D. Andrew White, 2009
It is perhaps now common knowledge that Earth owes its oxygen rich atmosphere to photosynthetic organisms, cynanobacteria, green algae and plants. About two thirds of the organic matter produced per year is by land plants, and the rest by aquatic plants. However, at least half of the oxygen in Earth's air comes from photosynthetic algae in the oceans, algae such as the diatoms and other planktonic plants. Oceanic algae are apparently more effective in liberating oxygen than land plants.
The atmospheres of Venus and Mars are very similar in composition, despite their vastly different temperatures. Earth has a special air about it.
James Lovelock, an atmospheric scientist at Princeton, proposed the now famous "Gaia Hypothesis" in the early 1970s. James Lovelock was driven to his hypothesis by the astounding observation that the Earth's atmosphere is not in chemical equilibrium. Rather, Earth's atmosphere is in dynamic equilibrium. It is held in an unstable mixture of nitrogen and oxygen by photosynthesis, which in turn is powered by the Sun. Lovelock made the prediction that Mars is either poorly stocked with life, or completely devoid of life. The atmosphere of Mars can be explained by abiotic chemical processes, Earth's atmosphere cannot. Both the Martian and Venusian atmospheres are dominated by carbon dioxide (98% +). If all of the carbon locked up in Earth's limestone, chalk, oil and coal were to be liberated, and to react with oxygen, Earth's atmosphere would then come to resemble that of Venus.
According to most theoretic models, the Sun should not have remained stable in radiant output during its long duration. Yet life has not been exterminated by any solar vagary. Also, the amount of carbon dioxide has never been so much that life was blotted out by a run-away greenhouse effect. Nor has the carbon dioxide level ever fallen so low that all living things were obliterated by an extreme glacial epoch. There are other details of the ecosystem that are likewise controlled by the collective action of algae and plants. These processes keep the Earth's biosphere in dynamic equilibrium.
Earth has rain. Venus is too hot for rain, and Mars is now too cold for rain. Rain, being slightly acidic, erodes surface rock and generates carbonates from the reaction of carbon dioxide and basic elements. These carbonates are washed into ocean sediments, forming limestone and dolomite. The net result being a culling of carbon dioxide from the air. Green plants play a major role also, they remove carbon dioxide, and release dioxygen. Many protozoa, with carbonate shells, store up carbon as they live. The precipitation of a portion of these testae to the ocean floor is another carbon sink. Vast sediments of chalk testify to the quantity of carbon removed from the air by these plankta. Furthermore, not every living thing is recycled in the food chain. Sometimes large plants are covered in water borne mud, hence coal is formed. Some of the fats and oils of organisms becomes trapped in sediments, resulting in the formation of petroleum. All of these processes also remove carbon dioxide from the air. Over the eons these carbon sinks have greatly reduced Earth's store of carbon dioxide gas.
Even though there are some abiotic processes that remove carbon dioxide, there are no simple chemical reactions that liberate free oxygen. Earth's atmosphere is about twenty one percent oxygen gas (dioxygen). Dioxygen is too reactive to endure for long in an atmosphere. It is now known, with a great deal of certainty, that photosynthesis is the driving force behind Earth's rich unstable atmosphere.
Science has tended to confirm the existence of the Gaian balance. However, Gaia's explanation has eluded science. It is difficult to comprehend how such a fine balance could have originated, through the independent natural selection of its constituent organisms. Perhaps, as Lovelock suggested, complex ecosystems tend to develop feedback loops that tend to self-stability. Some mathematical models seem to support this hypothesis. The latest climatic models suggest that the latent heat of the oceans keeps the Earth's temperatures within liveable bounds. And that even if the Earth had a much more elliptical orbit, the Earth's oceans and atmosphere would resist extreme seasonal swings. Gaia, according to this hypothesis, may be a fortunate consequence of both thermodynamics and ecodynamics on a grand scale. Others propose that the Gaian balance is evidence of a Creator Deity. Just because this Theistic evolution hypothesis falls outside of the scope of empirical science, at present, does not imply that it can be dismissed.
Some people have argued against the existence of a perfect Gaian balance. For example, there is evidence that there was an extreme glacial epoch just prior the Cambrian. This ice age was so extreme that it is possible that life had a "close call" with total obliteration. The oceanic ice may have become so thick that most photosynthetic bacteria and algae barely survived. However, within ten million years of ice age's end the frond-like Ediacaran creatures were thriving. A few tens of million more and the oceans were teaming with protozoa, plants and animals. Since life obviously did survive, perhaps there was not a total freeze-over. Or perhaps, algae survived near volcanic hot-spots. Even this close call, if it was one, was not an isolated disaster. There been seven or so major extinctions since the Cambrian. Some extinction events, evidence suggests, may have been due to asteroid/comet impacts. The mere existence of such 'close calls' has been used as evidence that the Earth was just plain lucky. On the other hand, the fact that life has survived all of these disasters has been interpreted as evidence for the robust resiliency of the Gaian balance.
The Gaia Hypothesis is not without its critics. As originally framed, the hypothesis was a little too wildly metaphorical - almost metaphysical. It left the impression that earth is demonstrably sentient. In later more detailed versions of the hypothesis Lovelock was less metaphoric. His mechanism for Gaia was in essence merely ‘feedback stabilisation’ - i.e. complex interconnected systems tend to resist change. This toned down version of Gaia was a far cry from claiming that Earth is literally alive.
The word Gaia comes from the Greek name of the Earth Goddress. This was originally intended to be taken metaphorically, not literally. Nevertheless, it is not surprising that the Gaia Hypothesis has become a doctrine of the so-called New Age religion(s). Believers in vital force, Earth energies, telluric forces, ley lines, dowser’s energy lines, earth lights, qi gong and feng shui, have all referenced the Gaia Hypothesis as 'evidence' of their claims. All of these claims go beyond the realm of science into speculative metaphysics. All involve suppositions that do not follow logically from the scientific evidence currently available. No doubt these concepts and wild fancies reflect the psychological appeal of the Earth Mother archetype. One can appreciate the mythopoetic allure of these ideas, but it is unwise to take them too literally.
What ever the true origin of the Gaian Process, it certainly now exists. Green plants are essential to it. We owe our every breath to them.
Since photosynthesis powers the majority of the Earth's biota, one would think that the Plantae Kingdom dominates the world. This is not exactly true.
In 1988 two oceanographers, Sallie W. Chrisholm and Robert J. Olson, discovered the most common genus of cyanobacterium. Prochlorococcus is probably the most important single taxon of photosynthetic organism alive on Earth today. By shear number and ubiquity the bacterial genus dominates the seven seas. This common blue-green alga is a genus of cyanobacterium or bacterial alga. The Prochlorococcus are only 0.5 to 0.7 microns wide, and live mostly within 200 metres of the surface, where there may be 1-3 x 105 cells per millilitre of sea water. They have a mere 1.7 x 106 base pairs in their DNA, which is very brief for an independent organism. This group of bacters accounts for roughly a quarter of all photosynthetic oxygen liberation, and a similar proportion of all biotic carbon dioxide absorption, on Earth. Together with other algae they account for more than half of all oxygen production and half of all carbon dioxide consumption. There are at least 35 species of Prochlorococcus known. Its cells are spread thinly, but widely, through all of the world's oceans. Indirectly the sea is powered by algae such as them. The sea could be considered like one big leaf.
Judging by what is visible to the human eye, the oceans seem top-heavy with carnivorous animals. Seaweeds and other visible plants comprise a minority of what is visible in the sea. In the sea there are several layers of carnivores eating carnivores before one reaches algae-eaters. Hence,the greater bulk of the oceanic flora is invisible to the human eye!
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Symbiosis is the co-existence of two different species, each of which mutually benefits the other. Lichens are symbiotic combinations of algae and fungi, allowing the fungus to photosynthesise like a plant! Many mammals use bacteria to aid in their digestion of foodstuffs. The fungus-plant interactions are one of the most interesting examples of symbiosis.
Vascular plants commonly have a symbiotic relationship with fungi. In this relationship the fungal mycelia intertwine with the rootlets of plants. Roots can absorb water and highly soluble minerals. Fungi are generally better at absorbing low mobility ions, such as phosphorous, than are plants. Fungi, however, do not produce sugar and are not themselves photosynthetic. Fungi must feed upon sugars which originate, ultimately, from green plants. The term mycorrhizae is given to those soil fungi commonly found in association with plant roots. Mycorrhizae gain sugar from the roots of plants and, as if by way of trade, pass dissolved minerals to the plants' rootlets. This system is symbiosis at its best. The plant gains enhanced mineral uptake. The fungus gains an energy source.
There are generally considered to be two main forms of mycorrhizae: the endomycorrhiza and the ectomycorrhiza. These terms refer to whether, or not, the fungal hyphae penetrate the plant's rootlets. Some plant species have strictly ecto- or endo-mycorrhizae, but not both. However, other plant species can have both forms of mycorrhizae. Some species can even switch mycorrhizal forms during their life cycle.
(A) Endomycorrhizae occur where fungal hyphae penetrate the root cells.
Arbuscular endomychorrhizae form branching hyphae inside cells of rootlets.
‘Arbusculars’ are most common in the Glomales order of the glomeromycetes.
(B) Ectomycorrhizae have hyphae which encircle rootlets and the spaces between the outer layer of rootlet cells.
Often these hyphae form complete sheaths around the tips of rootlets.
Ectomycorrhizae are commonly formed by the ascomycetes, basidiomycetes, and some of the zygomycetes.
Mycorrhizal associations were discovered by Professor A.G. Frank of the Landwirtschaftiche Hochschule in Berlin. A.G. Frank was commissioned by the King of Prussia in 1885 to find a way to grow truffles (Tuber spp.). Frank failed to develop a method of truffle cultivation. Frank did, however, discover that truffles are symbiotic with tree roots (Heinrich 1997) .
Truffles were not added to the list of agriculturally grown mushrooms. Nevertheless, the truffle is an excellent example of a mycorrhizal fungus. A typical truffle produces an edible, and delicious, fruiting body that grows underground. These always grow near trees, whether oaks or beech trees. Truffles have been esteemed delicacies in Europe for centuries. Pigs and dogs were, and are, used to find these ‘mushrooms’ by smell.
There are thousands of mycorrhizal fungi species. Some are specifically associated with particular plant hosts. This is why certain mushrooms are only found near specific tree species. Others are generalists. Mycorrhizae may be ascomycetes, basidiomycetes, glomeromycetes or zygomycetes. Some of these fungi are common mushrooms and toadstools. Mycorrhizal pin-moulds are especially widespread. Nature has apparently invented mycorrhizae several times. Plants with mycorrhizal associates occur in the mosses, conifers and angiosperms. Some mycorrhizae fungi even pass nutrients to both mosses and trees! So established is the phenomenon of mycorrhizae, that some plants exploit the fungi involved. Mycoheterotrophic parasites include young orchids, pinesaps, Cryptothallus liverworts and a host of other plant species. These parasites do not make fair exchanges of carbohydrates for mineral nutrients.
Study after study has shown that mycorrhizae really do enhance the health of mosses, herbs and trees. Mycorrhizae are just one example of the incredible relationships between living things.
Glomeromycota were, until recently, classified in the phylum Zygomycota. However, genetic comparisons in the 1990s suggested that the glomales are best considered a taxon unto themselves. The members of the Order Glomales are mostly, if not entirely, asexual. The glomales can produce branched sporangia, each sporangium with very few spores. Glomales are arbuscular endo-mychorrhizal fungi, their hyphae penetrate the cell walls of rootlets. The genus Glomus is one widespread example of an endo-mycorrhizal symbiont. Most of common glomale fungi are obligate mycorrhizals. A few of the glomeromycetes (e.g. Geosiphon spp) live symbiotically with cyanobacteria instead of roots.
Glomales are a very widespread clade of mycorrhizal fungi. Many plants have glomales as their main root associates. Fossil evidence suggests that the mycorrhizal association between glomale-like fungi and plants is very ancient. Probably both land-plants and fungi have had an association since times premordial.
As if to add superfluous complexity, there seems to be yet another layer of symbiosis in the glomales. The glomales often have little bladder-like extensions on their mycelia. In these chambers there are colonies of endobacteria. Some of the genes of the Candidatus Glomeribacter gigasorarum seems to be involved in mineral uptake. So quite possibly the ‘glomeribacters’ are symbiotic.
Mycorrhizal fungi in the genus Endogone are still often classed as zygomycetes. These fungi form ectomycorrhizal sheaths around rootlets. Endogones are not imperfect fungi. The sporocarps of the endogone fungi are small puffy masses under forest duff. Mice often feed on these fruiting bodies.
Confusingly, genetic studies show that the endogones are more closely related to some of the chytrid moulds than to other pin-moulds. However, unlike the chytridiomycetes they do not have flagellated gametes. Presently endogones are not considered to be true chytrids. Probably chytrids are simply those archaemycetes that have retained flagellated gametes. Those archaemycetes which are non-aquatic seem to have lost the need for flagellated gametes. Chytrids may be considered the more fully aquatic members of the Archaemycota.
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It is ‘normal’ for small planets to have atmospheres of carbon dioxide (CO2), nitrogen (N2), water vapour (H2O) and argon (Ar). Venus’ atmosphere of carbon dioxide and nitrogen is probably much like Earth’s ancient state. Venus is too hot for water molecules, they disassociate in its infernal air. Ancient Earth certainly had both water and rain showers. Nitrogen, carbon dioxide and water vapour were probably dominant in its original atmosphere.
Earth has rain and lifeforms, which together tend to scrub carbon dioxide from the air. Over the eons, apparently, earth has irregularly, and slowly, been losing its atmospheric carbon dioxide quota. Nevertheless, Earth’s small carbon dioxide level, along with water vapour, plays a crucial role for the biosphere. It is largely due to the greenhouse gasses that Earth's average surface temperature is above 0oC. Carbon dioxide is a greenhouse gas. It adsorbs higher frequency light waves, and re-emits the energy as infrared light. Infrared light does not easily escape into outer space. Consequently, it recycles thermal energy in the atmosphere which would otherwise radiate off into space, i.e. the temperature goes up. Greenhouse gasses, in effect, make the Earth darker on the infrared side of the spectrum. Least one think that this process is thermodynamically impossible, there is a concrete analogy. In full sunlight, a dark pavement can warm above freezing, even if the air temperature is sub-zero.
Now I have said that carbon dioxide has been falling through the ages. This has not been true in the past century and a half. Since the 1850s the total concentration of carbon dioxide in the atmosphere has risen from 0.0280 to 0.0355 percent by volume. The gas concentration can be calculated from bubbles dissolved in ice cores, as well as from old measurements made by chemists. During the same time period the average temperature of the lower atmosphere has risen by 0.6 Co. A less than one degree shift may seem small, but this is a global average. In reality, it has meant a rise of several degrees in the temperate and polar regions. It has also meant noticeably shorter and warmer winters.
Carbon dioxide levels have increased, temperatures have increased. It is not exactly rocket science to hypothesise that a surge in a 'greenhouse gas' could cause atmospheric temperature to rise. In fact, there is little reason to doubt that our current bout of global warming is due to the release of carbon dioxide from the burning of fossil fuels. Analyses of ice cores show that carbon dioxide levels have risen and fallen since the last ice age. The overall trend has been upward, with a gradual plateau at circa 280 p.p.m. But this then ‘spiked’ past 300 p.p.m. during the industrial era. These spikes do not appear to have been due to Neolithic farming or forest clearances. Isotope ratio analyses suggest that most spikes in carbon release had oceanic causes. This includes the spike and warm spell just after the last ice age. But the current spike appears to be different. It is actually higher than were any of the spikes since the last ice age. Isotope ratios suggest that it had an origin that was mostly terrestrial (Elsig et al 2009). Hence it is probable that the current carbon dioxide spike has an anthropogenic cause.
Earth has experienced fairly rapid climate swings before. Climate may change because of: solar variation, the precession of the equinox, axial wobble, asteroid impacts and periods of increased volcanism. The current rate of global temperature change is similar to the shifts which occurred at the beginning and end of each ice age. However, the temperature rise seems to be a bit more rapid than that which occurred after the last ice age. This climatic shift may have adverse affects on wildlife.
Global warming is expected to be a mixed blessing and curse. Global warming may increase crop yields overall. Almost certainly precipitation will increase globally. Nevertheless, certain regions could experience increased drought. Glaciers are already melting at an unprecedented rate. Global warming would also cause the sealevel to rise. Rising sealevel would be very bad news for lowlands such as the Maldives. Increased equatorial temperature could increase the frequency of hurricanes. Rising carbon dioxide levels would certainly increase the oceans' acidity. Of greatest concern is the probability that some ecosystems may not be able to respond quickly enough. Climate change may occur faster than some species can adapt. Some species may face extirpation or extinction if the climate shift is greater than they can tolerate. This stress is especially true today when human activity has fragmented and over-exploited too many ecosystems. In short, unstable temperatures could be very bad for wild ecosystems.
Elsig, J.; Schmitt, J.; Leuenberger, D.; Schneider, R.; Eyer, M.; Leuenberger, M.; Joos, F.; Fischer, H. and Stocker, T.F. 2009. Stable isotope constraints on Holocene carbon cycle changes from an Antarctic ice core. Nature. 461 (24): 507-510.
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Nilsson, Annika. 1992. Greenhouse Earth. John Wiley & Sons. Chichester.
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