File:Ocean carbon cycle and diatom carbon dioxide concentration mechanisms.jpg

English: Ocean carbon cycle and diatom carbon dioxide concentration mechanisms

(A) Schematic representation of the ocean carbon cycle depicting the role of marine diatoms in the biological carbon pump. The anthropogenic CO2 emission to the atmosphere (mainly generated by fossil fuel burning and deforestation) is nearly 11 Gigaton carbon (GtC) per year, of which almost 2.5 GtC is taken up by the surface ocean. In surface seawater (pH 8.1–8.4), bicarbonate (HCO3–) and carbonate ions (CO32–) constitute nearly 90 and <10% of dissolved inorganic carbon (DIC) respectively, while dissolved CO2 (CO2 aqueous) contributes <1%. Despite this low level of CO2 in the ocean and its slow diffusion rate in water, diatoms fix 10–20 GtC annually via photosynthesis thanks to their carbon dioxide concentration mechanisms (CCMs), allowing them to sustain food chains. In addition, 0.1–1% of this organic material produced in the euphotic layer sinks down as particles, thus transferring the surface carbon toward the deep ocean and sequestering atmospheric CO2 for thousands of years or longer. The remaining organic matter is remineralized through respiration. Thus, diatoms are one of the main players in this biological carbon pump, which is arguably the most important biological mechanism in the Earth System allowing CO2 to be removed from the carbon cycle for very long period. Based on data from Friedlingstein et al., 2020.
(B) Schematic representation of the CCMs in diatoms. The low levels of CO2 in the ocean and its slow diffusion rate in water have led diatoms and other photosynthetic organisms to evolve CCMs that utilize the higher concentrations of HCO3–. The biophysical CCM consists of various bicarbonate transporters and carbonic anhydrases (CAs) that serve to increase the CO2 flux balance toward the pyrenoid, a low CO2-permeable subcellular compartment in the chloroplast containing most of the Rubisco. In addition, some diatoms may also have a biochemical (C4-like) CCM involving phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate carboxykinase (PEPCK) and/or malic enzyme (ME).
(C) Schematic presentation of Rubisco activation by CbbX in diatoms and other phototrophs with red-type Rubisco. CbbX functions as a mechanochemical motor protein and uses the energy from ATP hydrolysis to modify the structure of Rubisco. This process facilitates the dissociation of inhibitory sugar phosphates [ribulose-1,5-bisphosphate (RuBP) and others] from the active site of Rubisco.

• Friedlingstein, P., O’Sullivan, M., Jones, M. W., Andrew, R. M., Hauck, J., Olsen, A., et al. (2020). Global carbon budget 2020. Earth Syst. Sci. Data 12, 3269–3340.