Fractionation of carbon isotopes in oxygenic photosynthesis values more similar to C3 plants. In contrast to terrestrial plants, where CO diffusion in air is relatively fast and typically not limiting, diffusion of dissolved CO in water is considerably slower and can often limit carbon fixation in phytoplankton. As gaseous CO is dissolved into aqueous CO, it is fractionated by both kinetic and equilibrium effects that are temperature-dependent. Relative to plants, the dissolved CO source for phytoplankton can be enriched in C by about 8‰ from atmospheric CO. Isotope fractionation of C by phytoplankton photosynthesis is affected by the diffusion of extracellular aqueous CO into the cell, the
Photosynthesis pigments present. For example, in green plants, the action spectrum resembles the absorption spectrum for chlorophylls and carotenoids with absorption peaks in violet-blue and red light. In red algae, the action spectrum is blue-green light, which allows these algae to use the blue end of the spectrum to grow in the deeper waters that filter out the longer wavelengths (red light) used by above ground green plants. The non-absorbed part of the light spectrum is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms. In