by David Lipson
As Elio has pointed out recently, the silica cycle indeed deserves respect, and it is probably responsible for major planetary events, such as driving the evolution of C4 photosynthesis in land plants since the formation of the Himalayas and the Tibetan Plateau helped drive down CO2 levels to their low, pre-industrial levels. Some explanation:
The first photosynthetic pathway to evolve is known as the C3 pathway because the primary product of photosynthesis by the enzyme, RuBisCO, is a pair of phosphoglycerate molecules, containing 3 C's each. C3 plants currently represent about 4/5th of global plant biomass. The early history of land plants took place in a high CO2 world, where atmospheric CO2 concentrations were greater than 1000 ppm, several times higher than the recent epoch, where CO2 has cycled between about 200 and 275 ppm with glacial/interglacial cycles (until, of course, the industrial period where CO2 has risen to over 400 ppm, a level not seen for over a million years). When the ratio of CO2 to O2 is low, C3 plants are vulnerable to photorespiration, in which O2 binds to the active site of RuBisCO, causing the oxidation of the substrate and a loss of photosynthetic efficiency (2).
C4 photosynthesis is a relatively recent evolutionary modification of the C3 pathway. C4 plants have morphological and biochemical adaptations that prevent photorespiration by concentrating CO2 into bundle sheath cells deep in leaf tissues using the enzyme, PEP carboxylase, which produces oxaloacetate, a 4-C compound, hence the name. C4 plants have an advantage at low CO2 levels, as well as hot and dry conditions. Hot and dry conditions were around for a long time before the appearance of C4 plants, whereas the decline of atmospheric CO2 that has occurred over the past 65 million years or so seems to be the best explanation for the selective pressure that led to the success of this pathway, which now makes up about 18% of the earth's plant biomass (1).
One of the major geological factors that led to the "CO2 starvation" felt by plants after the Cretaceous period was the collision of the Indian subcontinent into Asia, leading to the uplift of the Tibetan plateau and the formation of the Himalayas. These processes exposed new silicate minerals to the atmosphere, where they reacted with CO2 to form calcite and silicate: CaSiO3 + CO2 → CaCO3 + SiO2
These products were carried by rivers to the ocean and buried in sediments, with the net effect of removing CO2 from the atmosphere. These changes started around 65 million years ago, and CO2 had dropped precipitously to below 500 ppm by the time C4 plants took off, around 8 million years ago. Because C4 plants have a distinct 13C isotopic signature, their spread can be traced in the tooth enamel of fossil herbivores around this time. The isotope data show grazers of many ecosystems shifting toward a C4 diet from 6 – 8 million years ago (1).
And while we're on the subject, an influx of Si to the ocean from land due to increased winds could have fertilized the ocean's diatoms (and why not Pheodarians?), sucking down atmospheric CO2 levels and shifting the planet into the last glacial maximum about 18,000 years ago (3). But that's a story for another time…
(1) Cerling TE, Ehleringer JR, Harris JM. 1998. Carbon dioxide starvation, the development of C4 ecosystems, and mammalian evolution. Philosophical Transactions of the Royal Society of London B: Biological Sciences, 353 (1365), 159–171
(2) Ehleringer JR, Monson RK. 1993. Evolutionary and ecological aspects of photosynthetic pathway variation. Annual Review of Ecology and Systematics, 24 (1), 411–439
(3) Harrison KG. 2000. Role of increased marine silica input on paleo‐pCO2 levels. Paleoceanography, 15 (3), 292–298
David Lipson works on plant-microbe interactions and teaches microbiology as associate professor at USCD, San Diego. He also takes what he's learned as a microbiologist and applies it to the design of microtonal guitars that deviate from the standard 12-note system by removing the frets from various electric and acoustic guitars and repositioning them to create novel scales.