The kingdom of the eukaryotes rests on a very simple environmental economy. Plants are producers of carbohydrate through photosynthesis, thereby generating excess oxygen from the photo- and molecular chemistry involved. Animal consumers use up oxygen in their metabolism and return carbon dioxide, the ultimate source of carbohydrate, to the air. A simple view is that animals contribute to global warming, whereas plants help cool the world. Perhaps because of that “common sense” view, most environmental scientists take a very different line, linking it with volcanic exhalation of CO2, “capture of carbon through rock weathering and the burial of dead organic matter in the global carbon cycle. Greg Retallack of the University of Oregon is about to publish a reappraisal of the animal versus plant part of the C-cycle (in press, Journal of Geology) that is based on observed imbalances between the two opposed kinds of respiration. Specialists in the C-cycle hold that there is a an overall balance, taking all components into account, whose inevitable result is the build up of oxygen in the atmosphere of an inhabited world. Yet oxygen is extremely reactive and should quickly combine in mineral oxides and hydroxides – after all, the iron in an untended car reverts to its oxide ore in the space of a few decades at most.
Partly following James Lovelock’s Gaia hypothesis, Retallack focuses on the major fluctuations in atmospheric chemistry evidenced in the geochemical record, the most immediate being the see-saw fluctuation of modern levels of CO2 in the atmosphere – a 2% annual variation controlled by the waxing and waning of vegetation in the northern hemisphere (where plant cover is greatest) according to season. One of the largest shifts in atmospheric CO2 concentration followed the evolution of land plants from about 450 Ma ago. To thrive, they had to develop hard cellular material (lignin) that formed stems and trunks, which animals of the Palaeozoic were unable to oxidise efficiently. Both living biomass and burial of undigested lignin drew down CO2 and boosted oxygen levels. Animal evolution eventually exploited this “free lunch” through the humble termite and reptilian and then mammalian megafauns. Retallack believes that heavy breathing that resulted from lignin digestion reversed the declining CO2 trend for the 200 Ma following the Carboniferous to Permian glacial epoch in Gondwana. Though displaying some ups and downs, the Mesozoic saw a “greenhouse” world. Removal of the mighty and extremely abundant herbivorous dinosaurs by the K-T mass extinction provided and opportunity for plant diversification. Many Mesozoic plants evolved armour against browsing dinosaurs, exemplified by the surviving Andean “monkey puzzle” tree Araucaria. Their demise removed the need, and the plant Kingdom’s evolutionary response was the appearance of grasses. Reatallack points out that grass itself is not as good as lignin-rich plants in holding CO2, but grasslands encourage the development of thick carbon-rich soils that hold more than the soils of the forest floor. It is this development that Retallack believes lay at the base of the decline in average global temperature through the Cainozoic, to culminate in the present Ice Age. Unsurprisingly, proponents of the complexity and diversity of the C-cycle, particularly in the oceans, are disinclined to have truck with the hypothesis.
Source: Pearce, F. The Kingdoms of Gaia. New Scientist, 16 June 2001, p. 30-33.
Carbonates and biofilms
Above the low level that is essential for their role in molecular “information” transfer, calcium ions pose a fatal threat to cell processes. That is simply because excess calcium combines with carbonate ions to form minute calcium carbonate crystals within the cell when the solubility product of calcite is exceeded. The solubility product is the concentration of calcium ions multiplied by that of carbonate ions, so that increase in one or the other can lead to supersaturation of calcium carbonate and imminent precipitation. Because CO2 is an essential need for photosynthesis and a product of animal metabolism, this risk is always present. In the most common photosynthesising bacteria, the cyanobacteria that have been around for at least 3.6 billion years, the drawing in of CO2 in the form of carbonate (CO32-) or bicarbonate (HCO3–) ions in water can result in supersaturation immediately around the cell. When it occurs, the “blue-green” bacterial biofilms induce precipitation of calcium carbonate. That is why such micro-organisms can act as reef builders, as they did to great effect during the early Precambrian (stromatolites), and also from Cambrian to Cretaceous times.
Calcite mineralization by biofilms is, however, a complicated process. It is connected with highly reactive substances that cyanobacteria exude outside their cell walls. Depending on their degree of ordering and the supply of calcium ions, these substances control the manner in which calcium carbonate precipitates. The detailed biochemistry and the form of calcite biofilms obtained by study of modern cyanobacteria in different watery environments has allowed Gernot Arp and co-workers at the University of Göttingen to evaluate varying calcium and CO2 concentrations in ocean water since 540 Ma, and suggest differences in Precambrian oceans (Arp, G. et al. 2001. Photosynthesis-induced biofilm calcification and calcium concentrations in Phanerozoic oceans. Science, v. 292, p. 1701-1704).
Their studies suggest that up to the Cretaceous, the Phanerozoic oceans must have had higher calcium contents than they do today. Microbial reefs formed in that period preserve details of the “blue-green’s” cell structure, suggesting that calcite was nucleated directly by the extracellular substances. Vast burial of the calcite shells of planktonic metazoan organisms to form the Chalk deposits of Cretaceous age reduced very high levels to give the calcium-depleted oceans that prevailed during the Cainozoic. Microbial carbonates of these younger ages show no structure. The stromatolites that are so characteristic of Precambrian limestones are stuctureless too, although they show evidence of progressive build-up from myriads of thin layers. Irrespective of the Precambrian oceans’ calcium content, this lack of structure can be explained by more dissolved CO2 that resulted from its higher concentration in the atmosphere. About 700-750 Ma ago, stromatolites that contain calcified cyanobacterial cells appear, and that may signify the massive drawdown of CO2 from the atmosphere that is implicated in creating icehouse conditions on a global scale during the late Proterozoic Aeon.