Figure 2.14 An equilibrium reaction for carbon in water. The reaction converting CO₂ into bicarbonate ions (HCO₃-) is an equilibrium reaction. When photosynthetic organisms preferentially use CO₂, because it is used more efficiently, the amount of dissolved CO₂ in the water declines. As the amount of CO₂ declines, some of the bicarbonate ions are converted to CO₂ to replenish the supply. The size of each circle represents the relative size of the carbon pool.