Was there photosynthetic life on Earth 3.5 billion years ago
Was there photosynthetic life on Earth 3.5 billion years ago?
Viewpoint: Yes, morphological analyses and laser-Raman imaging have shown that photosynthetic life on Earth existed 3.5 billion years ago.
Viewpoint: No, it is more likely that life at the time was still using other, chemical, forms of energy to survive.
The two essays that follow debate claims concerning the possible early appearance of photosynthesis, which some scientists maintain originated on Earth 3.5 billion years ago (Gya). If this is true, it would be extraordinary, because Earth at that time was quite different from now, and indeed had only recently cooled after millions of years of battering by asteroids. For photosynthesis to have taken place at that time—and there is considerable evidence to suggest that it did—would be extremely remarkable, given the complexity of the photosynthetic process and the level of evolutionary advancement that it represents.
Photosynthesis is the biological conversion of light or electromagnetic energy from the Sun into chemical energy. It occurs in green plants, algae, and some types of bacteria, and requires a series of biochemical reactions. In photosynthesis, carbon dioxide and water react with one another in the presence of light and a chemical known as chlorophyll to produce a simple carbohydrate and oxygen. (A simple carbohydrate or simple sugar is one that, as its name suggests, cannot be broken down into any substance more basic. Examples include glucose or grape sugar, fructose or fruit sugar, and galactose.)
Though the thorough study of photosynthesis involves reference to exceedingly complex and (to the uninitiated) seemingly tedious biochemical information, in fact this process is one of the great miracles of life. In photosynthesis, plants take a waste product of human and animal respiration—carbon dioxide—and through a series of chemical reactions produce both food for the plant and oxygen for the atmosphere. Indeed, oxygen, which is clearly of utmost importance to animal life, is simply a waste by-product of photo-synthesis.
Much of the debate in the essays that follow concerns the fossil record, which is extremely problematic when one is discussing life forms that existed during Precambrian time. The latter term refers to the first three of the four eons into which Earth's geological history is divided, with the Cambrian period marking the beginning of the present eon, the Phanerozoic, about 545 million years ago. The early Cambrian period saw an explosion of invertebrate (lacking a spinal column) marine forms, which dominated from about 545 to 417 million years ago. By about 420 to 410 million years ago, life had appeared on land, in the form of algae and primitive insects. This helps to put into perspective the time period under discussion in the essays that follow—about 3 billion years before the beginning of the reliable fossil record.
The term fossil refers to the remains of any prehistoric life form, especially those preserved in rock prior to the end of the last ice age about 11,000 years ago. The fossils discussed in the following essays are the oldest on Earth, being the remains of single-celled organisms found in rock samples almost 80% as old as Earth itself. The process by which a once-living thing becomes a fossil is known as fossilization, wherein hard portions of the organism—such as bones, teeth, shells, and so on—are replaced by minerals. However, one of the controversial aspects of using the fossil record as a basis for forming judgments about the past (including the question of extremely early photosynthetic life) is that the fossil record is not exactly representative of all life forms that ever existed.
There is a famous story regarding a polling organization that produced disastrously inaccurate predictions regarding the outcome of the 1932 presidential elections. Their method of polling—using the telephone—would be perfectly appropriate today, but at the depths of the Depression, people who had phones were likely to be well-off. Those polled by this method reported overwhelmingly in favor of the incumbent, Herbert Hoover, but they were not a representative sample of the population. The vast majority of America—poor, uneducated, and hopeless—responded to the promises of Franklin D. Roosevelt, who won the election handily.
In the same way, the fossil record does not necessarily contain an accurate account of all life forms that existed at a particular point in the distant past. The majority of fossils come from invertebrates, such as mussels, that possess hard parts. Generally speaking, the older and smaller the organism, the more likely it is to have experienced fossilization, though other factors also play a part. One of the most important factors involves location: for the most part, the lower the altitude, the greater the likelihood that a region will contain fossils. Even so, all conditions must be right to ensure that a creature is preserved as a fossil. In fact, only about 30% of species are ever fossilized, a fact that scientists must take into account, because it could skew their reading of the paleontological record.
Also referenced in the arguments that follow is the study of DNA, or deoxyribonucleic acid, as a means of determining how life forms developed. All living cells contain DNA, which holds the genetic codes for inheritance—a blueprint for the organism. For organisms that reproduce asexually, through the splitting of cells, the DNA in ordinary cells also contains the blueprint for the offspring, whereas sexually reproducing organisms have special sex cells. The bacteria in question here would have reproduced asexually, as do bacteria today.
DNA is exceedingly complex, yet much of it varies little from organism to organism. For example, only about 0.2% of all human DNA differs between individuals, meaning that humans are 99.8% the same—and that all the variation that exists between people is a product of just 1/500th of the total DNA. Even between humans and chimpanzees, about 98% of the DNA is identical.
Photosynthesis, fossils, and DNA are just some of the concepts referenced in the lively discussion that follows. Compelling arguments are made for both sides, and while it might be easier, on the surface of it, to believe that photosynthesis could not have existed 3.5 Gya, the idea that it could have existed is too intriguing to be ignored. Furthermore, the fact that a number of prominent scientists have placed themselves on the affirmative side in this argument further recommends serious consideration of all the evidence.
Viewpoint: Yes, morphological analyses and laser-Raman imaging have shown that photosynthetic life on Earth existed 3.5 billion years ago.
It may be hard to believe, but a six billionth of a meter-long microbe imbedded in a rock for 3.5 billion years is undergoing an identity crisis. Is it really a microbe that proves photosynthetic life was flourishing on Earth about one billion years earlier than scientists previously suspected? Or is it an impostor, a mere bubble-like structure or rock flaw that looks similar to a microbe but may be volcanic glass from a hydrothermal vent formed under water? The fierce debate surrounding the answers to these questions involves complex geo-chemical analyses. It also pits primarily two camps of scientists against each other, each claiming that what they see when they look through their microscopes is the truth.
The hotly debated topic is an important one. If photosynthetic life was thriving on Earth 3.5 billion years ago (Gya), this fact would ultimately require scientists to rewrite much of what they believe about Earth's early history, including the evolution of both the planet and life. For example, the 3.5-billion-year date indicates that life appeared soon (at least in geological terms) after asteroids stopped colliding en masse into Earth 3.9 Gya. The appearance of photosynthetic life at such an early point in Earth's history would also indicate that life evolved rapidly afterwards, probably in scalding pools as opposed to the warm springs that would have abounded a million years later after the Earth had a chance to cool down a bit.
The Debate Begins
The discovery of the oldest known fossil bacteria dates back to 1993 when William Schopf, director of the Center for the Study of Evolution and the Origin of Life at the University of California, Los Angeles (UCLA), and coworkers announced that they had discovered a diverse bacterial flora in the 3.465-billion-year-old Apex cherts (flint-like rock) in a greenstone belt near Marble Bar in Western Australia. They ultimately identified 11 different bacterial species, including cyanobacteria, which are known as the "architects of the atmosphere" because they are photosynthetic and produce oxygen.
Although Schopf's identification of the fossils as cyanobacteria was widely accepted by many scientists, others have disputed the findings because they are based largely on morphology, that is, on the bacteria's form and structure. Unfortunately, bacteria have little in the way of concrete morphological features that can be used for identification purposes, especially after being imbedded in rock for billions of years. As a result, it is extremely difficult to discern between real bacterial fossils and pseudofossil look-alikes.
However, Schopf's finding was later bolstered by analyses of the isotopic composition of carbon in Archaean sediments. These analyses have supported the general proposition that life could have begun 3.5 Gya. Cyanobacteria live in the water and can manufacture their own food. Because they are bacteria, they are extremely small and usually unicellular. However, they often grow in colonies large enough to see. These colonies are like sticky mats that become finely layer red mounds of carbonate sediment called stromatolites. The specimens Schopf analyzed were associated with isotopically light carbon (C), which indicates photosynthetic activity. Living photosynthetic organisms preferentially incorporate 12C (six protons, six neutrons in the nucleus) as opposed to the rarer, heavier isotopes of 13C and 14C. Therefore, if a carbon-rich object has a greater amount of 12C relative to 14C than the surrounding rock, it is likely biologically produced carbon.
Although Schopf and his colleagues have been studying microscopic wisps of carbonaceous material, they carefully evaluated the evidence and came to the conclusion that this was indeed life. They based their reasoning on strict fundamental "rules" that Schopf and his colleagues had developed over years of studying and correctly identifying early small microorganisms. As Schopf stated in a presentation on microbe hunting and identification for a National Research Council workshop, it is not enough to deduce that an object must be biological because it does not look like a mineral or flaw. Positive evidence is needed; evidence strong enough to rule out plausible nonbiological sources. Schopf notes that "the best way to avoid being fooled by nonbiological structures is to accept as bona fide fossils only those of fairly complex form."
Although Schopf and his colleagues believed they had met these criteria and more, the morphological interpretation of data has kept the door open on a decade of debate. However, in the March 7, 2002, issue of Nature, Schopf and his colleagues presented evidence that strongly supports the view that life on Earth originated at least 3.5 Gya. The evidence is based on newly developed technology that enables scientists to look inside of rocks, determine what they are made of, and make a molecular map of any embedded structures.
For paleontologists, laser-Raman spectroscopy is a tremendous breakthrough that gives them Superman-like x-ray vision. As Schopf explains: "Because Raman spectroscopy is non-intrusive, non-destructive and particularly sensitive to the distinctive carbon signal of organic matter of living systems, it is an ideal technique for studies of ancient microscopic fossils. Raman imagery can show a one-toone correlation between cell shape and chemistry and prove whether fossils are biological."
In essence, Raman analysis provides both a two-dimensional image of the sample and its chemical composition. The spectrographic tool does this by bouncing laser beams off rock surfaces, creating a scattering pattern that is identical to the patterns created by organic molecules found in other fossils.
Schopf and his colleagues first tested the spectroscopic technique on four fossil specimens that had ages already established through other techniques and approaches. In their Nature article, Schopf and his colleagues state that the tests showed that laser-Raman spectroscopy can "extend available analytical data to a molecular level." Particularly, they note, "it can provide insight into the molecular make-up and the fidelity of preservation" of the kerogenous matter that makes up fossils. Kerogens are organic materials found in sedimentary rock.
Admittedly, the ancient samples under study are minute microscopic remnants, described by Schopf in the Nature paper as "graphitic, geo-chemically highly altered, dark brown to black carbonaceous filaments." But the laser-Raman analysis technique (developed by Schopf and his colleagues at UCLA and the University of Alabama) is highly sensitive to the distinct signal of carbonaceous matter so it can be used to characterize the molecular composition of fossils as small as 1 micron in diameter. The technique is so accurate that, as Schopf points out, it can achieve this analysis "in polished or unpolished petro-graphic thin sections, in the presence or absence of microscopy immersion oil, and whether the fossils analyzed are exposed at the upper surface of, or are embedded within, the section studied."
Schopf and his colleagues showed that the kerogen signal they identified did not result from contamination by immersion oil, which is used to enhance optical images. In fact, the laser-Raman technique could easily distinguish fossil kerogen signals from the possible spectral oil contaminants. Schopf and his colleagues also took great care to avoid contamination from graphite markers, which had been used to circle the fossils for identification. This source of contamination was further removed by the fact that some of the fossils were embedded as much as 65 microns in the quartz matrix, where they could not be affected by any surface contamination.
Schopf and his colleagues were also able to directly correlate molecular composition with filament morphology and establish that the filaments were composed of carbonaceous kerogen. Furthermore, they showed that the organic substance occurred in much greater concentrations in the fossils than in the "wispy, mucilage-like clouds of finely divided particulate kerogen in which the fossils … are preserved." These carbon molecules, which are the decay products of living bacterial cells, have firmly established the fossils' biogenicity.
Are Fossils in the Eyes of the Beholder?
Despite Schopf's further evidence, another faction of paleontologists, led by Martin Brasier of Oxford University, have sounded off loudly against Schopf's findings, publishing a paper on their theory in the same March 7, 2002, issue of Nature. They believe that Schopf's fossils are nothing more than "marks" resulting from an ancient and unusual geological process involving hydrothermal vents for volcanic gases and the formation of rock from magma coming in contact with the cooler earth surface. Nevertheless, Brasier's group agrees that the marks or fossils (depending on your point of view) have a chemical composition that appears biological in origin.
Brasier and his colleagues try to explain away this biological confirmation by saying that these seemingly biological molecules are actually the result of interactions between carbon dioxide and carbon monoxide, which is released by hot, metal-rich hydrothermal vents. The group argues that these molecules were trapped in bacteria-like filaments as the hot rocks cooled. However Schopf and others point out that, if Brasier's hypothesis is correct, this type of material should be found abundantly but is not, or at least has not been so far.
In an ABC Science Online article, Professor Malcom Walter of Macquarie University's Australian Centre for Astrobiology notes that the chemical evidence combined with the fossil shapes was "strongly suggestive" that the marks are actually fossil bacteria. However, Brasier and his colleagues argue that the squiggly marks do not even look like other ancient microbes in that their shapes are too complicated. Brasier went on to argue that Schopf illustrated selective parts of the fossils that looked like bacteria while ignoring the bulk of other carbonaceous material. Walter responds that such variations are to be expected. "Long experience shows," says Walter, "that with preserved or fossilized bacteria, only a few cells maintain their shape. Brasier's argument overlooks this."
Schopf's group believes that Brasier's interpretation of the data is simply mistaken. As Schopf and his colleagues point out, Brasier's group is not as adept at looking at Precambrian microfossils as Schopf's group. Furthermore, the depth of focus using the laser-Raman technique can be confusing. For example, Brasier and his colleagues noted that often something that looked like a bacterium at one focal depth merely became weird shapes at other depths. But Schopf points out that Brasier is misinterpreting the readings and that the nonbacterial-like branching of these bacterial cell chains is actually the folding of chains.
The Facts Are Clear
Schopf and his colleagues have used optical microscopy coupled with laser-Raman imagery and measurement of Raman point spectra to correlate chemical composition directly with observable morphology. As a result they have established biogenicity of the fossils studied through crucial indicators such as the fossils' kerogenous cell walls. The results will ultimately provide insights into the chemical changes that accompany organic metamorphosis.
A paleontologist, geologist, microbiologist, and organic geochemist, Schopf does not propose or follow an easy set of rules for verifying true microbial fossils as opposed to pseudofossils. When a recent claim for life arose involving fossils in a Martian meteorite, Schopf was one of the first skeptics to speak out and call for rigorous testing and analyses. He has carefully pointed out that techniques such as electron microscopy are not always reliable. However, he states unequivocally that the 3.5 billion-year-old fossils are real.
The fossils in the Apex cherts are structurally so similar to photosynthetic cyanobacteria that if one accepts they are indeed fossils, it is hard to argue that they are not cyanobacteria. "We have established that the ancient specimens are made of organic matter just like living microbes, and no non-biological organic matter is known from the geological record," says Schopf. "In science, facts always prevail, and the facts here are quite clear."
Viewpoint: No, it is more likely that life at the time was still using other, chemical, forms of energy to survive.
The main thrust of the argument that photosynthesis was already going strong 3.5 billion years ago (3.5 Gya) centers on several pieces of evidence indicating that the more complex form of photosynthesis using oxygen (aerobic photo-synthesis) was already in place at that time. However, all the evidence in favor of this conclusion is uncertain. At best, the less complex form of photosynthesis (anaerobic photosynthesis) could have existed then, but it is more likely that life at the time was still using other, chemical, forms of energy to survive.
There are four broad categories of evidence that the earliest life on Earth left behind to document its existence. The first is the fossil record. The second are biomarkers—characteristic signatures life leaves behind in some of the molecules it comes into contact with. The third category is the geological record, as life can also bring about changes in the structure of the rocks that make up the planet. The final category comes from the record of life written in the very genes that control it. Unfortunately, the record of this evidence becomes increasingly sparse and difficult to interpret the farther back in time we go. By the time we are looking at what happened 3.5 Gya, interpreting the evidence is fraught with many uncertainties.
The Fossil Record
The most obvious and supposedly strongest evidence for early aerobic photosynthesis comes from fossils. Fossils are any evidence of past life. Most people are familiar with fossils of bones or shells. However fossils of soft tissue are also possible. Fossils can be the unaltered original material of the organism, but more commonly the original material of the organism has been recrystallized or replaced by a different mineral.
The scanty fossil evidence that aerobic photosynthesis was thriving so early in Earth's history comes primarily from fossils found in 3.5 Gya rocks called the Warrawoona Group in Western Australia. J. William Schopf, Professor of Paleobiology at the University of California, Los Angeles, has matched the shapes and sizes of microscopic pieces of fossilized carbon with those of cyanobacteria, bacteria known to carry out aerobic photosynthesis. This assertion rests on the assumption that the external features of ancient bacteria indicate internal workings similar to modern bacteria, which is by no means certain. This is not the only problem, however. A recent paper published by Dr. Martin Brasier from the Faculty of Earth Sciences at the University of Oxford in the United Kingdom has shown that the shapes of the lumps of carbon supposed to be cells can all be formed by natural geological process acting on the carbon compounds that would have existed then.
Carbon Isotope Fractionation
Just because cell-shaped carbon lumps can be formed by geological processes does not necessarily mean that they were. The second strand of evidence cited to indicate that the fossilized carbon derives from early life comes from biomarkers created through the process of isotopic fractionation.
Isotopes are atoms of the same element that behave in an almost identical chemical fashion but have different weights. The carbon (C) from carbon dioxide in the air contains two isotopes:12C and 13C. Photosynthesizing bacteria take in carbon dioxide from the air to make compounds containing mainly hydrogen and carbon (carbohydrates). These compounds provide the bacteria with food, as well as the building blocks for most of the structures that make up their cells. In photosynthesis, the lighter 12C carbon reacts a little quicker than does the 13C, so the bacteria contain slightly more of it. The carbon dioxide from the air is also used to make the rocks that surround fossilized bacteria, but the process that forms the rocks skews the ratio in favor of 13C. Measuring the ratio of 12C to 13C between fossilized carbon and that of the surrounding rocks compared to a known 13C standard provides an indication as to whether the carbon came from a living (biotic) or inorganic (abiotic) source.
Unfortunately, different photosynthesizers can create widely varying 13C values that can overlap with abiotic values. There is also the added problem that over long periods of time, geological processes can alter the 12C/13C ratios, bringing yet more uncertainties into the equation. This has led some experts, including Dr. Roger Buick at the University of Washington, to question whether isotopic analysis qualifies as a valid means for testing for carbon from living organisms at all.
Were this not enough, several studies have also shown that the 13C values for biotic carbon compounds can be mimicked by a type of chemical reaction called Fischer-Tropsch synthesis. This reaction uses an iron or nickel catalyst to help react hydrogen gas with either carbon monoxide or carbon dioxide to create hydrocarbon compounds. Fischer-Tropsch synthesis has been shown to occur under the conditions that would prevail around underwater vents. At these high temperatures, water is broken down to provide the hydrogen, and carbon oxides and the metal catalysts come from the rocks. This provides a plentiful supply of hydrocarbon compounds with 13C values similar to those for biotic carbon compounds.
The Geological Record
The final pieces of evidence for early aerobic photosynthesis are contained in the geological record in the form of two types of layered structures: banded-iron formations (BIFs) and stromatolites.
Stromatolites are finely layered rocks that can form through the action of different groups of microorganisms living together in huge colonies. Living stromatolites exist today, with aerobic photosynthesizers inhabiting the top layer, anaerobic photosynthesizers populating the layers below them, and other microorganisms that cannot survive in the presence of oxygen toward the bottom. The microorganisms secrete mucus, which binds rocky particles together to form the stacked rocks. Stromatolites are found in the fossil record extending all the way back to 3.5 Gya, and some of the more recent ones have been shown to contain the fossilized remnants of the microorganisms that lived within them. We know very little about how the more ancient stromatolites formed, however, and work by Professors John Grotzinger and Daniel Rothman, of the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology (MIT), has shown that apparently identical structures can form easily through natural geological processes. Although this does not rule out microorganisms playing a part in early stromatolite formation, or at least inhabiting stromatolites, it cannot be relied upon without corroborating proof—from fossils, for example. However, no reliable fossil evidence has been found in any of the older stromatolites.
Banded-iron formations (BIFs) are brightly colored, layered rocks that are the source of most of the world's iron ore. Their distinctive red color comes from the oxidized iron they contain (the iron is described as oxidized because it has lost electrons, usually through the action of oxygen). BIFs first appear about 3.5 Gya and peter out around 2 Gya. Because of the particular way in which they form, BIFs are a good indicator of the level of oxygen in the atmosphere. The only mechanism previously thought able to explain the increase in BIFs was aerobic photosynthesis. A study by Professor Friedrich Widdel, Director of the Department of Microbiology at the Max-Plank-Institute for Marine Microbiology in Breman, Germany, has cast doubt on this by showing that anaerobic photosynthesis is also capable of oxidizing the iron and suggesting abiotic mechanisms that can do the same.
Evidence in Favor of Aerobic Photosynthesis
All the evidence postulating aerobic photosynthesis at 3.5 Gya can therefore be explained away by either abiotic means or through anaerobic photosynthesis. This would not be so damning were it not for positive evidence indicating that aerobic photosynthesis only arose around 2 Gya—conveniently coinciding with the large increase in oxygen documented in the geological record. This evidence comes firstly from a biomarker called 2-methyl-hopanoids and secondly from genetic clues.
2-methylhopanoids are compounds that are found in the cell membranes (outer skin) of cyanobacteria. A paper published by Roger Summons, a Professor at the Department of Earth, Atmospheric and Planetary Sciences at MIT, has shown that if cyanobacteria undergo fossilization, the hydrocarbon derivatives of the 2-methlyhopanoids created are exceptionally stable, surviving to provide a record of the existence of aerobic photosynthesis. These derivatives have been shown to be abundant in carbon sediments created after 2.5 Gya but not before.
The final piece of the puzzle comes from the evolutionary history contained within genes. Genes are the instruction manuals for life. Passed on through reproduction and contained in every cell in every living creature, they control the main features and traits of living organisms. The instructions contained in genes are written in the sequence of the building blocks of a molecule called DNA. As life has evolved, new genes have arisen, but always by making copies of and changes to the structure of DNA in preexisting genes. Because evolution builds on what has gone before, once an effective and successful trait has arisen, it tends to be passed on relatively unchanged to successive generations. For this reason, the genes controlling the fundamental processes of life arose in the earliest and most simple life and still control the basics of what happens inside life today. That is why humans have many genes in common with organisms as diverse as geraniums, slugs, and chickens.
The DNA in different organisms can therefore be compared to provide a record of how different organisms are related, and even to give a rough guide to when and how their evolutionary paths diverged. Analysis of this type is not an exact science, however. DNA in different organisms changes at different rates, making the timing of specific events very difficult to pinpoint. Bearing these caveats in mind, though, most genetic analysis seems to point to a relatively early evolution for anaerobic photosynthesis and the much later appearance for aerobic photosynthesis at around 2 Gya.
A More Timely Appearance for Photosynthesis
Taken together, this all suggests that aerobic photosynthesis did not exist 3.5 Gya. Proving that any form of photosynthesis did not exist at this time is much harder. Aside from fossils, the only evidence that could seemingly document its appearance is the carbon isotope record that has been shown to be so fallible in support of aerobic photosynthesis.
It is generally accepted that Earth formed about 4.6 Gya. However, until 3.9 Gya ago, evidence from the surface of the Moon shows us that the Earth's surface was continually bombarded by catastrophic impacts from meteors left over from the formation of the Solar System. These would almost certainly have wiped out any life that had managed to gain a foothold. If life could only have got started 3.9 Gya ago, the appearance of such a complicated trait as photosynthesis by 3.5 Gya implies a relatively rapid, early evolution for life on Earth. It seems far more reasonable to suggest that life was still using other chemical forms of energy production at 3.5 Gya. Anaerobic photosynthesis probably appeared some time after, culminating in the appearance of fully fledged aerobic photosynthesis sometime around 2 Gya.
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Not involving or derived from a living organism.
Not requiring oxygen.
Relating to the earliest eon of geological history called the Precambrian, in which there was no life on Earth.
Simple, single-celled life forms.
Involving or derived from living organisms.
Something that helps a chemical reaction on its way but is not used up in the process.
A hard, dark, opaque rock that looks like flint and consists primarily of a large amount of fibrous chalcedony (silica) with smaller amounts of cryptocrystalline quartz. Most often occurs as flint and sometimes in massive beds.
A photosynthetic microorganism related to bacteria but capable of photosynthesis. They are prokaryotic and represent the earliest known form of life on Earth.
The "instructions" coded in a chemical called DNA that governs the growth and reproduction of life.
Different atoms of the same element that are chemically identical but have different weights.
To undergo a reaction in which electrons are lost, usually by the addition of oxygen.
The description and systematic classification of rocks and their composition.
Anything that uses sunlight to synthesize foods from carbon dioxide and water and generates oxygen as a by-product.
Investigation and measurement of spectra produced when matter interacts with or emits electromagnetic radiation.