The significance of the "quantum yield" model's prediction of C3/C4 distributions is best viewed in the context of atmospheric C02 changes that have occurred over the past several hundred million years. The history of levels of atmospheric C02 is related to its production through volcanism relative to the losses associated with weathering, photosynthesis, and burial in the oceans (Berner, 1994, 1997). The important biogeochemical processes contributing to the change in atmospheric C02 are and
--> CH,O + O, where the first reaction describes weathering and the formation of carbonate sediments that are finally deposited in oceanic carbon sinks and the second reaction is an abbreviated description of the production and burial of organic matter in terrestrial sediments. The combination of these two reactions and the presence of liquid water on earth results in a long-term decline in atmospheric C02 values (Berner, 1991).
While there is uncertainty about the atmospheric C02 values prior to half a million years, most modeling and analytical approaches suggest that atmospheric C02 levels were substantially higher in the Cretaceous than they are today (Fig. 2, left). Modeled and experimental approaches further agree that atmospheric C02 levels began to decline during the late Cretaceous, eventually settling into a range of concentrations less than 500 ppmV. These
100 Age, Ma
400k 300k 200k 100k 0 Age, Ka b.p.
100 Age, Ma
400k 300k 200k 100k 0 Age, Ka b.p.
FIGURE 2 Patterns of atmospheric C02 concentrations through time. Left plate: reconstruction of paleo C02 levels between 200 Ma and present; adopted from Cerling et al. 1998a. Middle plate: reconstruction of atmospheric CO, from ice cores for the past 160,000 years; adopted from Petit et al (1999). Right plate: atmospheric CO, concentrations recorded at Mauna Loa, Hawaii since 1958; adopted from Keeling (1998). records at ORNL CDIAC
relatively low atmospheric C02 levels are thought to have characterized the earth's atmosphere from the late Miocene up to the dawn of the Industrial Revolution. Icecore data, particularly the lengthy Vostok ice core observations from Antarctica (Jouzel et al., 1987; Petit et al., 1999), indicate that over the past 420,000 years there have been oscillations in the atmospheric C02 from 180 to 280 ppmV, associated with glacial and interglacial periods, respectively (Fig. 2, middle). In contrast to this long-term historical pattern is an anthropogenically induced increase in atmospheric C02 levels, especially during the 20th century, to values well in excess of 350 ppmV in association with the continued combustion of fossil fuels (Fig. 2, right).
The answer to "why should natural selection favor the emergence of a second photosynthetic pathway?" is seen in the large decreases in atmospheric C02 that have occurred over the past 200 million years, particularly the changes in atmospheric C02 levels in the past 6-8 million year, while during the same interval atmospheric 02 levels are thought to have remained almost constant. It is the changes in the [C0,]/[02 ] ratio that result in decreased photosynthesis by C, plants as photorespiration rates increase, which favors the evolution and expansion of C4 photosynthesis. The higher activity of PEP carboxylase effectively creates a "CO, pump," resulting in a [C02]/ ratio inside the bundle-sheath cells of C4 plants that is several-fold greater than observed at sites of Rubisco activity in C, plants. The "quantum yield" model predicts how common C4 photosynthesis is expected to be for any global atmospheric CO, level. Specifically, the model predicts the temperature ranges that should have favored C4 over C, as atmospheric CO, declined over the last 200 million years.
The decreased atmospheric C02 levels have had enormous con sequences both for the distribution of plant communities across our planet and for animal evolution, as will be discussed below. While throughout much of history, earth had been subject to relatively high atmospheric C02 levels, the earth has now been in a "C02 -starved mode" for approximately 7 Ma with periods of exceptionally low atmospheric C02 levels (-180 ppmV) characterizing the atmosphere during recent glacial periods.
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