This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license http: This article has been cited by other articles in PMC. Abstract The diversity of extrasolar planets discovered in the last decade shows that we should not be constrained to look for life in environments similar to early or present-day Earth.
Super-Earth exoplanets are being discovered with increasing frequency, and some will be able to retain a stable, hydrogen-dominated atmosphere. We explore the possibilities for photosynthesis on a rocky planet with a thin H2-dominated atmosphere.
If a rocky, H2-dominated planet harbors life, then that life is likely to convert atmospheric carbon into methane. Outgassing may also build an atmosphere in which methane is the principal carbon species. We describe the possible chemical routes for photosynthesis starting from methane and show that less energy and lower energy photons could drive CH4-based photosynthesis as compared with CO2-based photosynthesis.
We find that a by-product biosignature gas is likely to be H2, which is not distinct from the hydrogen already present in the environment. Ammonia is a potential biosignature gas of hydrogenic photosynthesis that is unlikely to be generated abiologically.
We suggest that the evolution of methane-based photosynthesis is at least as likely as the evolution of anoxygenic photosynthesis on Earth and may support the evolution of complex life. Light is by far the most abundant source of chemical energy on the surface of the Earth, so any form of life that evolves the ability to capture light energy will be able to out-compete its non-photosynthetic sister species, at least for growth on the surface of the planet.
Light will be an abundant and accessible energy source on the surface of any planet with a sufficiently thin atmosphere. In this paper, we investigate, for the first time to our knowledge, how life can use light energy to capture atmospheric carbon in an environment that is dominated by hydrogen and methane.
We specifically address the energy requirements, possible photon wavelength requirements and whether such photosynthesis would generate as distinct a biosignature as oxygenic photosynthesis makes on Earth. Role and Evolution of Photosynthesis The ability to capture the energy of electromagnetic radiation has evolved at least three times on Earth, with mechanisms mediated by chlorophyll [ 123 ], rhodopsin-like proteins [ 45 ], which can capture light energy for biosynthesis [ 67 ], and fungal melanin-based systems that capture short wavelength light and ionizing radiation and use at least some of the captured energy to power ATP synthesis [ 8910 ].
The carbon isotope ratio at 3. The speed with which photosynthesis evolved on Earth suggests that analogous biochemistry might be expected to evolve on other inhabited worlds. Photosynthesis is a combination of two processes—the capture of light energy and the use of that energy to drive thermodynamically unfavorable redox reactions.
The redox reactions are needed to build biomass [ 13 ]. Living organisms build biomass by capturing environmental carbon, nitrogen, oxygen, phosphorus and sulfur and incorporating those elements into the complex molecules of biochemistry.
We note that a minority of organisms, including ourselves, build biomass by eating other organisms heterotrophy: However, clearly, the heterotrophic lifestyle cannot dominate the biosphere. In textbooks, terrestrial photosynthesis is usually illustrated by the chemistry needed to build carbohydrates compounds of the general formula CH2O from carbon dioxide CO2.
This requires energy input. In terrestrial plants, the energy captured from photons is used to power the reaction: Oxygenic photosynthesis produces molecular oxygen as a by-product, so the reaction becomes: This is of course a gross over-simplification of the complex molecular machinery of photosynthesis.
However it does illustrate the overall stoichiometry input and output of the process, from which the minimum energy input to make the process happen can be calculated. Photosynthesis beyond Earth No life, including photosynthesizing life, has been found beyond Earth. Theoretical studies of exoplanet photosynthesis have primarily addressed whether planets orbiting around different stars can have surface illumination consistent with terrestrial photosynthesis.
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Different groups have considered, at various levels of detail: For photosynthetic signals, all have assumed basically terrestrial biochemistry. The impact of photosynthesis on the atmosphere and the oxidation of the Earth has been the subject of extensive study, summarized in [ 27 ].
Other studies examine the impact of the evolution and the consequences of oxygenic photosynthesis on Earth [ 116172528293031 ]. These studies are all of terrestrial photosynthesis, i. The role of photosynthesis in allowing Earth to develop an abundant biosphere [ 253032 ] and its potential role in enabling complex life based on aerobic metabolism [ 230 ] have also been studied.
These obviously all assume O2 production.
In any case, the red edge is a less reliable signature than O2 and O3 [ 4243 ]. The previous studies cited above do not address the photosynthetic chemistry that might happen on a planet with a highly reducing atmosphere, the subject of this paper.
However a reduced atmosphere planet, with significant H2 content, is possible. Some Super-Earth planets loosely defined as rocky planets of up to 10 Earth masses may be able to retain hydrogen in their atmospheres.LPT: When writing an essay, put your weakest arguments in the middle and your strongest arguments in the start and end.
(plombier-nemours.comoTips) Introduction 2nd-strongest point Weakest point Strongest point Conclusion. By the way, for those who don't want to read the article, the idea is that people will remember the first and last thing they.
The building team consisted of fathers who included surveyors, building foremen, lorry drivers, draughtsmen, metal workers, carpenters, painters, electricians and a . The Stages of Writing. which involves many thought processes all going on at once. In order to produce written material more efficiently, these processes can be broken down into stages.
These are defined differently by various approaches, with anywhere between 4 and 10 stages. • Brainstorm and jot down any ideas, thoughts, arguments.
and writing strategies. Emphasis is placed on analyzing information and ideas and incorporating research findings into documented writing and research projects. The idea of an argument often conjures up images of two people yelling and screaming in anger. In writing, however, an argument is very different. An argument is a reasoned opinion supported and explained by evidence. To argue in writing is to advance knowledge and ideas in a positive way. Written. When you are doing a persuasive paper or speech, it is much easier to tackle this project if you create an outline first. Your outline should have a strong thesis statement, at least three main points, and then sub-points under the three main points, and finally a .
ENG Research and Writing Assignment: Research topics with explanation Name and Surname Professors Name Date: My first topic is should citizens be permitted to carry a concealed weapon to their workplace (e.g., office, school, construction site)?
86 5. The Research Essay The Research Essay The research essay requires students to use multiple sources in order to establish a context . 5 Do papers written collaboratively use different warrants from papers written from HUM at University of Phoenix.
What team writing processes produce the strongest arguments? Collaboration and Argument Paper and Presentation Instructions ENG/ Version 4 2 Annotated Bibliography.