Elevated atmospheric CO₂ will increase pCO₂ in seawater and reduce seawater pH, leading to ocean acidification. Rising CO₂ concentrations exert contrasting effects on algae. On one hand, they generate a fertilization effect: extra carbon supply stimulates algal photosynthesis. On the other hand, the accumulation of hydrogen ions and decreased pH impose acidification stress on marine algae. Whether elevated CO₂ ultimately benefits or hinders or neutrally influence algal growth is determined by the balance between these two competing effects.
Recent studies have demonstrated that high CO₂ can produce either positive or negative impacts on algae, and such divergent outcomes are mainly modulated by co-occurring environmental factors. For instance, elevated CO₂ yields an overall inhibitory effect under nutrient limitation or thermal stress from high temperatures. Similarly, high CO₂ exerts negative influences when intense light induces photoinhibition. However, under moderate light and optimal temperature conditions, increased CO₂ can accelerate the growth of micro and macroalgae even with concurrent acidification stress.  This phenomenon is fundamentally driven by the algal CO₂ concentrating mechanisms (CCMs). Upregulation or downregulation of the CCMs directly alters intracellular CO₂ concentrations within algal cells. Moreover, elevated CO₂ exhibits a diel effect: it boosts algal performance during the day but suppresses growth (cell division) at night. The underlying reason is that light upregulates carbon carboxylation pathways, while respiratory carbon loss pathways are enhanced in darkness.
When ambient CO₂ levels rise, algae downregulate their CCMs. This reduces intracellular CO₂ and lowers the cellular CO₂/O₂ ratio. As a result, the enzyme Rubisco favors oxygenation reactions, while its carboxylation activity for carbon fixation is suppressed.