Methane, oxygen, photosynthesis, rubisco and the regulation of the air through time

Abstract

Rubisco I's specificity, which today may be almost perfectly tuned to the task of cultivating the global garden, controlled the balance of carbon gases and O(2) in the Precambrian ocean and hence, by equilibration, in the air. Control of CO(2) and O(2) by rubisco I, coupled with CH(4) from methanogens, has for the past 2.9 Ga directed the global greenhouse warming, which maintains liquid oceans and sustains microbial ecology.Both rubisco compensation controls and the danger of greenhouse runaway (e.g. glaciation) put limits on biological productivity. Rubisco may sustain the air in either of two permissible stable states: either an anoxic system with greenhouse warming supported by both high methane mixing ratios as well as carbon dioxide, or an oxygen-rich system in which CO(2) largely fulfils the role of managing greenhouse gas, and in which methane is necessarily only a trace greenhouse gas, as is N(2)O. Transition from the anoxic to the oxic state risks glaciation. CO(2) build-up during a global snowball may be an essential precursor to a CO(2)-dominated greenhouse with high levels of atmospheric O(2). Photosynthetic and greenhouse-controlling competitions between marine algae, cyanobacteria, and terrestrial C3 and C4 plants may collectively set the CO(2) : O(2) ratio of the modern atmosphere (last few million years ago in a mainly glacial epoch), maximizing the productivity close to rubisco compensation and glacial limits.

Figure 1

Outline of rubisco phylogeny (time scale adapted from Nisbet & Sleep (2001). LCA, last common ancestor.

Figure 2

Outline geological record of the atmosphere (modified from Nisbet et al. 2007), showing timings of some key parts of the rock record.

Figure 3

Possible atmospheric interpretation of geological record: hypothesis modified and updated after Lovelock (1988).

Figure 4

Rubisco compensation controls on the atmosphere. O₂ Γ and CO₂ Γ lines plotted from data for tobacco seedlings in Tolbert et al. (1995). Note the extent of permitted zone in which oxygenic photosynthesis can occur (i.e. the limits to plant, algal or cyanobacterial growth). Extension to past conditions makes the large assumption that evolutionary tuning of rubisco specificity and compensation is ancient and was similar in those past times. (Note: Assumes modern CO₂ Γ and O₂ Γ—i.e. early evolutionary tuning or rubisco I specificity.)

Figure 5

Synopsis of possible Late Archaean atmospheric conditions.

Figure 6

CO₂ emission spikes compared with recovery from Snowball Events. (i) CO₂ emission spike (e.g. from eruption of a large igneous province). Strong release of CO₂ (e.g. doubling atmospheric CO₂) causes short-term warming and flourishing photosynthesis. However, when the CO₂ has been taken up and mole-equivalent O₂ released, the impact on the much larger O₂ inventory is small. The system thus regresses close to the starting point on the compensation line (probably within a few hundred thousand years). Control by the large O₂ inventory gives stability to the system. The Palaeocene/Eocene thermal maximum may be an example. (ii) During a snowball, CO₂ from volcanic gases builds up until greenhouse warming is sufficient to initiate melting. This may occur only when CO₂ is approximately 12% (Caldeira & Kasting 1992). Once warming begins, photosynthesis restarts and this large CO₂ inventory allows a mole-equivalent O₂ release. The system regresses to the compensation line at very much higher O₂ and thus at higher CO₂, allowing much warmer higher O₂ conditions to be sustained, compared with the system prior to the event.

Figure 7

Cyanobacterial and other microbial controls in the Archaean ocean/atmosphere system. AOM, anaerobic oxidation of methane.