplankton light responses, it could enhance predictions of ocean productivity, carbon export, and ecosystem resilience under future climate scenarios.
“This is a critical step toward more accurate and mechanistic modeling of marine photosynthesis,” said Amirian. “It helps us understand not just how much carbon phytoplankton fix, but how that process is shaped by the interplay of light, nutrients, and physiology.”
The authors emphasize that further refinement will require more field data, especially from under-sampled regions like the Southern Ocean and tropical gyres. They also call for more research into the molecular mechanisms of PSII repair and the role of photoprotective strategies such as non-photochemical quenching.
Story image: Unsplash
Publication:
M. Amirian, M., Z.V. Finkel, E. Devred, and A.J. Irwin (2025), Parameterization of photoinhibition for phytoplankton, Commun Earth Environ, doi: 10.1038/s43247-025-02686-3