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Friday, December 24, 2021

Tropical medicine

From Wikipedia, the free encyclopedia
 
Sir Patrick Manson, the father of Tropical Medicine
 
Tropical Medicine Physician
Occupation
Names
  • Physician
Occupation type
Specialty
Activity sectors
Medicine
Description
Education required
Fields of
employment
Hospitals, Clinics

Tropical medicine is an interdisciplinary branch of medicine that deals with health issues that occur uniquely, are more widespread, or are more difficult to control in tropical and subtropical regions.

Physicians in this field diagnose and treat a variety of diseases and ailments. Most infections they deal with are endemic to the tropics. A few of the most well-known include malaria, HIV/AIDS, and tuberculosis. They must be knowledgeable in the 18 lesser known neglected tropical diseases, which include Chagas disease, rabies, and dengue. Poor living conditions in underdeveloped tropical countries have led to a rising number of non-communicable diseases. These diseases include cancer and cardiovascular disease, which, in the past, have been more of a worry in developed countries. Physicians trained in tropical medicine must also be prepared to diagnose and treat these diseases.

Training for physicians wishing to specialize in tropical medicine varies widely over the different countries. They must study epidemiology, virology, parasitology, and statistics, as well as the training required of an ordinary MD. Research on tropical diseases and how to treat them comes from both field research and research centers, including those of the military.

Sir Patrick Manson is recognized as the father of tropical medicine. He founded the London School of Hygiene & Tropical Medicine in 1899. He is credited with discovering the vector by which elephantiasis was being passed to humans. He learned it was a microscopic nematode worm called filaria sanguinis hominis. He continued to study this worm and its life cycle and determined the worms underwent metamorphosis within female culex fatigans mosquitoes. Thus he discovered mosquitoes as a vector for elephantiasis. After this discovery he collaborated with Ronald Ross to examine the transmission of malaria via mosquito vector. His work with discovering vectors as modes of transmission was critical in the founding of tropical medicine and our current understanding of many tropical diseases.

Training

Training in tropical medicine is quite different between countries. Most physicians are trained at institutes of tropical medicine or incorporated into the training of infectious diseases.

The London School of Hygiene and Tropical Medicine, founded by Sir Patrick Manson

In the UK, if a physician wants to specialize in tropical medicine, they must first train in general internal medicine and get accepted into the Royal College of Physicians. They must simultaneously study the specialty of infectious diseases while completing a full-time course load to receive their Diploma of Tropical Medicine and Hygiene. Their studies are carried out at either the London or Liverpool schools of tropical medicine. Additionally, they must spend two years at one of the UK centers approved for tropical medicine (located in London, Liverpool, or Birmingham). Physicians in the UK who wish to be certified in tropical medicine must spend at least a year abroad in an area lacking resources. Only then can they become certified in tropical medicine.

The training of United States tropical doctors is similar, though it is not a board recognized specialty in America. Physicians must first complete medical school and a program focusing on infectious diseases. Once completed, physicians can take the certification exam from the American Society of Tropical Medicine and Hygiene in order to receive the Certificate of Knowledge in Clinical Tropical Medicine and Travelers' Health.

Challenges

HIV

In developing countries alone, 22 million people are living with HIV. Most infections are still in Africa, but Europe, Asia, Latin America, and the Caribbean are now seeing large numbers of infections as well.  95% of expected new infections will occur in the low income countries in the tropics. The expected number of new infections is 3-4 million per year. Risk factors such as needle use and unprotected sex are much more prevalent in tropical and underdeveloped areas. Once HIV is transmitted to a tropical area it is spread throughout the sexually active population. Though how fast and how far it spreads varies, some African countries have an HIV prevalence of 10%. More alarming still, in urban areas, prevalence among pregnant women can get as high as 30%. Healthcare professionals themselves are at great risk of exposure to HIV. An HIV prevalence of 10% means any given workforce will also have a 10% prevalence, and this does not exclude the healthcare team. Tuberculosis is thought to cause a more rapid disease progression. Tuberculosis is prevalent in tropical and under-developed countries, only making HIV more devastating. Without the expensive and high-tech medical equipment of developed, western countries, physicians in the tropics are left with few options. If they are able to catch an HIV-related bacterial or mycobacterial disease they can diagnose and manage the disease with basic drugs and standard treatment protocol. Many under-developed countries do not have a care strategy, and of those that do, they aren't as effective as they need to be to stop the spread of HIV.

Malaria

A map detailing areas where malaria is endemic.

Malaria is a parasitic disease transmitted by an Anopheles mosquito to a human host. The parasite that causes malaria belongs to the genus Plasmodium. Once infected, malaria can take a wide variety of forms and symptoms. The disease is placed into the uncomplicated category or the severe category. If quickly diagnosed and treated, malaria can be cured. However, some of the more serious symptoms, such as acute kidney failure, severe anemia, and acute respiratory distress syndrome can be fatal if not dealt with swiftly and properly. Certain types of Plasmodium can leave dormant parasites in the liver that can reawaken months or years later, causing additional relapses of the disease. In the World Malaria Report of 2016, the World Health Organization reported a malaria infection rate of 212 million, 90% of which occurred in the African region. However, malaria infection rates had fallen 21% since 2010 at the time of the report. The WHO also reported an estimated mortality rate of 429,000 deaths in the year 2015. The malaria mortality rate had fallen 29% globally since 2010. Children under 5 contract the malaria disease more easily than others, and in the year 2015, an estimated 303,000 children under the age of 5 were killed by malaria. Since the year 2010 however, the mortality rate of children under 5 fell by an estimated 35%.

Tuberculosis

Tuberculosis (TB) is an infectious bacterial disease that can affect any part of the body, though it primarily affects the lungs. It is a disease that affects the poor and weak, and is far more common in developing countries. TB can either be in its latent or active form. TB can be latent for years, sometimes over a decade. Though TB research receives a mere 1/6th the funding of HIV research, the disease has killed more people in the last 200 years than any other infectious disease. According to the Liverpool School of Tropical Medicine, an estimated 9 million people were infected with TB in the year 2013 alone. That same year 1.5 million people died from TB. Of those 1.5 million, 360,000 were HIV positive. Tuberculosis is extremely expensive to treat, and treatments are now becoming ineffective due to drug-resistant TB strains. In the year 2016, 1.3 million people died from TB. An additional 374,000 people died who were co-infected with both TB and HIV. Research has shown that if the subject is infected with HIV, the risk of latent TB becoming active TB is between 12 and 20 times higher.

Non-communicable diseases

Non-communicable diseases are a series of chronic illnesses such as cardiovascular disease, cancer, injuries, and respiratory diseases, among others. Historically these diseases have plagued developed countries far more than developing countries. In the Global Burden of Disease Study of 2001, it was discovered that 20% of deaths in sub-Saharan Africa were caused by non-communicable diseases. In 2005, the World Health Organization performed a study that showed 80% of chronic disease deaths occurred in low to middle income countries. Non-communicable disease prevalence has been rising in under-developed countries for a variety of reasons. Lack of education and preventive medicine in under-developed countries, along with malnutrition or poor diet lead to many risk factors for non-communicable diseases.

Neglected tropical diseases

Neglected tropical diseases (NTDs) have been identified by the World Health Organization (WHO) as 18 tropical diseases, affecting over a billion people worldwide, especially in developing countries. These diseases are heterogeneous, meaning originating outside the organism affected by the disease. NTDs are caused by parasites, viruses, and bacteria. NTDs are neglected because they are not normally fatal on their own but are disabling. Persons with these diseases become more susceptible to other NTDs and fatal diseases such as HIV or malaria.

Neglected tropical diseases effect can be measured in disability-adjusted life year (DALY). Each DALY corresponds to one lost year of healthy life, whether by death or disability. In the year 2010, it was estimated 26.6 million DALYs were lost. In addition to this, it is estimated NTDs cause a loss of 15-30% of productivity in countries that NTDs are endemic too. According to the CDC, 100% of countries categorized as 'low income' were affected by 5 different NTDs at once.

Interdisciplinary approach

Tropical medicine requires an interdisciplinary approach, as the infections and diseases tropical medicine faces are both broad and unique. Tropical medicine requires research and assistance from the fields of epidemiology, microbiology, virology, parasitology, and logistics. Physicians of tropical medicine must have effective communication skills, as many of the patients they interact with do not speak English comfortably. They must be proficient in their knowledge of clinical and diagnostic skills, as they are often without high-tech diagnostic tools when in the field. For example, in an attempt to manage the Chagas disease being brought into the almost Chagas-free Brazilian city São Paulo by Bolivian immigrants, an interdisciplinary team was set up. The Bolivian immigrant population in São Paulo had a prevalence of Chagas disease of 4.4%, while Chagas disease transmission in São Paulo has been under control since the 1970s. This influx of Chagas disease led to an interdisciplinary team being brought together, This team tested the feasibility of managing Chagas disease and transmission at the primary healthcare level. The interdisciplinary team consisted of community health agents and clerical healthcare workers to recruit Chagas infected persons for the study, physicians, nurses, lab workers, and community agents. A pediatrician and cardiologist were also on call. Each were trained in pathology, parasitology, ecoepidemiology, and how to prevent, diagnose, and control Chagas disease. Training from experts in these respective fields was required. They examined reasons for lack of adherence to treatment, and used this knowledge to improve the effectiveness of their interventions. This interdisciplinary approach has been used to train many teams across Brazil in the management of Chagas disease.

Tropical medicine also consists of a preventive approach, especially in an educational aspect. For example, from 2009 to 2011, the London School of Hygiene & Tropical Medicine did an interventional study on a cohort of female sex workers (FSW) in Ouagadougou, Burkina Faso, a country in Western Africa. 321 HIV-unaffected FSWs were provided with peer-led HIV/STI education, HIV/STI testing and care, psychological support, general healthcare, and services for reproductive health. The same cohort would continue to follow up, quarterly, for 21 months. At each follow-up they were tested for HIV and were able to utilize the preventive interventions if need be. Using models based on the same study population had their been no interventions, the expected prevalence of HIV infections was 1.23 infection per 100 person years. In the actual cohort with access to interventions not a single HIV infection was observed in the collective 409 person-years of follow-up.

Tropical research in the military

Throughout history, American military forces have been affected by many tropical diseases. In World War II alone, it was estimated almost one million soldiers had been infected by a tropical disease while serving. Most affected soldiers served in the Pacific, especially in the Philippines and New Guinea. Malaria was especially widespread in the Pacific, though soldiers in Southern Europe and Northern Africa also contracted tropical diseases. Many diseases now known as neglected tropical diseases affected America soldiers as well. These included helminthiasis, schistosomiasis, dengue, and lymphatic filariasis. Lymphatic filariasis was such a problem it caused a $100 million evacuation of U.S troops out of New Guinea and the Tonga Islands.

In both the Korean and Vietnam Wars the United States army continued to be affected by tropical diseases. The most prevalent diseases to affect their military were malaria and dengue. Hepatitis A, scrub typhus, and hookworm infections were among the other tropical infections picked up by troops in these conflicts.

To combat the significant effects tropical diseases were having on their forces, the United States Military worked hard to develop drugs, vaccines, and other methods of disease control. Research done at the Walter Reed Army Institute of Research (WRAIR), the Naval Medical Research Center (NMRC), and various affiliated research centers have greatly improved the military's preparedness against tropical disease. In 1967, Captain Robert Phillips earned the Lasker Award for developing a type of IV therapy that reduced cholera's fatality rate from 60% to less than 1%. Other interventions licensed by the US Army include vaccines for hepatitis A and Japanese encephalitis. They have also developed the drugs mefloquine and malarone, both used in the treatment of malaria.

Looking forward, the United States military currently has clinical trials testing for vaccines of malaria, adenovirus infection, dengue, and HIV/AIDS underway. However, with massive budget cuts to their military, these research centers are getting less and less funding and have lost many contractors already.

Bibliography

Organizations

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Life on Titan

From Wikipedia, the free encyclopedia
 
Multi-spectral view of Titan
 
MarsCuriosityRover-Drilling-Sol170++-2.jpg

Whether there is life on Titan, the largest moon of Saturn, is currently an open question and a topic of scientific assessment and research. Titan is far colder than Earth, but of all the places in the solar system, Titan is the only place besides Earth known to have liquids in the form of rivers, lakes, and seas on its surface. Its thick atmosphere is chemically active and rich in carbon compounds. On the surface there are small and large bodies of both liquid methane and ethane, and it is likely that there is a layer of liquid water under its ice shell. Some scientists speculate that these liquid mixes may provide prebiotic chemistry for living cells different from those on Earth.

In June 2010, scientists analyzing data from the Cassini–Huygens mission reported anomalies in the atmosphere near the surface which could be consistent with the presence of methane-producing organisms, but may alternatively be due to non-living chemical or meteorological processes. The Cassini–Huygens mission was not equipped to look directly for micro-organisms or to provide a thorough inventory of complex organic compounds.

Chemistry

Titan's consideration as an environment for the study of prebiotic chemistry or potentially exotic life stems in large part due to the diversity of the organic chemistry that occurs in its atmosphere, driven by photochemical reactions in its outer layers. The following chemicals have been detected in Titan's upper atmosphere by Cassini's mass spectrometer:

Study Magee, 1050 km Cui, 1050 km Cui, 1077 km Waite et al., 1000–1045 km
Density (cm−3) (3.18±0.71) x 109 (4.84±0.01) x 109 (2.27±0.01) x 109 (3.19, 7.66) x 109
Proportions of different species
Nitrogen (96.3±0.44)% (97.8±0.2)% (97.4±0.5)% (95.5, 97.5)%
14N15N (1.08±0.06)%


Methane (2.17±0.44)% (1.78±0.01)% (2.20±0.01)% (1.32, 2.42)%
13CH4 (2.52±0.46) x 10−4


Hydrogen (3.38±0.23) x 10−3 (3.72±0.01) x 10−3 (3.90±0.01) x 10−3
Acetylene (3.42±0.14) x 10−4 (1.68±0.01) x 10−4 (1.57±0.01) x 10−4 (1.02, 3.20) x 10−4
Ethylene (3.91±0.23) x 10−4 (5.04±0.04) x 10−4 (4.62±0.04) x 10−4 (0.72, 1.02) x 10−3
Ethane (4.57±0.74) x 10−5 (4.05±0.19) x 10−5 (2.68±0.19) x 10−5 (0.78, 1.50) x 10−5
Hydrogen cyanide (2.44±0.10) x 10−4


40Ar (1.26±0.05) x 10−5 (1.25±0.02) x 10−5 (1.10±0.03) x 10−5
Propyne (9.20±0.46) x 10−6 (9.02±0.22) x 10−6 (6.31±0.24) x 10−6 (0.55, 1.31) x 10−5
Propene (2.33±0.18) x 10−6

(0.69, 3.59) x 10−4
Propane (2.87±0.26) x 10−6 <1.84 x 10−6 <2.16e-6(3.90±0.01) x 10−6
Diacetylene (5.55±0.25) x 10−6 (4.92±0.10) x 10−6 (2.46±0.10) x 10−6 (1.90, 6.55) x 10−6
Cyanogen (2.14±0.12) x 10−6 (1.70±0.07) x 10−6 (1.45±0.09) x 10−6 (1.74, 6.07) x 10−6
Cyanoacetylene (1.54±0.09) x 10−6 (1.43±0.06) x 10−6 <8.27 x 10−7
Acrylonitrile (4.39±0.51) x 10−7 <4.00 x 10−7 <5.71 x 10−7
Propanenitrile (2.87±0.49) x 10−7


Benzene (2.50±0.12) x 10−6 (2.42±0.05) x 10−6 (3.90±0.01) x 10−7 (5.5, 7.5) x 10−3
Toluene (2.51±0.95) x 10−8 <8.73 x 10−8 (3.90±0.01) x 10−7 (0.83, 5.60) x 10−6

As mass spectrometry identifies the atomic mass of a compound but not its structure, additional research is required to identify the exact compound that has been detected. Where the compounds have been identified in the literature, their chemical formula has been replaced by their name above. The figures in Magee (2009) involve corrections for high pressure background. Other compounds believed to be indicated by the data and associated models include ammonia, polyynes, amines, ethylenimine, deuterium hydride, allene, 1,3 butadiene and any number of more complex chemicals in lower concentrations, as well as carbon dioxide and limited quantities of water vapour.

Surface temperature

Due to its distance from the Sun, Titan is much colder than Earth. Its surface temperature is about 94 K (−179 °C, or −290 °F). At these temperatures, water ice—if present—does not melt, evaporate or sublime, but remains solid. Because of the extreme cold and also because of lack of carbon dioxide (CO2) in the atmosphere, scientists such as Jonathan Lunine have viewed Titan less as a likely habitat for extraterrestrial life, than as an experiment for examining hypotheses on the conditions that prevailed prior to the appearance of life on Earth. Even though the usual surface temperature on Titan is not compatible with liquid water, calculations by Lunine and others suggest that meteor strikes could create occasional "impact oases"—craters in which liquid water might persist for hundreds of years or longer, which would enable water-based organic chemistry.

However, Lunine does not rule out life in an environment of liquid methane and ethane, and has written about what discovery of such a life form (even if very primitive) would imply about the prevalence of life in the universe.

Past hypothesis about the temperature

Titan – infrared view
(November 13, 2015).

In the 1970s, astronomers found unexpectedly high levels of infrared emissions from Titan. One possible explanation for this was the surface was warmer than expected, due to a greenhouse effect. Some estimates of the surface temperature even approached temperatures in the cooler regions of Earth. There was, however, another possible explanation for the infrared emissions: Titan's surface was very cold, but the upper atmosphere was heated due to absorption of ultraviolet light by molecules such as ethane, ethylene and acetylene.

In September 1979, Pioneer 11, the first space probe to conduct fly-by observations of Saturn and its moons, sent data showing Titan's surface to be extremely cold by Earth standards, and much below the temperatures generally associated with planetary habitability.

Future temperature

Titan may become warmer in the future. Five to six billion years from now, as the Sun becomes a red giant, surface temperatures could rise to ~200 K (−70 °C), high enough for stable oceans of a water–ammonia mixture to exist on its surface. As the Sun's ultraviolet output decreases, the haze in Titan's upper atmosphere will be depleted, lessening the anti-greenhouse effect on its surface and enabling the greenhouse effect created by atmospheric methane to play a far greater role. These conditions together could create an environment agreeable to exotic forms of life, and will persist for several hundred million years. This was sufficient time for simple life to evolve on Earth, although the presence of ammonia on Titan could cause the same chemical reactions to proceed more slowly.

Absence of surface liquid water

The lack of liquid water on Titan's surface was cited by NASA astrobiologist Andrew Pohorille in 2009 as an argument against life there. Pohorille considers that water is important not only as the solvent used by "the only life we know" but also because its chemical properties are "uniquely suited to promote self-organization of organic matter". He has questioned whether prospects for finding life on Titan's surface are sufficient to justify the expense of a mission that would look for it.

Possible subsurface liquid water

Laboratory simulations have led to the suggestion that enough organic material exists on Titan to start a chemical evolution analogous to what is thought to have started life on Earth. While the analogy assumes the presence of liquid water for longer periods than is currently observable, several hypotheses suggest that liquid water from an impact could be preserved under a frozen isolation layer. It has also been proposed that ammonia oceans could exist deep below the surface; one model suggests an ammonia–water solution as much as 200 km deep beneath a water ice crust, conditions that, "while extreme by terrestrial standards, are such that life could indeed survive". Heat transfer between the interior and upper layers would be critical in sustaining any sub-surface oceanic life. Detection of microbial life on Titan would depend on its biogenic effects. For example, the atmospheric methane and nitrogen could be examined for biogenic origin.

Data published in 2012 obtained from NASA's Cassini spacecraft, have strengthened evidence that Titan likely harbors a layer of liquid water under its ice shell.

Formation of complex molecules

Titan is the only known natural satellite (moon) in the Solar System that has a fully developed atmosphere that consists of more than trace gases. Titan's atmosphere is thick, chemically active, and is known to be rich in organic compounds; this has led to speculation about whether chemical precursors of life may have been generated there. The atmosphere also contains hydrogen gas, which is cycling through the atmosphere and the surface environment, and which living things comparable to Earth methanogens could combine with some of the organic compounds (such as acetylene) to obtain energy.

Trace organic gases in Titan's atmosphere—HNC (left) and HC3N (right).

The Miller–Urey experiment and several following experiments have shown that with an atmosphere similar to that of Titan and the addition of UV radiation, complex molecules and polymer substances like tholins can be generated. The reaction starts with dissociation of nitrogen and methane, forming hydrogen cyanide and acetylene. Further reactions have been studied extensively.

In October 2010, Sarah Hörst of the University of Arizona reported finding the five nucleotide bases—building blocks of DNA and RNA—among the many compounds produced when energy was applied to a combination of gases like those in Titan's atmosphere. Hörst also found amino acids, the building blocks of protein. She said it was the first time nucleotide bases and amino acids had been found in such an experiment without liquid water being present.

In April 2013, NASA reported that complex organic chemicals could arise on Titan based on studies simulating the atmosphere of Titan. In June 2013, polycyclic aromatic hydrocarbons (PAHs) were detected in the upper atmosphere of Titan.

Research has suggested that polyimine could readily function as a building block in Titan's conditions. Titan's atmosphere produces significant quantities of hydrogen cyanide, which readily polymerize into forms which can capture light energy in Titan's surface conditions. As of yet, the answer to what happens with Titan's cyanide is unknown; while it is rich in the upper atmosphere where it is created, it is depleted at the surface, suggesting that there is some sort of reaction consuming it.

Hypotheses

Hydrocarbons as solvents

See also: Hypothetical types of biochemistry
 
Hydrocarbon lakes on Titan (Cassini radar image from 2006)

Although all living things on Earth (including methanogens) use liquid water as a solvent, it is conceivable that life on Titan might instead use a liquid hydrocarbon, such as methane or ethane. Water is a stronger solvent than hydrocarbons; however, water is more chemically reactive, and can break down large organic molecules through hydrolysis. A life-form whose solvent was a hydrocarbon would not face the risk of its biomolecules being destroyed in this way.

Titan appears to have lakes of liquid ethane or liquid methane on its surface, as well as rivers and seas, which some scientific models suggest could support hypothetical non-water-based life. It has been speculated that life could exist in the liquid methane and ethane that form rivers and lakes on Titan's surface, just as organisms on Earth live in water. Such hypothetical creatures would take in H2 in place of O2, react it with acetylene instead of glucose, and produce methane instead of carbon dioxide. By comparison, some methanogens on Earth obtain energy by reacting hydrogen with carbon dioxide, producing methane and water.

In 2005, astrobiologists Chris McKay and Heather Smith predicted that if methanogenic life is consuming atmospheric hydrogen in sufficient volume, it will have a measurable effect on the mixing ratio in the troposphere of Titan. The effects predicted included a level of acetylene much lower than otherwise expected, as well as a reduction in the concentration of hydrogen itself.

Evidence consistent with these predictions was reported in June 2010 by Darrell Strobel of Johns Hopkins University, who analysed measurements of hydrogen concentration in the upper and lower atmosphere. Strobel found that the hydrogen concentration in the upper atmosphere is so much larger than near the surface that the physics of diffusion leads to hydrogen flowing downwards at a rate of roughly 1025 molecules per second. Near the surface the downward-flowing hydrogen apparently disappears. Another paper released the same month showed very low levels of acetylene on Titan's surface.

Chris McKay agreed with Strobel that presence of life, as suggested in McKay's 2005 article, is a possible explanation for the findings about hydrogen and acetylene, but also cautioned that other explanations are currently more likely: namely the possibility that the results are due to human error, to a meteorological process, or to the presence of some mineral catalyst enabling hydrogen and acetylene to react chemically. He noted that such a catalyst, one effective at −178 °C (95 K), is presently unknown and would in itself be a startling discovery, though less startling than discovery of an extraterrestrial life form.

The June 2010 findings gave rise to considerable media interest, including a report in the British newspaper, the Telegraph, which spoke of clues to the existence of "primitive aliens".

Cell membranes

A hypothetical cell membrane capable of functioning in liquid methane was modeled in February 2015. The proposed chemical base for these membranes is acrylonitrile, which has been detected on Titan. Called an "azotosome" ('nitrogen body'), formed from "azoto", Greek for nitrogen, and "soma", Greek for body, it lacks the phosphorus and oxygen found in phospholipids on Earth but contains nitrogen. Despite the very different chemical structure and external environment, its properties are surprisingly similar, including autoformation of sheets, flexibility, stability, and other properties. According to computer simulations azotosomes could not form or function under the weather conditions found on Titan.

An analysis of Cassini data, completed in 2017, confirmed substantial amounts of acrylonitrile in Titan's atmosphere.

Comparative habitability

In order to assess the likelihood of finding any sort of life on various planets and moons, Dirk Schulze-Makuch and other scientists have developed a planetary habitability index which takes into account factors including characteristics of the surface and atmosphere, availability of energy, solvents and organic compounds. Using this index, based on data available in late 2011, the model suggests that Titan has the highest current habitability rating of any known world, other than Earth.

Titan as a test case

While the Cassini–Huygens mission was not equipped to provide evidence for biosignatures or complex organics, it showed an environment on Titan that is similar, in some ways, to ones theorized for the primordial Earth. Scientists think that the atmosphere of early Earth was similar in composition to the current atmosphere on Titan, with the important exception of a lack of water vapor on Titan. Many hypotheses have developed that attempt to bridge the step from chemical to biological evolution.

Titan is presented as a test case for the relation between chemical reactivity and life, in a 2007 report on life's limiting conditions prepared by a committee of scientists under the United States National Research Council. The committee, chaired by John Baross, considered that "if life is an intrinsic property of chemical reactivity, life should exist on Titan. Indeed, for life not to exist on Titan, we would have to argue that life is not an intrinsic property of the reactivity of carbon-containing molecules under conditions where they are stable..."

David Grinspoon, one of the scientists who in 2005 proposed that hypothetical organisms on Titan might use hydrogen and acetylene as an energy source, has mentioned the Gaia hypothesis in the context of discussion about Titan life. He suggests that, just as Earth's environment and its organisms have evolved together, the same thing is likely to have happened on other worlds with life on them. In Grinspoon's view, worlds that are "geologically and meteorologically alive are much more likely to be biologically alive as well".

Panspermia or independent origin

An alternate explanation for life's hypothetical existence on Titan has been proposed: if life were to be found on Titan, it could have originated from Earth in a process called panspermia. It is theorized that large asteroid and cometary impacts on Earth's surface have caused hundreds of millions of fragments of microbe-laden rock to escape Earth's gravity. Calculations indicate that a number of these would encounter many of the bodies in the Solar System, including Titan. On the other hand, Jonathan Lunine has argued that any living things in Titan's cryogenic hydrocarbon lakes would need to be so different chemically from Earth life that it would not be possible for one to be the ancestor of the other. In Lunine's view, presence of organisms in Titan's lakes would mean a second, independent origin of life within the Solar System, implying that life has a high probability of emerging on habitable worlds throughout the cosmos.

Planned and proposed missions

See also: Titan (moon) § Proposed or conceptual missions

The proposed Titan Mare Explorer mission, a Discovery-class lander that would splash down in a lake, "would have the possibility of detecting life", according to astronomer Chris Impey of the University of Arizona.

The planned Dragonfly rotorcraft mission is intended to land on solid ground and relocate many times. Dragonfly will be New Frontiers program Mission #4. Its instruments will study how far prebiotic chemistry may have progressed. Dragonfly will carry equipment to study the chemical composition of Titan's surface, and to sample the lower atmosphere for possible biosignatures, including hydrogen concentrations.

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Bioindicator

From Wikipedia, the free encyclopedia
 
Caddisfly (order Trichoptera), a macroinvertebrate used as an indicator of water quality.

A bioindicator is any species (an indicator species) or group of species whose function, population, or status can reveal the qualitative status of the environment. For example, copepods and other small water crustaceans that are present in many water bodies can be monitored for changes (biochemical, physiological, or behavioural) that may indicate a problem within their ecosystem. Bioindicators can tell us about the cumulative effects of different pollutants in the ecosystem and about how long a problem may have been present, which physical and chemical testing cannot.

A biological monitor or biomonitor is an organism that provides quantitative information on the quality of the environment around it. Therefore, a good biomonitor will indicate the presence of the pollutant and can also be used in an attempt to provide additional information about the amount and intensity of the exposure.

A biological indicator is also the name given to a process for assessing the sterility of an environment through the use of resistant microorganism strains (e.g. Bacillus or Geobacillus). Biological indicators can be described as the introduction of a highly resistant microorganisms to a given environment before sterilization, tests are conducted to measure the effectiveness of the sterilization processes. As biological indicators use highly resistant microorganisms, any sterilization process that renders them inactive will have also killed off more common, weaker pathogens.

Overview

A bioindicator is an organism or biological response that reveals the presence of pollutants by the occurrence of typical symptoms or measurable responses and is, therefore, more qualitative. These organisms (or communities of organisms) can be used to deliver information on alterations in the environment or the quantity of environmental pollutants by changing in one of the following ways: physiologically, chemically or behaviourally. The information can be deduced through the study of:

  1. their content of certain elements or compounds
  2. their morphological or cellular structure
  3. metabolic biochemical processes
  4. behaviour
  5. population structure(s).

The importance and relevance of biomonitors, rather than man-made equipment, are justified by the observation that the best indicator of the status of a species or system is itself. Bioindicators can reveal indirect biotic effects of pollutants when many physical or chemical measurements cannot. Through bioindicators, scientists need to observe only the single indicating species to check on the environment rather than monitor the whole community.

The use of a biomonitor is described as biological monitoring and is the use of the properties of an organism to obtain information on certain aspects of the biosphere. Biomonitoring of air pollutants can be passive or active. Experts use passive methods to observe plants growing naturally within the area of interest. Active methods are used to detect the presence of air pollutants by placing test plants of known response and genotype into the study area.

The use of a biomonitor is described as biological monitoring. This refers to the measurement of specific properties of an organism to obtain information on the surrounding physical and chemical environment.

Bioaccumulative indicators are frequently regarded as biomonitors. Depending on the organism selected and their use, there are several types of bioindicators.

Use

In most instances, baseline data for biotic conditions within a pre-determined reference site are collected. Reference sites must be characterized by little to no outside disturbance (e.g. anthropogenic disturbances, land use change, invasive species). The biotic conditions of a specific indicator species are measured within both the reference site and the study region over time. Data collected from the study region are compared against similar data collected from the reference site in order to infer the relative environmental health or integrity of the study region.

An important limitation of bioindicators in general is that they have been reported as inaccurate when applied to geographically and environmentally diverse regions. As a result, researchers who use bioindicators need to consistently ensure that each set of indices is relevant within the environmental conditions they plan to monitor.

Plant and fungal indicators

The lichen Lobaria pulmonaria is sensitive to air pollution.

The presence or absence of certain plant or other vegetative life in an ecosystem can provide important clues about the health of the environment: environmental preservation. There are several types of plant biomonitors, including mosses, lichens, tree bark, bark pockets, tree rings, and leaves. Fungi too may be useful as indicators.

Lichens are organisms comprising both fungi and algae. They are found on rocks and tree trunks, and they respond to environmental changes in forests, including changes in forest structure – conservation biology, air quality, and climate. The disappearance of lichens in a forest may indicate environmental stresses, such as high levels of sulfur dioxide, sulfur-based pollutants, and nitrogen oxides. The composition and total biomass of algal species in aquatic systems serve as an important metric for organic water pollution and nutrient loading such as nitrogen and phosphorus. There are genetically engineered organisms that can respond to toxicity levels in the environment; e.g., a type of genetically engineered grass that grows a different colour if there are toxins in the soil.

Animal indicators and toxins

Populations of American crows (Corvus brachyrhynchos) are especially susceptible to the West Nile Virus, and can be used as a bioindicator species for the disease's presence in an area.

Changes in animal populations, whether increases or decreases, can indicate pollution. For example, if pollution causes depletion of a plant, animal species that depend on that plant will experience population decline. Conversely, overpopulation may be opportunistic growth of a species in response to loss of other species in an ecosystem. On the other hand, stress-induced sub-lethal effects can be manifested in animal physiology, morphology, and behaviour of individuals long before responses are expressed and observed at the population level. Such sub-lethal responses can be very useful as "early warning signals" to predict how populations will further respond.

Pollution and other stress agents can be monitored by measuring any of several variables in animals: the concentration of toxins in animal tissues; the rate at which deformities arise in animal populations; behaviour in the field or in the laboratory; and by assessing changes in individual physiology.

Frogs and toads

Amphibians, particularly anurans (frogs and toads), are increasingly used as bioindicators of contaminant accumulation in pollution studies. Anurans absorb toxic chemicals through their skin and their larval gill membranes and are sensitive to alterations in their environment. They have a poor ability to detoxify pesticides that are absorbed, inhaled, or ingested by eating contaminated food. This allows residues, especially of organochlorine pesticides, to accumulate in their systems. They also have permeable skin that can easily absorb toxic chemicals, making them a model organism for assessing the effects of environmental factors that may cause the declines of the amphibian population. These factors allow them to be used as bioindicator organisms to follow changes in their habitats and in ecotoxicological studies due to humans increasing demands on the environment.

Knowledge and control of environmental agents is essential for sustaining the health of ecosystems. Anurans are increasingly utilized as bioindicator organisms in pollution studies, such as studying the effects of agricultural pesticides on the environment. Environmental assessment to study the environment in which they live is performed by analyzing their abundance in the area as well as assessing their locomotive ability and any abnormal morphological changes, which are deformities and abnormalities in development. Decline of anurans and malformations could also suggest increased exposure to ultra-violet light and parasites. Expansive application of agrochemicals such as glyphosate have been shown to have harmful effects on frog populations throughout their lifecycle due to run off of these agrochemicals into the water systems these species live and their proximity to human development.

Pond-breeding anurans are especially sensitive to pollution because of their complex life cycles, which could consist of terrestrial and aquatic living. During their embryonic development, morphological and behavioral alterations are the effects most frequently cited in connection with chemical exposures. Effects of exposure may result in shorter body length, lower body mass and malformations of limbs or other organs. The slow development, late morphological change, and small metamorph size result in increased risk of mortality and exposure to predation.

Crustaceans

Crayfish have also been hypothesized as being suitable bioindicators, under the appropriate conditions.

Microbial indicators

Chemical pollutants

Microorganisms can be used as indicators of aquatic or terrestrial ecosystem health. Found in large quantities, microorganisms are easier to sample than other organisms. Some microorganisms will produce new proteins, called stress proteins, when exposed to contaminants such as cadmium and benzene. These stress proteins can be used as an early warning system to detect changes in levels of pollution.

In oil and gas exploration

Microbial Prospecting for oil and gas (MPOG) is often used to identify prospective areas for oil and gas occurrences. In many cases, oil and gas is known to seep toward the surface as a hydrocarbon reservoir will usually leak or have leaked towards the surface through buoyancy forces overcoming sealing pressures. These hydrocarbons can alter the chemical and microbial occurrences found in the near-surface soils or can be picked up directly. Techniques used for MPOG include DNA analysis, simple bug counts after culturing a soil sample in a hydrocarbon-based medium or by looking at the consumption of hydrocarbon gases in a culture cell.

Microalgae in water quality

Microalgae have gained attention in recent years due to several reasons including their greater sensitivity to pollutants than many other organisms. In addition, they occur abundantly in nature, they are an essential component in very many food webs, they are easy to culture and to use in assays and there are few if any ethical issues involved in their use.

Gravitactic mechanism of the microalgae Euglena gracilis (A) in the absence and (B) in the presence of pollutants.

Euglena gracilis is a motile, freshwater, photosynthetic flagellate. Although Euglena is rather tolerant to acidity, it responds rapidly and sensitively to environmental stresses such as heavy metals or inorganic and organic compounds. Typical responses are the inhibition of movement and a change of orientation parameters. Moreover, this organism is very easy to handle and grow, making it a very useful tool for eco-toxicological assessments. One very useful particularity of this organism is gravitactic orientation, which is very sensitive to pollutants. The gravireceptors are impaired by pollutants such as heavy metals and organic or inorganic compounds. Therefore, the presence of such substances is associated with random movement of the cells in the water column. For short-term tests, gravitactic orientation of E. gracilis is very sensitive. Other species such as Paramecium biaurelia (see Paramecium aurelia) also use gravitactic orientation.

Automatic bioassay is possible, using the flagellate Euglena gracilis in a device which measures their motility at different dilutions of the possibly polluted water sample, to determine the EC50 (the concentration of sample which affects 50 percent of organisms) and the G-value (lowest dilution factor at which no-significant toxic effect can be measured).

Macroinvertebrates

Macroinvertebrates are useful and convenient indicators of the ecological health of water bodies and terrestrial ecosystems. They are almost always present, and are easy to sample and identify. This is largely due to the fact that most macro-invertebrates are visible to the naked eye, they typically have a short life-cycle (often the length of a single season) and are generally sedentary. Pre-existing river conditions such as river type and flow will affect macro invertebrate assemblages and so various methods and indices will be appropriate for specific stream types and within specific eco-regions. While some benthic macroinvertebrates are highly tolerant to various types of water pollution, others are not. Changes in population size and species type in specific study regions indicate the physical and chemical state of streams and rivers. Tolerance values are commonly used to assess water pollution and environmental degradation, such as human activities (e.g. selective logging and wildfires) in tropical forests.

An integrative biological assessment of sites in the Custer National Forest, Ashland Ranger District

Benthic indicators for water quality testing

Benthic macroinvertebrates are found within the benthic zone of a stream or river. They consist of aquatic insects, crustaceans, worms and mollusks that live in the vegetation and stream beds of rivers. Macroinvertebrate species can be found in nearly every stream and river, except in some of the world's harshest environments. They also can be found in mostly any size of stream or river, prohibiting only those that dry up within a short timeframe. This makes the beneficial for many studies because they can be found in regions where stream beds are too shallow to support larger species such as fish. Benthic indicators are often used to measure the biological components of fresh water streams and rivers. In general, if the biological functioning of a stream is considered to be in good standing, then it is assumed that the chemical and physical components of the stream are also in good condition. Benthic indicators are the most frequently used water quality test within the United States. While benthic indicators should not be used to track the origins of stressors in rivers and streams, they can provide background on the types of sources that are often associated with the observed stressors.

Global context

In Europe, the Water Framework Directive (WFD) went into effect on October 23, 2000. It requires all EU member states to show that all surface and groundwater bodies are in good status. The WFD requires member states to implement monitoring systems to estimate the integrity of biological stream components for specific sub-surface water categories. This requirement increased the incidence of biometrics applied to ascertain stream health in Europe A remote online biomonitoring system was designed in 2006. It is based on bivalve molluscs and the exchange of real-time data between a remote intelligent device in the field (able to work for more than 1 year without in-situ human intervention) and a data centre designed to capture, process and distribute the web information derived from the data. The technique relates bivalve behaviour, specifically shell gaping activity, to water quality changes. This technology has been successfully used for the assessment of coastal water quality in various countries (France, Spain, Norway, Russia, Svalbard (Ny-Ålesund) and New Caledonia).

In the United States, the Environmental Protection Agency (EPA) published Rapid Bioassessment Protocols, in 1999, based on measuring macroinvertebrates, as well as periphyton and fish for assessment of water quality.

In South Africa, the Southern African Scoring System (SASS) method is based on benthic macroinvertebrates, and is used for the assessment of water quality in South African rivers. The SASS aquatic biomonitoring tool has been refined over the past 30 years and is now on the fifth version (SASS5) in accordance with the ISO/IEC 17025 protocol. The SASS5 method is used by the South African Department of Water Affairs as a standard method for River Health Assessment, which feeds the national River Health Programme and the national Rivers Database.

The imposex phenomenon in the dog conch species of sea snail leads to the abnormal development of a penis in females, but does not cause sterility. Because of this, the species has been suggested as a good indicator of pollution with organic man-made tin compounds in Malaysian ports.

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