Here is a little background on this paper:

Most of the carbon that is fixed on Earth is processed through the Calvin cycle. Rubisco, the carboxylating enzyme of the Calvin cycle, performs the heavy lifting. It is extremely abundant, making up ≥ 50% of the protein in most leaves, and is most concentrated within the chloroplasts.

Rubisco (full name: ‘d-ribulose-1,5-bisphosphate carboxylase/oxygenase’) has been stereotyped as a ‘bad enzyme’ for quite some time. This is because it not only acts as a carboxylase, but also as an oxygenase. Plants have a catalytic cycle that allows carbon to be fixed will regenerate the four-carbon intermediates, but if Rubisco attempts to fix O2 instead of CO2 the cycle is broken. Instead, plants must recover some—but not all!—of the carbon through the C2 oxidative photosynthetic carbon cycle, often called photorespiration. It has been estimated that leaves under ambient conditions have a rate of oxygenation to carboxylation of 0.4. This is a major loss of carbon in terrestrial ecosystems.

There are many issues we could address, but two issues are worth keeping in mind while reading this paper. First, is there a way to increase Rubisco’s affinity for CO2, or reduce its affinity for O2? This has been a major goal for agronomists and plant physiologists for the last few decades, for obvious reasons: dramatic increases in plant yields could be had. This paper argues that it is not possible. Second, if estimates of changes in atmospheric O2 and CO2 concentrations are in any way accurate over the 460 million years of terrestrial plant evolution, how would Rubisco have functioned in these environments? For example, several authors have estimated that atmospheric oxygen increased to over 30% during the Carboniferous Period. What would that mean for Rubisco?