The global mass and average rate of rubisco

Abstract

Photosynthetic carbon assimilation enables energy storage in the living world and produces most of the biomass in the biosphere. Rubisco (d-ribulose 1,5-bisphosphate carboxylase/oxygenase) is responsible for the vast majority of global carbon fixation and has been claimed to be the most abundant protein on Earth. Here we provide an updated and rigorous estimate for the total mass of Rubisco on Earth, concluding it is ≈0.7 Gt, more than an order of magnitude higher than previously thought. We find that >90% of Rubisco enzymes are found in the ≈2 × 10¹⁴ m² of leaves of terrestrial plants, and that Rubisco accounts for ≈3% of the total mass of leaves, which we estimate at ≈30 Gt dry weight. We use our estimate for the total mass of Rubisco to derive the effective time-averaged catalytic rate of Rubisco of ≈0.03 s^-1 on land and ≈0.6 s^-1 in the ocean. Compared with the maximal catalytic rate observed in vitro at 25 °C, the effective rate in the wild is ≈100-fold slower on land and sevenfold slower in the ocean. The lower ambient temperature, and Rubisco not working at night, can explain most of the difference from laboratory conditions in the ocean but not on land, where quantification of many more factors on a global scale is needed. Our analysis helps sharpen the dramatic difference between laboratory and wild environments and between the terrestrial and marine environments.

Keywords: Rubisco; primary productivity; quantitative biology.

Fig. 1.

A schematic representation of our methodology for estimating the mass of Rubisco. Our methodology for estimating the mass of terrestrial Rubisco comprises two parts: estimating the total mass of leaves and estimating the mean mass fraction Rubisco out of the dry leaf mass. Our methodology for estimating the mass of marine Rubisco is based on an estimate of the total biomass of marine autotrophs, as well as on reports on the fraction of Rubisco out of the dry weight of autotrophs.

Fig. 2.

The distribution of the fraction of Rubisco out of the dry mass of leaves. Each color denotes a different category of plant growth form, with dark-green values representing woody plants and the lighter-green values representing C3 herbaceous plants. The vertical line indicates the average fraction for each growth form used for the analysis.

Fig. 3.

Estimating the effective rate of an average Rubisco. (A) We use estimates for the total annual rate of carbon fixation—the gross primary productivity—in the terrestrial and marine environments, in conjunction with our estimates of the total mass of Rubisco in those environments. We convert the units of the total rate of carbon fixation to reactions per second using the fact that each reaction of Rubisco fixes one carbon atom, which has a mass of 12 Da. We also convert our estimates for the total mass of Rubisco into the number of active sites by dividing the total mass by the molecular weight of an active site. By dividing the total amount of reactions that all the active sites are performing each second by the total number of active sites, we get an estimate of the average rate of a single active site. B and C show the calculations for terrestrial and marine Rubisco, respectively.