User:Peterju2/Anoxygenic photosynthesis

Anoxygenic photosynthesis is the metabolic process of reducing carbon without producing oxygen. The community of anoxygenic phototrophs is diverse even though such organisms require niche ecological conditions to thrive. Anoxygenic photosynthesizing bacteria employ a variety of strategies with differing photosynthetic complexes and pigments. Studies suggest that anoxygenic photosynthesis was the first anabolic process of harnessing solar energy to evolve. Certain organisms were likely utilizing anoxygenic photosynthesis 3 billion years ago when the planet had extremely low levels of oxygen and organisms thrived under reduced conditions. Anoxygenic phototrophs contain one of two enzymes called Reaction Centers (RC) 1 and 2. RC 1 and 2 are the ancient analogs to Photosystem I and II found in oxygenic phototrophs and likely evolved in sequence as the availability of Iron (II) Sulfide (FeS) became more limited and the planet more oxidized.

Pigments
The pigments used to carry out anaerobic photosynthesis are similar to chlorophyll but differ in molecular detail and peak wavelength of light absorbed. Bacteriochlorophylls (Bchl) a through g absorb electromagnetic radiation maximally in the near-infrared within their natural membrane milieu. This differs from chlorophyll a, the predominant plant and cyanobacteria pigment, which has peak absorption wavelength approximately 100 nanometers shorter (in the red and blue portion of the visible spectrum).

Reaction Centers
There are two main types of anaerobic photosynthetic electron transport chains in bacteria. The type I reaction centers (RC1) are found in green sulfur bacteria, Chloracidobacterium. The type II reaction centers (RC2) are found in FAPs and purple bacteria. RC1 uses low-potential FeS clusters as electron acceptors to reduce NADP+ to NADPH. RC2 receives electrons from small soluble proteins such as cytochrome c, cupredoxins and ferredoxins. RC2 uses electron donors such as ferrous iron, reduced sulfur compounds, and molecular hydrogen. It is similar to Photosystem II (PSII), but without the oxygen-evolving complex. Detailed below are the reaction centers found in green sulfur bacteria and purple bacteria.

The electron transport chain of green sulfur bacteria, present in model organism, Chlorobaculum tepidum — uses the reaction center bacteriochlorophyll pair, P840. When light is absorbed by the reaction center, P840 enters an excited state with a large negative reduction potential, and so readily donates the electron to bacteriochlorophyll 663 which passes it on down the electron chain. The electron is transferred through a series of electron carriers and complexes until it is used to reduce ferredoxin. P840 regeneration is accomplished with the oxidation of sulfide ion from hydrogen sulfide (or hydrogen or ferrous iron) by cytochrome c555.

The electron transport chain of purple non-sulfur bacteria begins when the reaction center bacteriochlorophyll pair, P870, becomes excited from the absorption of light. Excited P870 will then donate an electron to bacteriopheophytin, which then passes it on to a series of electron carriers down the electron chain. In the process, it will generate an electrochemical gradient which can then be used to synthesize ATP by chemiosmosis. P870 has to be regenerated (reduced) to be available again for a photon reaching the reaction-center to start the process anew.

Bacteria
While oxygenic photosynthesis only exists in one bacterial phylum (cyanobacteria), anoxygenic photosynthesis is widespread in the bacterial kingdom. Some examples of phyla that can utilize anoxygenic photosynthesis are Chlorobi, Chloroflexi, and Proteobacteria. These bacteria use different chemical compounds as electron donors. Green Sulfur Bacteria (phyla: Chlorobi) utilize sulfur or sulfide instead of water. Other species of Green Bacteria such as Filamentous Anoxygenic Phototrophic Bacteria (Green Non-sulfur Bacteria), use molecular hydrogen or reduced iron as their electron donors. Most anoxygenic phototrophs use one or multiple of the listed electron donors and the specific electron donor used is dependent on the type of pigment and reaction centers employed by the specific bacteria.

There are a variety of pigments among anoxygenic phototrophs. Green Sulfur Bacteria use Bchl-a and chlorophyll a, absorbing visible light in the blue and red, and reflecting green light (which gives them their green color). Purple Bacteria use Bchl-a or -b, resulting in a color ranging from purple to orange-red. Green Non-sulfur Bacteria use similar bacteriochlorophyll as Green Sulfur Bacteria but use RC2 like Purple Bacteria. Heliobacteria are nitrogen fixers. They are similar to Green Sulfur Bacteria in that they conduct a type 1 photosynthesis reaction (RC1), except they use bacteriochlorophyll g unlike any other bacterium.

Some cyanobacteria can do both anoxygenic and oxygenic photosynthesis. When these organisms are in an environment with abundant sulfur, they will preferentially oxidize sulfide rather than water. A similar phenomenon was observed in a study by Shiba et al 1979. They isolated a subclass of the Proteobacteria in oxic conditions and found they could synthesize Bchl-a without producing oxygen in the process. This bacterium is part of a different category of aerobic anoxygenic photosynthetic bacteria.

Archaea
Some archaea (e.g. Halobacterium) capture light energy for metabolic function and are thus phototrophic but none are known to "fix" carbon (i.e. be photosynthetic). Instead of a chlorophyll-type receptor and electron transport chain, proteins such as bacteriorhodopsin and halorhodopsin capture light energy with the aid of diterpenes to move ions against the gradient and produce adenosine triphosphate (ATP) via chemiosmosis in the manner of mitochondria.

Marine
Anoxygenic photosynthesis by marine cyanobacteria, dinoflagellates and bacteria is potentially of great ecological importance to the world's oceans where they are abundant. Anoxygenic photosynthetic organisms live in photic, anoxic marine environments where hydrogen sulfide is available and light intensity is low. Optimal habitats include stratified seas, sediment layers, intertidal microbial mats, and even within the guts of zooplankton (though not their fecal pellets), salt marshes, anoxic microzones associated with particulate matter. Organisms  capable of anoxygenic photosynthesis are more abundant than previously thought, as evidenced by widespread detection of bacteriochlorophyll a in the worlds aerobic open oceans oceans. Photoheterotrophic alpha-bacteria capable of anoxygenic photosynthesis account for as much as 10% of the bacterial biomass off the coast of Washington and Oregon. Their ability for anoxygenic photosynthesis may be more important in oligotrophic waters where photoheterotrophic bacteria are more likely to be limited by the availability of reduced carbon, as photoheterotrophs do not need to use light at all if all organic compounds are sufficiently available, however their abundance throughout the oceans suggest that they may contribute to marine sulfur, nitrogen, and carbon cycles.

One of the best known examples of sustained anoxygenic photosynthesis is the Black Sea. The feedback mechanisms described in the Canfield Ocean are evident in the Black Sea. Though it is 2000m deep, the majority of the water column is euxinic and there is a steep chemocline with an overlying oxic surface layer of approximately 100m.

Intertidal
Anoxygenic photosynthetic organisms are abundant in intertidal mudflats and salt marsh systems. These habitats can have large sulfur and iron redox gradients in near surface sediments. Coupled with a reliable availability of light, interidal zones are ideal environments for anoxygenic photosynthesis.

Freshwater
Anoxygenic photosynthetizing bacteria typically stratify in anoxic water, such as shallow ponds of stagnant water, hot springs, or stratified sulfuric lakes, forming thick brown, green, or pink aggregations at the chemocline, where very sharp gradients in temperature, light, and chemical concentrations (e.g. oxygen and sulfide) favor their growth. The chemocline depth within lakes may vary seasonally and cause changes in anaerobic primary production due to the intensity of ambient light. Optimal growth conditions may be self-limiting as a reduction of light intensity from self-shading can limit growth even when nutrients are non-limiting.

Terrestrial
Aerobic anoxygenic phototrophic bacteria have been reported to exist in biological soils, comprising 0.1–5.9% of the cultivable bacterial community in moss, lichen and cyanobacteria‐dominated crust from sand dunes and sandy soils, and could accelerate organic carbon cycling in nutrient-poor arid soils. Their effects will be especially important as global climate change enhances soil erosion and consequent nutrient loss.

History
Anoxygenic photosynthesis likely evolved in bacteria approximately 3.5 billion years ago during the Archaen. One billion years later, oxygenic photosynthesis evolved and the Great Oxidation Event occurred. While this produced substantial levels of atmospheric oxygen for the first time in Earth's history, it took approximately 900 million years before the atmosphere contained oxygen concentrations conducive for the life we see today (often called the Boring Billion). The Canfield Ocean is a theorized model that explains how the feedback mechanisms associated with anoxygenic photosynthesis made it difficult for the planet to fully oxidize for nearly 1 billion years following the onset of oxygenic photosynthesis. During the Proterozoic, anoxygenic cyanobacteria maintained highly euxinic conditions in the world’s oceans. Molecular fossils from the Proterozoic oceans show pigments derived from anoxygenic photosynthesizers which is direct evidence for photic zone euxinia as Canfield theorizes. Anoxygenic photosynthesizers were the dominant photosynthesizers in the ocean at that time.

Geologic Timescale
The Canfield Ocean is one theory on why oxygen levels were so low in the ocean during the Boring Billion. Though the Great Oxidation Event that occurred approximately 2.7 billion years ago produced substantial atmospheric oxygen concentrations, the ocean did not experience substantial oxidation and rather stayed reduced. During this time, surface waters were nitrogen-limited because of the efficient utilization of nitrogen by anoxygenic phototrophs. As nitrogen-rich deep waters upwelled, anoxygenic phototrophs were likely to utilize it first, consequently making the upper photic zone a difficult place for oxygenic phototrophs to thrive. This would have limited oxygenic photosynthesis in the surface waters and produced a severe chemocline.The Canfield Ocean model explains how the oxygen levels in the ocean were intertwined with the marine sulfide budget. The weathering of pyrite and sulfate brought high amounts of iron and sulfur to the ocean. The high oxygen rates in the atmosphere led to oxidative weathering of sulfides which filled the ocean with sulfates. The substantial input of sulfur to the marine environment provided anoxygenic phototrophs with the ingredients they needed to continue to thrive. This healthy input lasted several thousands of years. In the mid-Proterozoic, subchemocline anoxygenic photosynthesizers dominated the ocean which created a sulfide rich ocean.

Feedback mechanisms helped maintain the euxinic environment that lasted through the Boring Billion. Euxinic conditions would have allowed for high rates of anoxygenic photosynthesis and thus high production of elemental sulfur. Consequently, there would be an increased flux of organic material and sulfur to the ocean floor. In the deep ocean, this exported sulfur would have reacted with dissolved iron in the water to form pyrite (pyritization). Iron pyrite would also have been used by organisms to produce their outer layer. A positive feedback loop occurred because as anoxygenic photosynthesizers increased, the abundant sulfur in the oceans also increased. This was followed by a strong negative feedback loop, because an increase in sulfur production led to more sinking biomass, which increased carbon burial in the sediment. The more carbon burial led to more pyritization which in turn lowered the sulfur budget in the ocean. The dynamic between these two feedbacks is how a reduced ocean with limited oxygenic photosynthesis could be sustained for such a long time after the Great Oxidation Event