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The age of Rubisco: the evolution of ..

The age of RubisCO: the evolution of oxygenic ..

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The age of Rubisco: the evolution of Plant Cell ..

Two contributions deal with the role of CO2 as a driver of the evolution of terrestrial ecosystems in which grasses with the C4 photosynthetic pathway play an ecologically important role. In plants with the C4 photosynthetic pathway, inorganic carbon from the atmosphere is initially fixed into four-carbon organic acids via the enzyme phosphoenolpyruvate carboxylase []. C4 acids are then shuttled into a specialized compartment of the cell or leaf and the CO2 is liberated to feed the Rubisco-driven C3 photosynthetic pathway []. A major adaptation of C4 plants is the physical isolation from the atmosphere of the compartment where C3 photosynthesis occurs, typically in bundle sheaths, to enrich CO2 at the active site of Rubisco and suppress the oxygenation reaction. However, the C4 CCM has an energetic cost and C4 grasses are typically only more photosynthetically efficient, in terms of productivity and nitrogen-use efficiency, than C3 plants under high-light, warm temperature environments that promote high rates of photorespiration in C3 plants. These considerations generally confine C4 grasses to open habitats in the warm tropics and sub-tropics where they form important components of flammable biomes, including savannahs.

The age of Rubisco: the evolution of oxygenic ..

as Venus and Mars did, which saved all life on Earth. An atmosphere of as little as two percent oxygen may have been adequate to form the ozone layer, and that level was likely first attained during the first GOE. The ozone layer absorbs most of . Ultraviolet light carries more energy than visible light and breaks covalent and other bonds and , particularly to DNA and RNA. Before the ozone layer formed, life would have had a challenging time surviving near the ocean’s surface. Ultraviolet light damage presented a formidable evolutionary hurdle, and proteins and enzymes that assist cellular division . Life has adapted to many hostile conditions in Earth’s past, but if conditions change too rapidly, life cannot adapt in time to survive. that dot Earth’s past were probably the result of conditions changing too rapidly for most organisms to adapt, if they could have adapted at all. During the , which was the greatest extinction event yet known, there is evidence that the ozone layer was depleted and . From the formation of to mass extinction events, ultraviolet light has played a role.

The age of Rubisco: The evolution of oxygenic photosynthesis…

2007 x The age of Rubisco: the evolution of oxygenic photosynthesis

in the late Carboniferous. Arthropods became dominant predators once again, although cephalopods patrolled the reefs as apex predators. at that time, although the succeeding Devonian Period has been called the Golden Age of Brachiopods. As oxygen levels rose, trilobites lost segments and, hence, gill surface area, which may have been an ultimately extinctive gamble. When the Devonian extinction happened during anoxic events, trilobites steeply declined and thereafter only eked out an existence until the Permian extinction finally eliminated them from the fossil record. Fish began in the Silurian, which was a great evolutionary leap and arguably the most important innovation in vertebrate history. Jaws, tentacles, claws… features were advantageous, as animals could more effectively manipulate their environments and acquire energy. On land the colonization began, as mossy “forests” abounded, and made their appearance, although they were generally less than a hand-width tall when the Silurian ended, and nothing reached even waist-high.

But the branch of the that readers might find most interesting led to humans. Humans are in the phylum, and the last common ancestor that founded the Chordata phylum is still a mystery and understandably a source of controversy. Was our ancestor a ? A ? Peter Ward made the case, as have others for a long time, that it was the sea squirt, also called a tunicate, which in its larval stage resembles a fish. The nerve cord in most bilaterally symmetric animals runs below the belly, not above it, and a sea squirt that never grew up may have been our direct ancestor. Adult tunicates are also highly adapted to extracting oxygen from water, even too much so, with only about 10% of today’s available oxygen extracted in tunicate respiration. It may mean that tunicates adapted to low oxygen conditions early on. Ward’s respiration hypothesis, which makes the case that adapting to low oxygen conditions was an evolutionary spur for animals, will repeatedly reappear in this essay, as will . Ward’s hypothesis may be proven wrong or will not have the key influence that he attributes to it, but it also has plenty going for it. The idea that fluctuating oxygen levels impacted animal evolution has been gaining support in recent years, particularly in light of recent reconstructions of oxygen levels in the eon of complex life, called and , which have yielded broadly similar results, but their variances mean that much more work needs to be performed before on the can be done, if it ever can be. Ward’s basic hypotheses is that when oxygen levels are high, ecosystems are diverse and life is an easy proposition; when oxygen levels are low, animals adapted to high oxygen levels go extinct and the survivors are adapted to low oxygen with body plan changes, and their adaptations helped them dominate after the extinctions. The has a pretty wide range of potential error, particularly in the early years, and it also tracked atmospheric carbon dioxide levels. The challenges to the validity of a model based on data with such a wide range of error are understandable. But some broad trends are unmistakable, as it is with other models, some of which are generally declining carbon dioxide levels, some huge oxygen spikes, and the generally relationship between oxygen and carbon dioxide levels, which a geochemist would expect. The high carbon dioxide level during the Cambrian, of at least 4,000 PPM (the "RCO2" in the below graphic is a ratio of the calculated CO2 levels to today's levels), is what scientists think made the times so hot. (Permission: Peter Ward, June 2014)

The age of Rubisco: the evolution of oxygenic photosynthesis

(2007) The age of Rubisco: The evolution of oxygenic photosynthesis Geobiology

In the earliest days of life on Earth, it had to solve the problems of how to reproduce, how to separate itself from its environment, how to acquire raw materials, and how to make the chemical reactions that it needed. But it was confined to those areas where it could take advantage of briefly available potential energy as . The earliest process of skimming energy from energy gradients to power life is called respiration. That earliest respiration is today called because there was virtually no free oxygen in the atmosphere or ocean in those early days. Respiration was life’s first energy cycle. A biological energy cycle begins by harvesting an energy gradient (usually by a proton crossing a membrane or, in photosynthesis, directly capturing photon energy), and the acquired energy powered chemical reactions. The cycle then proceeds in steps, and the reaction products of each step sequentially use a little more energy from the initial capture until the initial energy has been depleted and the cycle’s molecules are returned to their starting point and ready for a fresh influx of energy to repeat the cycle.

Carbon-concentrating mechanisms (CCMs) induced with biophysical and/or biochemical mechanisms that suppress the oxygenation reaction of Rubisco are an important evolutionary response to falling CO2. Indeed, the 1.1 Ga burst of Rubisco evolution detected by Young et al. appears linked to an endosymbiotic event that produced a CCM and relaxed constraints on the need for high CO2 specificity. The origin and function of CCMs in photosynthetic marine algae is reviewed by Raven et al. []. CCMs are widely distributed among algae and very likely evolved as an adaptation to low CO2 by effectively accumulating CO2 in the compartment containing Rubisco to a higher steady-state concentration than in the growth medium, thereby increasing its efficiency. As Raven et al. [] emphasize, there are numerous documented episodes of low CO2 in the past when such adaptations could confer obvious advantages, including the Permo-Carboniferous glaciation (300–270 Ma) and repeated glacial intervals of the Pleistocene (2.1 Ma).

(2007) The age of Rubisco: the evolution of oxygenic photosynthesis
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What is oxygenic photosynthesis | scholarly search

During that “,” , , and the rise of grazing and predation had eonic significance. While many critical events in life’s history were unique, one that is not is multicellularity, , and some prokaryotes have multicellular structures, some even with specialized organisms forming colonies. There are , but the primary advantage was size, which would become important in the coming eon of complex life. The rise of complex life might have happened faster than the billion years or so after the basic foundation was set (the complex cell, oxygenic photosynthesis), but geophysical and geochemical processes had their impacts. Perhaps most importantly, the oceans probably did not get oxygenated until just before complex life appeared, as they were sulfidic from 1.8 bya to 700 mya. Atmospheric oxygen is currently thought to have remained at only a few percent at most until about 850 mya, although there are recent arguments that it remained low until only about 420 mya, when large animals began to appear and animals began to colonize land. Just as the atmospheric oxygen content began to rise, then came the biggest ice age in Earth’s history, which probably played a major role in the rise of complex life.

timing of evolution of Rubisco and its ..

Perhaps a few hundred million years after the first mitochondrion appeared, as the oceanic oxygen content, at least on the surface, increased as a result of oxygenic photosynthesis, those complex cells learned to use oxygen instead of hydrogen. It is difficult to overstate the importance of learning to use oxygen in respiration, called . Before the appearance of aerobic respiration, life generated energy via and . Because oxygen , aerobic respiration generates, on average, about per cycle as fermentation and anaerobic respiration do (although some types of anaerobic respiration can get ). The suite of complex life on Earth today would not have been possible without the energy provided by oxygenic respiration. At minimum, nothing could have flown, and any animal life that might have evolved would have never left the oceans because the atmosphere would not have been breathable. With the advent of aerobic respiration, became possible, as it is several times as efficient as anaerobic respiration and fermentation (about 40% as compared to less than 10%). Today’s food chains of several levels would be constrained to about two in the absence of oxygen. Some scientists have and oxygen and respiration in eukaryote evolution. is controversial.

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