Phytoplankton is an important primary producer in the ocean and one of the key components of the marine biological pump. The cellular carbon content and chlorophyll a (Chl a) content are the two most important biomass indices for phytoplankton. Recording and monitoring these biomasses are important tasks for studying phytoplankton, and C:Chl a is a key conversion tool between them. Although C:Chl a varies over a wide range, it is tightly regulated by external environmental factors, which mainly include light, nutrients and temperature, etc. Modelling studies of C:Chl a are important for understanding the ocean carbon cycle.

The history of the development of the C:Chl a model from the early empirical models to the present mechanistic models is outlined. Using the Geider empirical model and the Cloern model as examples, the empirical model is based on a large amount of experimental data on incubation and is obtained by summarising the data by linear or non-linear fitting. Empirical models have the advantage of being easy to use, but they cannot help us understand the specific effects of environmental factors on the physiological state of phytoplankton because they ignore the biological processes of phytoplankton. Mechanism models are represented by the Geider and Pahlow models, which are based on phytoplankton cellular carbon and nitrogen sources and expenditure, and optimal uptake kinetics, respectively. Mechanism models are popular in biogeochemical modelling because they can assist in tracing the flow of carbon and nitrogen elements.

With regard to the shortcomings and future challenges of the current C:Chl a model, the first is the various externally regulated environmental factors, such as photoinhibition of phytoplankton in the surface ocean, the common limitation of nutrients, and high temperature stress brought about by the context of global warming. The second is the taxonomic groups, in particular the current C:Chl a model may generally underestimate the carbon biomass of dinoflagellates with multiple growth strategies.

Finally, it is proposed that future C:Chl a models should strike a balance between reliability and universality. Taking the Pahlow model as an example, although the optimality-based assumption makes it better than the Monod equation in describing the nutrient uptake of phytoplankton cells, it is seldom used in marine biogeochemical modelling due to its complicated computational process. Therefore, the structure of the model should not be too complex and difficult to use.