The N:Chl-a ratio defines the optimal physiological state in marine microalgae

Several factors may influence the growth of microalgae and their biochemical composition. Thus, bioassays can contribute to understanding how the environment influences the chemical element:chlorophyll-a ratio and the metabolic processes in different microalgae. The objective of this study, under laboratory conditions, was to study the variation in the Carbon-to-Chlorophyll-a (C:Chl-a), Nitrogen-to-Chlorophyll-a (N:Chl-a), and Hydrogen-to-Chlorophyll-a (H:Chl-a) ratios as indicators of the physiological state of species representative of different functional groups of phytoplankton: Isochrysis galbana, Chaetoceros calcitrans, and Dunaliella tertiolecta. The results suggest that changes in the content of intracellular pigments can be directly attributed to maximizing phytoplankton growth through the metabolic requirements, where Nitrogen and Carbon uptake and assimilation are critical drivers related to their physiological needs rather than their photosynthesis processes per se. Our observations documented the behavior of phytoplanktonic microalgae, where I. galbana showed the lowest Chl-a values associated with metabolic processes to produce complex molecules for storage and as reserve compounds. In C. calcitrans, metabolic processes are mainly addressed to generate complex compounds of Carbon and Ni-trogen inside the cell rather than for population increase. Finally, D. tertiolecta showed that these elements are used gradually but progressively, generating more photosynthetic units during the exponential growth phase. This work provides new evidence to understand metabolic mechanisms between different groups of phyto-plankton, essential knowledge to produce macromolecules and bioactive metabolites with different applications.