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1.
When did oxygenic photosynthesis evolve? 总被引:1,自引:0,他引:1
Buick R 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2008,363(1504):2731-2743
The atmosphere has apparently been oxygenated since the 'Great Oxidation Event' ca 2.4 Ga ago, but when the photosynthetic oxygen production began is debatable. However, geological and geochemical evidence from older sedimentary rocks indicates that oxygenic photosynthesis evolved well before this oxygenation event. Fluid-inclusion oils in ca 2.45 Ga sandstones contain hydrocarbon biomarkers evidently sourced from similarly ancient kerogen, preserved without subsequent contamination, and derived from organisms producing and requiring molecular oxygen. Mo and Re abundances and sulphur isotope systematics of slightly older (2.5 Ga) kerogenous shales record a transient pulse of atmospheric oxygen. As early as ca 2.7 Ga, stromatolites and biomarkers from evaporative lake sediments deficient in exogenous reducing power strongly imply that oxygen-producing cyanobacteria had already evolved. Even at ca 3.2 Ga, thick and widespread kerogenous shales are consistent with aerobic photoautrophic marine plankton, and U-Pb data from ca 3.8 Ga metasediments suggest that this metabolism could have arisen by the start of the geological record. Hence, the hypothesis that oxygenic photosynthesis evolved well before the atmosphere became permanently oxygenated seems well supported. 相似文献
2.
The advent of oxygenic photosynthesis represents the most prominent biological innovation in the evolutionary history of the Earth. The exact timing of the evolution of oxygenic photoautotrophic bacteria remains elusive, yet these bacteria profoundly altered the redox state of the ocean–atmosphere–biosphere system, ultimately causing the first major rise in atmospheric oxygen (O2)—the so-called Great Oxidation Event (GOE)—during the Paleoproterozoic (~2.5–2.2 Ga). However, it remains unclear how the coupled atmosphere–marine biosphere system behaved after the emergence of oxygenic photoautotrophs (OP), affected global biogeochemical cycles, and led to the GOE. Here, we employ a coupled atmospheric photochemistry and marine microbial ecosystem model to comprehensively explore the intimate links between the atmosphere and marine biosphere driven by the expansion of OP, and the biogeochemical conditions of the GOE. When the primary productivity of OP sufficiently increases in the ocean, OP suppresses the activity of the anaerobic microbial ecosystem by reducing the availability of electron donors (H2 and CO) in the biosphere and causes climate cooling by reducing the level of atmospheric methane (CH4). This can be attributed to the supply of OH radicals from biogenic O2, which is a primary sink of biogenic CH4 and electron donors in the atmosphere. Our typical result also demonstrates that the GOE is triggered when the net primary production of OP exceeds >~5% of the present oceanic value. A globally frozen snowball Earth event could be triggered if the atmospheric CO2 level was sufficiently small (<~40 present atmospheric level; PAL) because the concentration of CH4 in the atmosphere would decrease faster than the climate mitigation by the carbonate–silicate geochemical cycle. These results support a prolonged anoxic atmosphere after the emergence of OP during the Archean and the occurrence of the GOE and snowball Earth event during the Paleoproterozoic. 相似文献
3.
The rise of oxygen ca. 2.3 billion years ago (Ga) is the most distinct environmental transition in Earth history. This event was enabled by the evolution of oxygenic photosynthesis in the ancestors of Cyanobacteria. However, long‐standing questions concern the evolutionary timing of this metabolism, with conflicting answers spanning more than one billion years. Recently, knowledge of the Cyanobacteria phylum has expanded with the discovery of non‐photosynthetic members, including a closely related sister group termed Melainabacteria, with the known oxygenic phototrophs restricted to a clade recently designated Oxyphotobacteria. By integrating genomic data from the Melainabacteria, cross‐calibrated Bayesian relaxed molecular clock analyses show that crown group Oxyphotobacteria evolved ca. 2.0 billion years ago (Ga), well after the rise of atmospheric dioxygen. We further estimate the divergence between Oxyphotobacteria and Melainabacteria ca. 2.5–2.6 Ga, which—if oxygenic photosynthesis is an evolutionary synapomorphy of the Oxyphotobacteria—marks an upper limit for the origin of oxygenic photosynthesis. Together, these results are consistent with the hypothesis that oxygenic photosynthesis evolved relatively close in time to the rise of oxygen. 相似文献
4.
J. William Schopf 《Biochemistry. Biokhimii?a》2014,79(3):165-177
Fossil evidence of photosynthesis, documented in the geological record by microbially laminated stromatolites, microscopic fossils, and carbon isotopic data consistent with the presence of Rubisco-mediated CO2-fixation, extends to ~3500 million years ago. Such evidence, however, does not resolve the time of origin of oxygenic photosynthesis from its anoxygenic photosynthetic evolutionary precursor. Though it is evident that cyanobacteria, the earliest-evolved O2-producing photoautotrophs, existed before ~2450 million years ago — the onset of the “Great Oxidation Event” (GOE) that forever altered Earth’s environment — O2-producing photosynthesis seems certain to have originated hundreds of millions of years earlier. How did Earth’s biota respond to the GOE? Four lines of evidence are here suggested to reflect this major environmental transition: (1) rRNA phylogeny-correlated metabolic and biosynthetic pathways document evolution from an anaerobic (pre-GOE) to a dominantly oxygen-requiring (post-GOE) biosphere; (2) consistent with the rRNA phylogeny of cyanobacteria, their fossil record evidences the immediately post-GOE presence of cyanobacterial nostocaceans characterized by specialized cells that protect their oxygen-labile nitrogenase enzyme system; (3) the earliest known fossil eukaryotes, obligately aerobic phytoplankton and putative algae, closely post-date the GOE; and (4) microbial sulfuretums are earliest known from rocks deposited during and immediately after the GOE, their apparent proliferation evidently spurred by an increase of environmental oxygen and a resulting upsurge of metabolically useable sulfate and nitrate. Though the biotic response to the GOE is a question new to paleobiology that is yet largely unexplored, additional evidence of its impact seems certain to be uncovered. 相似文献
5.
Olson JM 《Photosynthesis research》2006,88(2):109-117
The earliest reductant for photosynthesis may have been H2. The carbon isotope composition measured in graphite from the 3.8-Ga Isua Supercrustal Belt in Greenland is attributed to H2-driven photosynthesis, rather than to oxygenic photosynthesis as there would have been no evolutionary pressure for oxygenic photosynthesis in the presence of H2. Anoxygenic photosynthesis may also be responsible for the filamentous mats found in the 3.4-Ga Buck Reef Chert in South Africa. Another early reductant was probably H2S. Eventually the supply of H2 in the atmosphere was likely to have been attenuated by the production of CH4 by methanogens, and the supply of H2S was likely to have been restricted to special environments near volcanos. Evaporites, possible stromatolites, and possible microfossils found in the 3.5-Ga Warrawoona Megasequence in Australia are attributed to sulfur-driven photosynthesis. Proteobacteria and protocyanobacteria are assumed to have evolved to use ferrous iron as reductant sometime around 3.0 Ga or earlier. This type of photosynthesis could have produced banded iron formations similar to those produced by oxygenic photosynthesis. Microfossils, stromatolites, and chemical biomarkers in Australia and South Africa show that cyanobacteria containing chlorophyll a and carrying out oxygenic photosynthesis appeared by 2.8 Ga, but the oxygen level in the atmosphere did not begin to increase until about 2.3 Ga. 相似文献
6.
Phototrophic bacteria are among the most biogeochemically significant organisms on Earth and are physiologically related through the use of reaction centers to collect photons for energy metabolism. However, the major phototrophic lineages are not closely related to one another in bacterial phylogeny, and the origins of their respective photosynthetic machinery remain obscured by time and low sequence similarity. To better understand the co‐evolution of Cyanobacteria and other ancient anoxygenic phototrophic lineages with respect to geologic time, we designed and implemented a variety of molecular clocks that use horizontal gene transfer (HGT) as additional, relative constraints. These HGT constraints improve the precision of phototroph divergence date estimates and indicate that stem green non‐sulfur bacteria are likely the oldest phototrophic lineage. Concurrently, crown Cyanobacteria age estimates ranged from 2.2 Ga to 2.7 Ga, with stem Cyanobacteria diverging ~2.8 Ga. These estimates provide a several hundred Ma window for oxygenic photosynthesis to evolve prior to the Great Oxidation Event (GOE) ~2.3 Ga. In all models, crown green sulfur bacteria diversify after the loss of the banded iron formations from the sedimentary record (~1.8 Ga) and may indicate the expansion of the lineage into a new ecological niche following the GOE. Our date estimates also provide a timeline to investigate the temporal feasibility of different photosystem HGT events between phototrophic lineages. Using this approach, we infer that stem Cyanobacteria are unlikely to be the recipient of an HGT of photosystem I proteins from green sulfur bacteria but could still have been either the HGT donor or the recipient of photosystem II proteins with green non‐sulfur bacteria, prior to the GOE. Together, these results indicate that HGT‐constrained molecular clocks are useful tools for the evaluation of various geological and evolutionary hypotheses, using the evolutionary histories of both genes and organismal lineages. 相似文献
7.
Palaeoproterozoic ice houses and the evolution of oxygen-mediating enzymes: the case for a late origin of photosystem II 总被引:1,自引:0,他引:1
Kirschvink JL Kopp RE 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2008,363(1504):2755-2765
Two major geological problems regarding the origin of oxygenic photosynthesis are (i) identifying a source of oxygen pre-dating the biological oxygen production and capable of driving the evolution of oxygen tolerance, and (ii) determining when oxygenic photosynthesis evolved. One solution to the first problem is the accumulation of photochemically produced H(2)O(2) at the surface of the glaciers and its subsequent incorporation into ice. Melting at the glacier base would release H(2)O(2), which interacts with seawater to produce O(2) in an environment shielded from the lethal levels of ultraviolet radiation needed to produce H(2)O(2). Answers to the second problem are controversial and range from 3.8 to 2.2 Gyr ago. A sceptical view, based on the metals that have the redox potentials close to oxygen, argues for the late end of the range. The preponderance of geological evidence suggests little or no oxygen in the Late Archaean atmosphere (less than 1 ppm). The main piece of evidence for an earlier evolution of oxygenic photosynthesis comes from lipid biomarkers. Recent work, however, has shown that 2-methylhopanes, once thought to be unique biomarkers for cyanobacteria, are also produced anaerobically in significant quantities by at least two strains of anoxygenic phototrophs. Sterane biomarkers provide the strongest evidence for a date 2.7 Gyr ago or above, and could also be explained by the common evolutionary pattern of replacing anaerobic enzymes with oxygen-dependent ones. Although no anaerobic sterol synthesis pathway has been identified in the modern biosphere, enzymes that perform the necessary chemistry do exist. This analysis suggests that oxygenic photosynthesis could have evolved close in geological time to the Makganyene Snowball Earth Event and argues for a causal link between the two. 相似文献
8.
Hydrogen peroxide and the evolution of oxygenic photosynthesis 总被引:1,自引:0,他引:1
The early atmosphere of the Earth is considered to have been reducing (H2 rich) or neutral (CO2-N2). The present atmosphere by contrast is highly oxidizing (20% O2). The source of this oxygen is generally agreed to have been oxygenic photosynthesis, whereby organisms use water as the electron donor in the production of organic matter, liberating oxygen into the atmosphere. A major question in the evolution of life is how oxygenic photosynthesis could have evolved under anoxic conditions — and also when this capability evolved. It seems unlikely that water would be employed as the electron donor in anoxic environments that were rich in reducing agents such as ferrous or sulfide ions which could play that role. The abiotic production of atmospheric oxidants could have provided a mechanism by which locally oxidizing conditions were sustained within spatially confined habitats thus removing the available reductants and forcing photosynthetic organisms to utilize water as the electron donor. We suggest that atmospheric H2O2 played the key role in inducing oxygenic photosynthesis because as peroxide increased in a local environment, organisms would not only be faced with a loss of reductant, but they would also be pressed to develop the biochemical apparatus (e.g., catalase) that would ultimately be needed to protect against the products of oxygenic photosynthesis. This scenario allows for the early evolution of oxygenic photosynthesis while global conditions were still anaerobic. 相似文献
9.
Activation of methanogenesis in arid biological soil crusts despite the presence of oxygen 总被引:1,自引:0,他引:1
Methanogenesis is traditionally thought to occur only in highly reduced, anoxic environments. Wetland and rice field soils are well known sources for atmospheric methane, while aerated soils are considered sinks. Although methanogens have been detected in low numbers in some aerated, and even in desert soils, it remains unclear whether they are active under natural oxic conditions, such as in biological soil crusts (BSCs) of arid regions. To answer this question we carried out a factorial experiment using microcosms under simulated natural conditions. The BSC on top of an arid soil was incubated under moist conditions in all possible combinations of flooding and drainage, light and dark, air and nitrogen headspace. In the light, oxygen was produced by photosynthesis. Methane production was detected in all microcosms, but rates were much lower when oxygen was present. In addition, the δ(13)C of the methane differed between the oxic/oxygenic and anoxic microcosms. While under anoxic conditions methane was mainly produced from acetate, it was almost entirely produced from H(2)/CO(2) under oxic/oxygenic conditions. Only two genera of methanogens were identified in the BSC-Methanosarcina and Methanocella; their abundance and activity in transcribing the mcrA gene (coding for methyl-CoM reductase) was higher under anoxic than oxic/oxygenic conditions, respectively. Both methanogens also actively transcribed the oxygen detoxifying gene catalase. Since methanotrophs were not detectable in the BSC, all the methane produced was released into the atmosphere. Our findings point to a formerly unknown participation of desert soils in the global methane cycle. 相似文献
10.
Mitochondria have been put forward as the saviours of anaerobes when their environment became oxygenated. However, despite oxygenic photosynthesis evolving around 2.7 billion years ago (Ga), followed by the "Great Oxidation" of the atmosphere ~ 2.4 Ga, the deep oceans remained largely anoxic and either iron-enriched or sulphidic until 580 million years ago, when the eukaryotic radiation was well underway. Atmospheric oxygen probably remained at an intermediate concentration (1-10% of the present level) from ~ 2.4 until ~ 0.8 Ga when a "lesser oxidation" began. This drastically changes the textbook view of the ecological conditions under which the mitochondrial endosymbiont established itself. It could explain the widespread distribution of anaerobic biochemistry in every eukaryotic supergroup: anaerobic biochemistry is hard-wired into the eukaryotes. 相似文献
11.
E. G. NISBET N. V. GRASSINEAU C. J. HOWE P. I. ABELL M. REGELOUS R. E. R. NISBET 《Geobiology》2007,5(4):311-335
The evolutionary history of oxygenesis is controversial. Form I of ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in oxygen‐tolerant organisms both enables them to carry out oxygenic extraction of carbon from air and enables the competitive process of photorespiration. Carbon isotopic evidence is presented from ~2.9 Ga stromatolites from Steep Rock, Ontario, Canada, ~2.9 Ga stromatolites from Mushandike, Zimbabwe, and ~2.7 Ga stromatolites in the Belingwe belt, Zimbabwe. The data imply that in all three localities the reef‐building autotrophs included organisms using Form I Rubisco. This inference, though not conclusive, is supported by other geochemical evidence that these stromatolites formed in oxic conditions. Collectively, the implication is that oxygenic photosynthesizers first appeared ~2.9 Ga ago, and were abundant 2.7–2.65 Ga ago. Rubisco specificity (its preference for CO2 over O2) and compensation constraints (the limits on carbon fixation) may explain the paradox that despite the inferred evolution of oxygenesis 2.9 Ga ago, the Late Archaean air was anoxic. The atmospheric CO2:O2 ratio, and hence greenhouse warming, may reflect Form I Rubisco's specificity for CO2 over O2. The system may be bistable under the warming Sun, with liquid oceans occurring in either anoxic (H2O with abundant CH4 plus CO2) or oxic (H2O with more abundant CO2, but little CH4) greenhouse states. Transition between the two states would involve catastrophic remaking of the biosphere. Build‐up of a very high atmospheric inventory of CO2 in the 2.3 Ga glaciation may have allowed the atmosphere to move up the CO2 compensation line to reach stability in an oxygen‐rich system. Since then, Form I Rubisco specificity and consequent compensation limits may have maintained the long‐term atmospheric disproportion between O2 and CO2, which is now close to both CO2 and O2 compensation barriers. 相似文献
12.
Electrons, life and the evolution of Earth's oxygen cycle 总被引:1,自引:0,他引:1
Falkowski PG Godfrey LV 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2008,363(1504):2705-2716
The biogeochemical cycles of H, C, N, O and S are coupled via biologically catalysed electron transfer (redox) reactions. The metabolic processes responsible for maintaining these cycles evolved over the first ca 2.3 Ga of Earth's history in prokaryotes and, through a sequence of events, led to the production of oxygen via the photobiologically catalysed oxidation of water. However, geochemical evidence suggests that there was a delay of several hundred million years before oxygen accumulated in Earth's atmosphere related to changes in the burial efficiency of organic matter and fundamental alterations in the nitrogen cycle. In the latter case, the presence of free molecular oxygen allowed ammonium to be oxidized to nitrate and subsequently denitrified. The interaction between the oxygen and nitrogen cycles in particular led to a negative feedback, in which increased production of oxygen led to decreased fixed inorganic nitrogen in the oceans. This feedback, which is supported by isotopic analyses of fixed nitrogen in sedimentary rocks from the Late Archaean, continues to the present. However, once sufficient oxygen accumulated in Earth's atmosphere to allow nitrification to out-compete denitrification, a new stable electron 'market' emerged in which oxygenic photosynthesis and aerobic respiration ultimately spread via endosymbiotic events and massive lateral gene transfer to eukaryotic host cells, allowing the evolution of complex (i.e. animal) life forms. The resulting network of electron transfers led a gas composition of Earth's atmosphere that is far from thermodynamic equilibrium (i.e. it is an emergent property), yet is relatively stable on geological time scales. The early coevolution of the C, N and O cycles, and the resulting non-equilibrium gaseous by-products can be used as a guide to search for the presence of life on terrestrial planets outside of our Solar System. 相似文献
13.
Geological records of atmospheric oxygen suggest that pO2 was less than 0.001% of present atmospheric levels (PAL) during the Archean, increasing abruptly to a Proterozoic value between 0.1% and 10% PAL, and rising quickly to modern levels in the Phanerozoic. Using a simple model of the biogeochemical cycles of carbon, oxygen, sulfur, hydrogen, iron, and phosphorous, we demonstrate that there are three stable states for atmospheric oxygen, roughly corresponding to levels observed in the geological record. These stable states arise from a series of specific positive and negative feedbacks, requiring a large geochemical perturbation to the redox state to transition from one to another. In particular, we show that a very low oxygen level in the Archean (i.e., 10?7 PAL) is consistent with the presence of oxygenic photosynthesis and a robust organic carbon cycle. We show that the Snowball Earth glaciations, which immediately precede both transitions, provide an appropriate transient increase in atmospheric oxygen to drive the atmosphere either from its Archean state to its Proterozoic state, or from its Proterozoic state to its Phanerozoic state. This hypothesis provides a mechanistic explanation for the apparent synchronicity of the Proterozoic Snowball Earth events with both the Great Oxidation Event, and the Neoproterozoic oxidation. 相似文献
14.
Gantt E 《Photosynthesis research》2011,107(1):1-6
The integrated functioning of two photosystems (I and II) whether in cyanobacteria or in chloroplasts is the outstanding sign
of a common ancestral origin. Many variations on the basic theme are currently evident in oxygenic photosynthetic organisms
whether they are prokaryotes, unicellular, or multicellular. By conservative estimates, oxygenic photosynthesis has been around
for at least ca. 2.2–2.7 billions years, consistent with cyanobacteria-type microfossils, biomarkers, and an atmospheric rise
in oxygen to less than 1.0% of the present concentration. The presumptions of chloroplast formation by the cyanobacterial
uptake into a eukaryote prior to 1.6 BYa ago are confounded by assumptions of host type(s) and potential tolerance of oxygen
toxicity. The attempted dating and interrelationships of particular chloroplasts in various plant or animal lineages has relied
heavily on phylogenomic analysis and evaluations that have been difficult to confirm separately. Many variations occur in
algal groups, involving the type and number of accessory pigments, and the number(s) of membranes (2–4) enclosing a chloroplast,
which can both help and complicate inferences made about early or late origins of chloroplasts. Integration of updated phylogenomics
with physiological and cytological observations remains a special challenge, but could lead to more accurate assumptions of
initial and extant endosymbiotic event(s) leading toward stable chloroplast associations. 相似文献
15.
Judith M. Klatt Mohammad A. A. Al-Najjar Pelin Yilmaz Gaute Lavik Dirk de Beer Lubos Polerecky 《Applied and environmental microbiology》2015,81(6):2025-2031
Before the Earth''s complete oxygenation (0.58 to 0.55 billion years [Ga] ago), the photic zone of the Proterozoic oceans was probably redox stratified, with a slightly aerobic, nutrient-limited upper layer above a light-limited layer that tended toward euxinia. In such oceans, cyanobacteria capable of both oxygenic and sulfide-driven anoxygenic photosynthesis played a fundamental role in the global carbon, oxygen, and sulfur cycle. We have isolated a cyanobacterium, Pseudanabaena strain FS39, in which this versatility is still conserved, and we show that the transition between the two photosynthetic modes follows a surprisingly simple kinetic regulation controlled by this organism''s affinity for H2S. Specifically, oxygenic photosynthesis is performed in addition to anoxygenic photosynthesis only when H2S becomes limiting and its concentration decreases below a threshold that increases predictably with the available ambient light. The carbon-based growth rates during oxygenic and anoxygenic photosynthesis were similar. However, Pseudanabaena FS39 additionally assimilated NO3− during anoxygenic photosynthesis. Thus, the transition between anoxygenic and oxygenic photosynthesis was accompanied by a shift of the C/N ratio of the total bulk biomass. These mechanisms offer new insights into the way in which, despite nutrient limitation in the oxic photic zone in the mid-Proterozoic oceans, versatile cyanobacteria might have promoted oxygenic photosynthesis and total primary productivity, a key step that enabled the complete oxygenation of our planet and the subsequent diversification of life. 相似文献
16.
Oxygenic photosynthesis uses light as energy source to generate an oxidant powerful enough to oxidize water into oxygen, electrons and protons. Upon linear electron transport, electrons extracted from water are used to reduce NADP(+) to NADPH. The oxygen molecule has been integrated into the cellular metabolism, both as the most efficient electron acceptor during respiratory electron transport and as oxidant and/or "substrate" in a number of biosynthetic pathways. Though photosynthesis of higher plants, algae and cyanobacteria produces oxygen, there are conditions under which this type of photosynthesis operates under hypoxic or anaerobic conditions. In the unicellular green alga Chlamydomonas reinhardtii, this condition is induced by sulfur deficiency, and it results in the production of molecular hydrogen. Research on this biotechnologically relevant phenomenon has contributed largely to new insights into additional pathways of photosynthetic electron transport, which extend the former concept of linear electron flow by far. This review summarizes the recent knowledge about various electron sources and sinks of oxygenic photosynthesis besides water and NADP(+) in the context of their contribution to hydrogen photoproduction by C. reinhardtii. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts. 相似文献
17.
We use a 1‐D numerical model to study the atmospheric photochemistry of oxygen, methane, and sulfur after the advent of oxygenic photosynthesis. We assume that mass‐independent fractionation (MIF) of sulfur isotopes – characteristic of the Archean – was best preserved in sediments when insoluble elemental sulfur (S8) was an important product of atmospheric photochemistry. Efficient S8 production requires three things: (i) very low levels of tropospheric O2; (ii) a source of sulfur gases to the atmosphere at least as large as the volcanic SO2 source today; and (iii) a sufficiently high abundance of methane or other reduced gas. All three requirements must be met. We suggest that the disappearance of a strong MIF sulfur signature at the beginning of the Proterozoic is better explained by the collapse of atmospheric methane, rather than by a failure of volcanism or the rise of oxygen. The photochemical models are consistent in demanding that methane decline before O2 can rise (although they are silent as to how quickly), and the collapse of a methane greenhouse effect is consistent with the onset of major ice ages immediately following the disappearance of MIF sulfur. We attribute the decline of methane to the growth of the oceanic sulfate pool as indicated by the widening envelope of mass‐dependent sulfur fractionation through the Archean. We find that a given level of biological forcing can support either oxic or anoxic atmospheres, and that the transition between the anoxic state and the oxic state is inhibited by high levels of atmospheric methane. Transition from an oxygen‐poor to an oxygen‐rich atmosphere occurs most easily when methane levels are low, which suggests that the collapse of methane not only caused the end of MIF S and major ice ages, but it may also have enabled the rise of O2. In this story the early Proterozoic ice ages were ended by the establishment of a stable oxic atmosphere, which protected a renewed methane greenhouse with an ozone shield. 相似文献
18.
Judith Krause Bui Quang Minh Achim Heck Stephan Endres Vera Svensson Astrid Wirtz Arndt von Haeseler Karl‐Erich Jaeger Thomas Drepper Ulrich Krauss 《Molecular microbiology》2014,93(5):1066-1078
In all photosynthetic organisms, chlorophylls function as light‐absorbing photopigments allowing the efficient harvesting of light energy. Chlorophyll biosynthesis recurs in similar ways in anoxygenic phototrophic proteobacteria as well as oxygenic phototrophic cyanobacteria and plants. Here, the biocatalytic conversion of protochlorophyllide to chlorophyllide is catalysed by evolutionary and structurally distinct protochlorophyllide reductases (PORs) in anoxygenic and oxygenic phototrophs. It is commonly assumed that anoxygenic phototrophs only contain oxygen‐sensitive dark‐operative PORs (DPORs), which catalyse protochlorophyllide reduction independent of the presence of light. In contrast, oxygenic phototrophs additionally (or exclusively) possess oxygen‐insensitive but light‐dependent PORs (LPORs). Based on this observation it was suggested that light‐dependent protochlorophyllide reduction first emerged as a consequence of increased atmospheric oxygen levels caused by oxygenic photosynthesis in cyanobacteria. Here, we provide experimental evidence for the presence of an LPOR in the anoxygenic phototrophic α‐proteobacterium Dinoroseobacter shibae DFL12T. In vitro and in vivo functional assays unequivocally prove light‐dependent protochlorophyllide reduction by this enzyme and reveal that LPORs are not restricted to cyanobacteria and plants. Sequence‐based phylogenetic analyses reconcile our findings with current hypotheses about the evolution of LPORs by suggesting that the light‐dependent enzyme of D. shibae DFL12T might have been obtained from cyanobacteria by horizontal gene transfer. 相似文献
19.
Iron (Fe) is an essential element for life, and its geochemical cycle is intimately linked to the coupled history of life and Earth's environment. The accumulated geologic records indicate that ferruginous waters existed in the Precambrian oceans not only before the first major rise of atmospheric O2 levels (Great Oxidation Event; GOE) during the Paleoproterozoic, but also during the rest of the Proterozoic. However, the interactive evolution of the biogeochemical cycles of O2 and Fe during the Archean–Proterozoic remains ambiguous. Here, we develop a biogeochemical model to investigate the coupled biogeochemical evolution of Fe–O2–P–C cycles across the GOE. Our model demonstrates that the marine Fe cycle was less sensitive to changes in the production rate of O2 before the GOE (atmospheric pO2 < 10−6 PAL; present atmospheric level). When the P supply rate to the ocean exceeds a certain threshold, the GOE occurs and atmospheric pO2 rises to ~10−3–10−1 PAL. After the GOE, the marine Fe(II) concentration is highly sensitive to atmospheric pO2, suggesting that the marine redox landscape during the Proterozoic may have fluctuated between ferruginous conditions and anoxic non-ferruginous conditions with sulfidic water masses around continental margins. At a certain threshold value of atmospheric pO2 of ~0.3% PAL, the primary oxidation pathway of Fe(II) shifts from the activity of Fe(II)-utilizing anoxygenic photoautotrophs in sunlit surface waters to abiotic process in the deep ocean. This is accompanied by a shift in the primary deposition site of Fe(III) hydroxides from the surface ocean to the deep sea, providing a plausible mechanistic explanation for the observed cessation of iron formations during the Proterozoic. 相似文献
20.
Dependence of phytoplankton assimilation quotients on light and nitrogen source: implications for oceanic primary productivity 总被引:1,自引:0,他引:1
Megard R. O.; Berman T.; Curtis P. J.; Vaughan P. W. 《Journal of plankton research》1985,7(5):691-702
Photosynthetic production of oxygen by phytoplankton assemblagedominated by Peridinium in Lake Kinneret, Israel, generallyexceeds the molar equivalent rate of carbon assimilation. Carbonassimilation occurs only if oxygenic photosynthesis exceedsa light-dependent threshold. Assimilation quotients (mol C molO21) are a variable function of irradiance, and typicallyonly about one-half of the photoreductant produced during oxygenicphotosynthesis is used for reduction of carbon dioxide. Mostof the residual oxygenic photoreductant probably is used forlight-dependent reduction of nitrate, which competes with carbondioxide for oxygenic photoreductant. Nitrate is an importantsource of nitrogen for this algal assemblage, and light-dependentnitrate reduction probably is much larger than carbon dioxidereduction at lowest irradiances in the euphotic zone. Oxygenproduction also may be much larger than carbon assimilationat low light levels in other environments where oxidized formsof nitrogen are important nitrogenous nutrients for phytoplankton,as in the lower euphotic zone of the sea, where low rates ofcarbon assimilation by phytoplankton have been thought to beinconsistent with the amount of oxygen that accumulates duringsummer. 相似文献