Of Mountains and Microbes…

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).
 

(Click to enlarge )
Histograms comparing δ¹³C values for fossil tooth enamel older than 8 Ma (lower half of charts) with those that are younger than 6 Ma (upper half of charts) for (a) Pakistan, (b) East Africa (Kenya), (c) South America, (d) southwes­tern North Ame­ri­ca, (e) northwestern America, and (f) Europe. All regions have a C3-domina­ted, and perhaps ex­clu­sive C3 diet before 8 Ma. In Pakistan the younger fauna is almost exclu­sively C4-grazers; in East Af­rica the younger fauna has mammals that are gene­rally either C3-dominated or C4-dominated with a few mixed feeders; southwestern North America shows mixed feeders, as well as exclusive C3- and C4-diets; in Europe and in the northwest­ern America the younger fauna retains a C3-diet: in South America both C3 and C4 diets are evident after 6 Ma. For details see Source. Frontpage: Leaf cross section of a C4 Plant (corn) 400× mag­nification Source

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…
 

References

(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.