Chemosynthesis by bacteria is normally very low in the pelagic of streams, infertile waters, and lakes of intermediate productivity and becomes a significant contribution to the whole only in productive or meromictic lakes exhibiting steep redox gradients.
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The ratio of dark CO 2 fixation almost all by heterotrophic metabolism of bacteria; see Gerletti, ; Romanenko, to photosynthetic fixation of CO 2 and bacterial chemosynthesis generally is small in oligotrophic waters. For example, the yr average of dark CO 2 fixation was This percentage is much less for the lake, about one-tenth, if the littoral photosynthesis of this lake is considered in addition to the carbon fixation of the phytoplankton.
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The ratio of photosynthetic to dark CO 2 fixation decreases in the transition to planktonic eutrophy and hypereutrophy. In lakes that are deeper than Mirror Lake z m , 10 m , planktonic bacterial production probably accounts for a greater proportion of the total decomposition simply because particulate detritus would have a longer residence time in the water column. With further transition of the system or in lakes with a predominance of productivity by emergent macroflora and associated attached and littoral microflora, the relative contribution of dark CO 2 fixation to the DOC pool apparently decreases cf.
Wetzel, TABLE While a voluminous nomenclature is available to differentiate among variations in these processes, for simplicity, chemosynthesis and photosynthesis are used here. In photosynthesis, sunlight captured by proteins provides energy for the conversion of inorganic carbon carbon dioxide, CO 2 and water H 2 O into organic carbon carbohydrates, [CH 2 O] and oxygen O 2 eqn . Photosynthesis by plants is the basis for consumer and degradative food webs both on land and, as a rain of organic detritus derived from surface phytoplankton productivity, on the seabed.
In the deep sea, detrital inputs of organic carbon are exceedingly small, accounting for the paucity of consumer biomass in abyssal muds. At hydrothermal vents, the supply of surface-derived organic material is overwhelmed by the supply of new organic carbon generated through chemical oxidation of hydrogen sulfide H 2 S eqn . Metabolic fixation pathways for carbon can be identical in photosynthetic plants and chemosynthetic microorganisms, namely the Calvin—Benson cycle, but the energy-yielding processes that fuel the Calvin—Benson cycle photon capture versus chemical oxidation are distinctive.
High biomass at hydrothermal vents is in part a consequence of the aerobic nature of the process described in eqn .
Oxygen is used to oxidize the hydrogen sulfide, generating a large energy yield that in turn can fuel the production of large amounts of organic carbon Figure 1. Nonaerobic chemical reactions, such as oxidation of vent-supplied hydrogen H 2 by carbon dioxide CO 2 , can also support chemosynthesis at vents, but energy yields under such anaerobic conditions are much lower than from aerobic oxidation.
Microorganisms using these anaerobic reactions cannot by themselves support complex food webs and large invertebrates. Figure 1. Photosynthetic and chemosynthetic processes in the ocean. Sunlight fuels the generation of organic material CH 2 O from inorganic carbon dioxide CO 2 and water H 2 O by phytoplankton in surface, illuminated waters.
In place of sunlight, the chemical oxidation of sulfide H 2 S by oxygen O 2 fuels the conversion of carbon dioxide to organic carbon by chemosynthetic bacteria.
The physiological mechanisms for capturing chemical energy during chemosynthesis are diverse, and there are several descriptive qualifiers that define an organism based on its carbon and energy sources. Typically, chemolithoautotrophs use compounds present in rocks or groundwater. Chemical electron donors include, but are not limited to, molecular hydrogen, reduced sulfur compounds, metals, and so on. Organisms that gain cellular energy from chemical transformations but use organic carbon compounds for their carbon source are chemoorganotrophs , and heterotrophs use organic carbon for cellular energy and carbon sources.
Several studies have shown that chemolithoautotrophs can grow if organic carbon is present as mixotrophs , in which both chemolithoautotrophy and heterotrophy are expressed simultaneously. Microorganisms also have oxygen requirements, and can respire aerobically, anaerobically, or ferment, all of which relates to electron acceptor utilization. Oxygen is the terminal electron acceptor for aerobic metabolic processes Table 1. In reducing environments, microbes that do not require oxygen anaerobes use a variety of alternative electron acceptors for respiration in a sequence of energetic, reduction reactions that occur along thermodynamic and redox gradients, from nitrate to carbon dioxide Table 1.
There are several other energetically favorable, chemolithoautotrophic pathways that occur in the absence of oxygen, including sulfide oxidation via nitrate reduction and anaerobic ammonium oxidation anammox Table 1. Table 1. Louise M. Prockter, Robert T. Based on our terrestrial view, the primary ingredients for life are water, organic compounds, and chemical energy. Europa may have all three: water of the ocean, organic compounds that have been delivered to the satellite, and chemical energy from radiolysis and possibly chemosynthesis.
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The evidence for liquid water within Europa is strong, as discussed earlier, and Europa's sub-ice ocean may have a greater volume than that of all Earth's surface water. Cometary and asteroidal impactors have rained onto the surfaces of the Galilean satellites throughout solar system history. Just as Ganymede and Callisto have been darkened by impactor material, similar material must have been delivered to Europa, where its young and bright surface implies that much of this material is now incorporated into the ice shell and ocean.
Metabolic reactions within living cells depend upon chemical reactions between oxidants and reductants. For animals, this depends on taking in oxygen, which is combined with sugars to produce CO 2 and water. For plants, CO 2 is combined with water to form sugars and oxygen. The key is that chemical disequilibrium must exist, which organisms then exploit to create the energy needed for life. Whether Europa has sufficient chemical energy to support life is the most significant unknown in understanding Europa's potential for life. Irradiation of surface ice can form molecules of oxygen and hydrogen, with most of the hydrogen floating away but much of the oxygen and other oxidants remaining behind, like a condensed out atmosphere frozen into the uppermost centimeters of ice.
If these oxidants can be delivered to the ice shell and ocean, they maybe able to power the chemical reactions necessary for life. Some of these oxidants will be churned into the upper meter of ice by small impacts. Geological processes such as chaos formation may be able to deliver near-surface materials to the ocean, but the means of surface-ocean communication remain poorly understood.
Some oxygen and hydrogen is also produced within the ice shell and ocean by radioactive decay of potassium, but this alone could not provide much energy for life. If Europa's rocky mantle is tidally heated, then hydrothermal systems could exist on Europa's ocean floor. On Earth, hot chemical-laden water pours into the oceans, delivering organic materials and reductants into the water.
If hydrothermal systems exist at the bottom of Europa's ocean, and if oxidants are delivered from the ice shell above, then the necessary chemical disequilibrium that could be used by life exists. Another important consideration is whether Europa's interior environment is stable enough through time, such that if life ever developed it would still exist today. Europa's ocean may have persisted for aeons thanks to internal radioactive heating and the warming resulting from Jupiter's gravitational tug.
However, the internal heating induced by the Laplace resonance is not necessarily ancient, and as discussed earlier the intensity of tidal heating may have varied perhaps cyclically through time. It is an open question whether chemical energy sources for life exist within Europa and have been sufficiently stable to support life through time. Even if life does not exist within Europa today, it may have existed in the past.
Most ecosystems on earth ultimately rely on photosynthesis, with the energy source being solar. In marked contrast, deep-sea hydrothermal ecosystems are based predominantly on chemosynthesis , with the energy source being geothermal. Many of the chemosynthetic microbes are fueled by hydrogen sulfide, which is present at low-temperature vents in concentrations up to several hundred micromoles per liter and at high-temperature vents in concentrations up to milimoles per liter.
The tubeworms have no mouth, no digestive system, and no anus; in short, no opening to the external environment. Hydrogen sulfide diffuses across cell membranes and is transported via the hemoglobin-containing circulatory system to the trophosome, where it is utilized by the associated symbionts. Mussels Bathymodiolus thermophilus Figures 7 and 8 and vesicomyid clams Calyptogena magnifica Figure 9 , common along both the Galapagos Rift and East Pacific Rise EPR , represent two of the other dominant members of the vent megafauna that house chemosynthetic symbionts.
In the case of each of these bivalves, the symbionts are associated with the gills and both species have modified feeding apparatuses relative to those of shallow-water related species likely a result of their predominant reliance on the associated symbionts for nutrition. Closely related mussels and clams within the families Mytilidae and Vesicomyidae are common constituents of the fauna associated with vents along mid-oceanic ridge and back-arc spreading centers as well as at many cold-water hydrocarbon seeps throughout the world's oceans.
Figure 4. The top two-thirds of the edifice is covered with vestimentiferan tubeworms, both Riftia pachyptila larger organisms and Tevnia jerichonana smaller organisms , as well as numerous brachyuran crabs Bythograea thermydron and zoarcid fish Thermarces andersoni.
Figure 5. Higher magnification of the side of Tubeworm Pillar depicted in Figure 4 , showing tubeworms Riftia pachyptila and Tevnia jerichonana and scavenging brachyuran crabs Bythograea thermydron. Figure 6. Close-up image of a cluster of Tevnia jerichonana , together with a brachyuran crab Bythograea thermydron and a zoarcid fish Thermarces andersoni. Figure 7. A dense population of mussels Bathymodiolus thermophilus inhabiting a low-temperature hydrothermal vent field along the East Pacific Rise. Associated fauna in the field of view include tubeworms Riftia pachyptila , brachyuran crabs Bythograea thermydron , zoarcid fish Thermarces andersoni , and a galatheid crab Munidopsis subsquamosa lower left.
Figure 8. Close-up of mussels Bathymodiolus thermophilus attached to the tubes of the tubeworm Riftia pachyptila. Limpets Lepetodrilus elevatus are seen attached to the external surfaces of both the mussel shells and tubeworm tubes. Figure 9. Walter H. All organisms and therefore their ecosystems require energy to function. For most higher plants and algae, that energy source is solar, through the process of photosynthesis. Energy from chemosynthesis , and particularly from volcanic vents along mid-ocean ridges, is quite interesting, but globally is very small as compared to photosynthesis.
For many animals and bacteria, the energy source, through food webs, is based directly in higher plants and algae. However, for some very large water ecosystems e. It is well known that small, planktonic algae, protozoa, and bacteria which can be considered particulates are fed on by a wide variety of larger filter feeders, and provide the base of open-water food webs.
These particulates are not, by any means, the end point. They continue to be an energy source in mid-water detrital food webs. Ultimately, every living thing uses the molecule ATP adenosine triphosphate for this purpose. In turn, to derive energy from molecules, those molecules, called nutrients, must be easy to find and simple to break down.
Glucose fits this description for most life on Earth. Some organisms get glucose by digesting what they eat; others have to make it or make other carbohydrates.
Far under the ocean's surface, where pressures are extreme and nutrients scarce, certain communities of organisms are able to not merely survive but thrive. Not by accident, in fact, they do so while clustering around hydrothermal vents , openings in the sea floor that emit extreme heat and chemicals that many species cannot tolerate like miniature volcanoes.
These chemosynthetic organisms represent both a curiosity and a triumph of evolution in terms of how they make food.
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