Importance and Evolution of Photosynthesis - Essay Example

Importance and Evolution of Photosynthesis The very existence of human life fully depends upon photosynthesis, which is a significant biochemical process of energy production in plants with the help of chloroplasts, their most essential cell organelle containing chlorophyll, the green pigment essential for capturing sunlight. The importance of photosynthesis is deep seated in our lives with implications in energy production, agriculture, environmental control and health issues. The novel mechanism of conversion of light energy to chemical energy not only affects our life but also the life of the apparently most insignificant forms of organisms, like the coral animal that takes the help of this biochemical process to deposit calcium skeleton at a much faster rate in light than in darkness. (Institute of Materials, Great Britain, P. 223) The importance is far reaching, from environmental O2 /CO2 balance to the synthesis of artificial herbicides that act on unwanted herbs by blocking some important steps in this energy producing process. The chlorophyll pigment alone has lead to new avenues for thoughts and research on the importance of photosynthetic processes and has paved the way for the synthesis of certain medicinal drugs exploiting its photoprotective mechanisms for preventing light induced damage of cells. The chlorophyll research has added to the significance of this novel energy yielding life process and has led to the foundation of drug therapy for treatment of tumors, carcinoma and related maladies. In fact the earliest forms of photosynthetic plants principally inhabited the water bodies, especially in the warm mineral rich pools, mainly due to the intense effect of ultraviolet radiation on the land due to the absence of the ozone layer. Instead of oxidizing, with a highly reducing nature of environment, principally due to the sparse oxygen count, the warm pools full of evolving life forms probably utilized the massive energy resource to convert the simple inorganic compounds to complex organic biomolecules, like, purines, pyrimidines, nucleosides, nucleotides, etc, paving the way for the synthesis of nucleic acids and complex proteins and enzymes. (Pearson, P. 10) This definitely potentiated the production of biomolecules required oxygen production. As the oxygen level in the atmosphere increased slowly with the passage of time, plants reached out and slowly colonized the lands progressively transforming their semi-aquatic habitats to terrestrial by nature. With the evolution of the earliest microbes that resembled cyanobacteria, algae and lichens till the further differentiated poikilohydric bryophytes evolved. About 25 million years ago, these non vascular bryophytes were soon followed with vascular homioihydric plants capable of maintaining their internal water content at an optimum level irrespective of the external environmental conditions. However before the origin of these photosynthesizing life forms the challengingly low levels of atmospheric oxygen as hypothesized by the geologists, required an explanation of unknown factors that increased the atmospheric oxygen count at a considerable extent. This unknown process of oxygen accumulation in the atmosphere through an unexplained time gap of hundred million years can be associated with a number of presumable theories each of which has its own investigational platform. "The red line shows the inferred level of atmospheric oxygen bounded by the constraints imposed by the proxy record of atmospheric oxygen variation over Earth's history. The signature of mass-independent sulphur-isotope behaviour sets an upper limit for oxygen levels before 2.45 billion years ago and a lower limit after that time. The record of oxidative weathering after 2.45 billion years ago sets a lower limit for oxygen levels at 1% of PAL, whereas an upper limit of 40% of PAL is inferred from the evidence for anoxic oceans during the Proterozoic. The tighter bounds on atmospheric oxygen from 420 million years ago to the present is set by the fairly continuous record of charcoal accumulation: flames cannot be sustained below an oxygen level of 60% of PAL, and above about 160% of PAL the persistence of forest ecosystems would be unlikely because of the frequency and vigour of wildfires". (Nature, 2008) The earth scientists and geologists proposed that the earliest photosynthesizing microbes gave out oxygen by utilizing carbondioxide and thereby producing organic carbon in the process. The organic carbon deposited at the sea bed soon hardened to form rocks rich in organic resources. The dying and decomposing organisms released the oxygen, most of which dissolved in the water and a part escaped to the atmosphere. However, the carbon sedimentation rate was found to be constant over the years and therefore, couldn't directly account for the oxygen surge in the atmosphere. It was then proposed that the volcanoes belched out considerable amounts of hydrogen in the early anaerobic environment. The increased levels of hydrogen in a way increased the oxygen count in the atmosphere. The theorists suggest that most of the oxygen produced by early photosynthesis deposited as oxides on the surface of the earth, at sea beds and within the earths mantle, thereby reacting with CO, H2S and getting recycled into the Earth. These gases were released into the atmosphere through volcanic eruptions to increase the oxygen count. During the early photosynthetic processes by the microbial life forms much of the oxygen produced chemically combined with sulfur, iron, etc through the raindrops in dissolved form and remained deposited in the sea beds, crust and mantle of the earth. These can be verified by the banded iron- formations (BIFS) due to the oxidation of the iron minerals giving sediments of red colored iron oxide deposits on the water bed. (William, P. 44) Whatever is the story of the accumulation of oxygen in the atmosphere; the early process of photosynthesis evolved much earlier with the cyanobacteria like microbial ancestry with considerably lower count of atmospheric oxygen at an anaerobic level. There were oxygen sinks to drain out the oxygen from the atmosphere. The most important were the biological scavengers, the early microbes undergoing photosynthesis. Though they produced oxygen by combining light, water and carbondioxide in order to make sugar, they utilized it in their aerobic respiratory process by oxidizing these organic compounds in order to derive energy. The facultative aerobes that were commonly found at that time when the oxygen levels in the atmosphere where insufficient for a regular and steady supply used the considerably rare oxygen surges for carrying out the occasional aerobic processes. (William, P. 47 - 48) Thus, these oxygen sinks maintained rigid control on restraining probable atmospheric oxygen surges in the early course of evolution. To this day the volcanic gases still devour oxygen and the aerobic form of respiration further depletes its levels; but the BIF layers at the ocean beds have been saturated enough to scavenge further oxygen, leading to an uncontrolled outflow of the gas into the atmosphere for an oxygen rich environment. So let us address the critical question at this point regarding the onset of the true evolution of photosynthesis, perfectly going along with the complex aerobic life forms in this earth. The proposed conjectures by the existing schools of thoughts can even be refuted by the recent arguments that the BIFs may have been formed by the ultraviolet photochemical mechanisms or by the oxygen independent photoferrotrophic bacterial population. This definitely establishes a strong standing against the conventional oxygen scavenging theory by the red iron oxide deposits over millions of evolved years. Further, the sedimentations on the ocean beds could have been attributed to the post-depositional hydrothermal activity, rather than the oxygen dependent organic carbon accumulation by photosynthetic microbes. The supporting microfossil record of the Archaean era is limited for drawing any definitive long term conclusion based on the exact onset of the evolution of oxygenic photosynthesis in earth. (Philosophical Transactions of the Royal Society B, 2008) In order to get a clear idea of the evolution of oxygenic photosynthesis, potential biomarkers are used for tracking down the novel geochemical and biological changes that marked the beginning of an aerobic world. The potential biomarkers include a number of geological proxies for predicting the paleo-environmental changes connected with the Great Oxidative Event and the evolutionary history of photosynthesis. There are definitive hydrocarbon biomarkers, fossil records, traceable redox reactive metals and sulfur deposits that can hint upon the incidents having positive correlation with the evolutionary events. Before the oxidative environment was fully established, the facultative aerobes depended on fermentation and only switched to oxygen dependent means for metabolism during the discreet phases of limited oxygen surges in the atmosphere. The foundation of photosynthesis was laid in the organelle of the cyanobacteria from where the ability to utilize light as the principal energy source had transcended to the more complex and advanced eukaryotic forms. (Baltscheffsky, P. 177-178) The process used then was obviously different from the cofactors and metallozyme dependent biochemical processes in the mechanistically complex photosystems. Long before the development of the manganese-cofactor dependent photosystem II, the hydrogen and hydrogen sulfide were the principal reducing agents of CO2 used in photosynthetic process for which O2 was the byproduct. These were low potential donors of electrons and acted as the primary source of fuel in the earlier phases of geochemical evolution. An isotopic analysis of the carbon content in the carbonates of the sedimentary rock deposits clearly indicated that 20% of the volcanic carbondioxide was reduced to carbon in its organic form, which explained the phenomenon of phosphate limitation in photosynthesis and the carbon burial process in the course of evolution. The intermediate redox potential in the Fe (II) with respect to the other electron donors clearly indicates without doubt that the earlier form photosynthetic mechanism must have resembled the biochemical pathways of the Fe (II) - dependent phototrophy. Though it was known that ferrous salts are oxidized to Ferric by the anoxygenic phototrophs, there was no clear cut indication of the process, till the bacterium was discovered in 1993. The bacterium that could potentially use the Fe (II) photosynthetic mechanism was eventually isolated and after careful analysis of the rock records clearly indicated the actual reason behind the banded iron formations in the ocean bed, refuting the conventional conjectures that the oxygen from the oxygenic metabolic processes was responsible for the said formations. (Lovely, P. 47) On culturing and analyzing various forms of the Fe (II) utilizing phototrophs a comparative idea of their rates of Fe (II) oxidation was established. The understanding of the function of these phototrophs were extended to the paleo-environmental context and isotopic studies were conducted for measuring the 54/56 Fe (II) oxidizing ratio in the Fe (III) product obtained from aquatic sources. The ratio was found to be independent of the rate of Fe (II) oxidation by the phototrophs and was compared to the fractionation data obtained for the Fe (III) reducing bacteria. This led an interesting discovery that by to some extent, established the general idea about the biochemical pathway of Fe turnover within the bacterial cells. This definitely throws light on the fact that there are common cellular machinery for carrying out the mutually reverse processes and maintaining the unique fractionation rate in both the organisms capable of oxidizing Fe (II) and reducing Fe (III). The presence of C-type cytochromes in the cellular machinery of these two organisms indicated the presence of two different operon systems, one of which has been found to code a b-barrel protein helping in the Fe transportation mechanism. Another strongly indicative factor behind the evolutionary story of the photosynthetic activity is the discovery of the fossilized sample of a component of organic origin, known as 2-methyl hopane. The same indications are found in the fluctuating salinity studies in ancient sediments with Cyanobacterial remains where presence of 2-methyl hopanes can be traced. (Warren P. 655) This finding indicates the molecular fossil to be the precursor of 2-methyl bacterialhopanoids, the commonly found functionally significant isoprenoids in Cyanobacteria, which had practiced the oxygenic modes of photosynthesis since the earlier phases of evolution. This definitely tags us with the proposition that oxygenic photosynthesis evolved quite a while ago as supported by the fossil evidence. There is a unique natural control on the evolutionary process of a definite biochemical pathway involving a series of enzymes, proteins and cofactors for the proper expression of an evolved series of connecting functions. The exact sequence of biochemical events needed for the evolution of the photosynthetic process requires the presence of newly synthesized chains of enzymes that can act in converting one biochemical intermediary component to another, thereby preventing the shortfalls and fatalities due to unprecedented evolutionary lags. The evolutionary process for a series of functionally connected enzymes needed for the synthesis of chlorophyll had taken place in such a well coordinated way that the nature's control can be exercised to prevent the fatality due to the lack of the obviously detoxifying next enzyme in the series. The intermediates synthesized by the progressively evolving enzymes lead to the formation of a number of biochemical intermediates that are otherwise toxic and new to the evolving system, provided the next evolved enzyme selectively detoxifies them with their subsequent insertion in the apoproteins for an overall beneficial functionality in favor of the system. So, in the overall process of photosynthetic evolution several controls were found to be functional in timely expression of the oxygen disarming complexes for preventing the lethal effects of ground state oxygen on the cell walls of the evolving aerobes. There were also sufficient controls to hinder the expression of certain genes leading to the formation of junk proteins to alter the course of the desired biochemical event. In spite of the indication about such nature specified controls the overall process of photosynthetic evolution is clouded by several contextual gaps regarding the exact nature of expression of the ATP synthase motor and the rubisco complex. The nature's exact controls on the evolutionary process of the system of seventeen enzymes in the chlorophyll dependent protein assembly have not been properly explained. The evolutionary mechanism involved in the expression of the functional triplet states of the chlorophyll complex have not been understood yet and researchers are trying to draw a positive conclusion regarding the timely expression of the phototoxins in between each consecutive expression of the involved enzymes. The researchers are disturbed by fundamental questions like, the reason for the expression of useless intermediates till the entire machinery of the biochemical process have been formed and the exact process by which the evolutionary mechanism selectively brings about the expression of a pigment binding factor before the original pigment molecule has been expressed. So the key questions regarding the importance and evolution of photosynthetic process are yet to be answered. References 1. Pearson, Lorentz C, 1995, The Diversity and Evolution of Plants, CRC Press, Boca Raton 2. Kump, Lee R, 2008, The Rise of Atmospheric Oxygen, Nature 3. William J, 1992, Major Events in the History of Life, Jones and Bartlett Publishers, Sudbury 4. Buick, Roger, 2008, When Did Oxygenic Photosynthesis Evolve, Philosophical Transactions of the Royal Society B 5. Baltscheffsky, Herrick, 1996, Origin and Evolution of Biological Energy Conversion, Wiley-VCH, Weinheim 6. Lovley Derek R, 2000, Environmental Microbe-metal Interactions, ASM Press, Washington DC 7. Warren, John K, 2006, Evaporites: Sediments, Resources and Hydrocarbons, Birkhuser, Basel 8. Bohinski, Robert C, 1976, Modern Concepts in Biochemistry, Allyn and Bacon, Upper Saddle River Read More