Introduction to Symbiogenesis

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Symbiogenesis is the theory that describes speciation that arises by symbiosis. Symbiosis is the beneficial association of two or more different species of organisms. For convenience, this discussion will just consider the cases of the symbiosis of two species with the understanding that situations exist in nature that involve more than two species. This association differs in intimacy from simply living in close proximity where each species derives some benefit from the other to becoming so closely intertwined that neither species can survive for long without the other except through artificial means. A cleaner shrimp that pick tiny food scraps from the mouth of a moray eel is one example of a very loose symbiotic relationship. The shrimp gains a safe an easy source of food and the eel gains good oral hygiene. Some species of lichen are the archetypal representatives of a very intimate association between an alga (or cyanobacterium) and fungus where if the two are separated they would only survive for a short time in the wild.

This theory originated in tsarist Russia in the latter half of the 19^th century and early part of the 20^th century. Andrey Famintsyn did much of the seminal research and was one of the first to enunciate the theory. Konstantin Merezhkovsky first publish the theory in 1905 in his paper, "The Nature and Origins of Chromatophores in the Plant Kingdom." Famintsyn published his version two years later also on the topic of chromatophores. Although the two men communicated frequently prior to the publication of either paper and Merezhkovsky referred to experimental work done by Famintsyn, the two men appear to have arrived independently at the conclusion that chromatophores in plants were the result of symbiotic incorporation of bacteria into another species.

Russia's plentiful and varied lichen biota provided a natural laboratory for the investigation of symbiogenesis. Gelatinous lichens in particular were found to range from loose associations to symbiosis so intimate that it was difficult to separate the alga and fungus as separate organisms. In the process of cataloging and studying the rich variety of lichens, scientists were able to see the progression from two species into a new third species. In addition Famintsyn's and Merezhkovsky's studies of plastids, such as chloroplasts, in plant cells when compared to certain very simple species of cyanobacteria provided direct evidence that these plastids had once lived as separate species and had been symbiotically incorporated into another species to give rise to a third species. Russian botanists in particular continued to investigate and refine this theory into the 1960s and 1970s. However, their papers were only published in Russian and so this theory was unknown in Western Europe and the USA.

Lynn Margulis rediscovered this important mechanisms of how evolution can proceed. It augments but does not replace the accumulation of mutations as a mechanism of speciation. Margulis is a microbiologist and developed her own hypothesis of symbiogenesis - speciation through symbiosis - based on her research into the structure of protists and protoctists where she was able to establish that chloroplasts, mitochondria, flagella, and other cell organelles had once been independently living organisms that had been incorporated by another to produce new species. Her work in many ways paralleled the work of the two Russians, Famintsyn and Merezhkovsky. It was only after she had published her work that she became aware of the earlier work in Russia.

In the case of simple organisms, protoctists in particular, symbiogenesis is well established as a scientific fact. The chlorophyllic plastids in alga, for example, were once independent cyanobacteria. The nuclei of eukaryotes have been demonstrated to have arisen from flagellant bacteria as have cilia (see Acquiring Genomes, A Theory of the Origins of Species by Lynn Margulis and Dorian Sagan for more information). This mechanism of speciation is in addition to the gradual accumulation of mutations that is one of the first proposed mechanisms. Natural selection works on both of these sources of variation, symbiosis and mutation, to favor the individuals and populations that adapt best to the environment in which they live.

The fossil record actually supports this and similar phenomena as important mechanisms of evolution. The punctuated equilibrium theory developed by Stephen Jay Gould and Niles Eldredge is more consistent with symbiogenesis than phyletic gradualism, in my opinion. This theory describes long periods of stasis (little change in the biota as evidenced by fossils) interrupted by periods of relatively rapid speciation (think of the demise of nonavian dinosaurs and the adaptive radiation of mammals in the few million years around the Cretaceous-Tertiary boundary demarcated by the iridium-rich layer signifying the asteroid impact at about 65 mya (million years ago).

In addition to the research concerning microbes, there are well documented cases of underwater slugs (Elysia viridis, for example). The ancestors of this newly evolved slug are gray animals that eat algae. In some of them, instead of being digested, the alga became part of the animal - a plant animal hybrid in fact. Elysia viridis are always green and they do not eat as adults. They derive all their nourishment from the symbiotic algae and so spend their time sunbathing instead of grazing on algae like their gray ancestors. Based on studies of the genomes of the green and gray slugs, it is clear that the green slugs evolved from the gray slugs by symbiotically incorporating the same algae that serve as the fodder of the gray slugs. This is a very clear case of evolution by symbiogenesis.

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What are Safe Radiation Limits?

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The short answer is that there is no absolute level that can be certified as safe.  What is known is that the average annual dose that a person receives during the year from naturally occurring radiation, mostly from airborne radon with smaller contributions from cosmic rays and internal body radiation, is about 300 millirems at sea level.  In Denver, one mile above sea level, the annual background radiation is about 400 millirem.  The 300 millirems is roughly equivalent to 30 chest x-rays or one-third of a whole-body CT scan.  Radiation dosage is accumulative throughout one's life.  So if you are 50 years old and have received no radiation other than average background radiation, your life-time dose would be 50 x 300 mrems = 15,000 mrems or 15 rems total.  The fact that the cancer rate is lower in Denver than the average in the rest of the USA (Bulletin of the Atomic Scientist, Letters, Bertram Wolfe, President of the American Nuclear Society, San Jose, Ca, November 1986, p55) indicates that it is probably reasonable to assume that this level of radiation is relatively safe, that is, the number of deaths due to background radiation is likely small.


At the other extreme, based on data from the aftermath of the atomic bombs that were dropped on Nagasaki and Hiroshima, 600,000 millirems is 100% fatal.  Anyone receiving this dose of ionizing radiation will die of radiation sickness.  50% of the people exposed to a total dose of 450,000 millirems died.  You can determine your approximate annual exposure at this handy website: Radiation Dose Chart.


Each nation sets what the limits it considers safe for its citizens.  For the USA, dosage limits are as follows:
  Astronauts:  25,000 millirems per mission
  Occupational, adult:  5,000 millirems per year (reduced from 15,000 mrems/yr. in 
                                 1957, reduced from 25,000 mrems/yr. in 1950)
  Lifetime exposure:  1,000 mrem/yr. multiplied by ones age in years
  Occupational, minor:  500 mrem/yr.
  Fetus of pregnant worker:  500 mrems total dose; 
                   recommended:  maximum of 50 mrem/month. (limit created in 1994)


  Note:  All these levels are in addition to the background radiation one receives.


So how much additional radiation is a person living on the west coast of the USA likely to receive?  Most likely, one will receive about 300 microrems per year, roughly one-thousandth of the normal background radiation exposure coastal residents experience.  So fear not for the radiation Americans will receive from Japan; hope for the best for the Japanese living within 50 miles of the Fukushima nuclear plant.  The radiation levels are continuing to increase.  Most recently, traces of plutonium were found in soil samples around the plant site.  This can only occur when the shielding around the core has been breached.  Reports indicate that this nuclear situation is approaching the severity of Chernobyl.
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Conversion of Radiation Units, a Follow up to the 15 March 2011 Post

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The media have begun using a variety of units to measure radiation: sieverts, becquerels, millirems, etc.  How many people know all these units and whether 10 sieverts/hour is more or less dangerous than 100 becquerels/hr.?  To help with the confusion, here is a conversion table that may be of use:

Here is a conversion table for various other units of measure:

Conversion Factors

To convert from	To	Multiply by
Curies (Ci)	becquerels (Bq)	3.7 x 1010
millicuries (mCi)	megabecquerels (MBq)	37
microcuries (µCi)	megabecquerels (MBq)	0.037
millirads (mrad)	milligrays (mGy)	0.01
millirems (mrem)	microsieverts (µSv)	10
milliroentgens (mR)	microcoulombs/kilogram (µC/kg)	0.258
becquerels (Bq)	curies (Ci)	2.7 x 10-11
megabecquerels (MBq)	millicuries (mCi)	0.027
megabecquerels (MBq)	microcuries (µCi)	27
milligrays (mGy)	millirads (mrad)	100
microsieverts (µSv)	millrems (mrem)	0.1
microcoulombs/kilogram (µC/kg)	milliroentgens (mR)	3.88

From: http://orise.orau.gov/reacts/guide/measure.htm#Conversions

Radiation Measurements

	Radioactivity	Absorbed Dose	Dose Equivalent	Exposure
Common Units	curie (Ci)	rad	rem	roentgen (R)
SI Units	becquerel (Bq)	gray (Gy)	sievert (Sv)	coulomb/kilogram (C/kg)

From: http://orise.orau.gov/reacts/guide/measure.htm#Conversions

So the answer to the question is neither.  A becquerel (Bq) is a measure of the radioactivity in counts per second of a given sample where one count represents the decay of one nucleus. It is an SI (Systeme Internationale) unit of measure related to the older, non-SI unit of Curies (Ci).  One Ci is equal to the activity of one gram of radium-226.  A sample must be normalized to the atomic mass of the isotope being measured.

The absorbed dose is what is actually important when determining if a radiation level is of concern or not.  But the radioactivity is the number that is most easily measured.  Different types of ionizing radiation (radiation that will change a neutral atom to an ion by removing an electron - similar to creating a free radical that can then damage cells and DNA and such) are absorbed at different levels.  Absorbed Dose is measured in joules per kilogram (grays [Gy]), where joules measure the energy absorbed from the radiation and kilograms is the measure of the mass of the absorbing medium (a person's tissue).  The effective dose is determined by multiplying a conversion factor,  a weighted average of absorption of different organs (heart, liver, skin, e.g.), times the amount of radiation exposure.  It is not a simple calculation nor is it exact.

So when the reporter talks about there being so many becquerels of radiation, he is reporting how many counts are being recorded.  It is an indication of how much radioactive material is present.  By also determining what isotopes of what elements are present, one can evaluate the relative risk to humans and other living things.

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Relative Risk of Power Generation Methods

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The nuclear disaster in Japan has focused the world's attention on the dangers of nuclear power generation.  Germany has temporarily shut down seven reactors, China has suspended approval of new facilities, and the usual hew and cry has arisen from the anti-nuclear energy crowd in the USA.  No one denies that when a nuclear reactor malfunctions that the risk of a large number of deaths is possible and that it is essential to assure that proper precautions be taken to prevent such disasters.  However, when assessing the dangers associated with nuclear power to determine if pursuing this form of power generation is worthwhile, one must compare the risks to those associated with other forms of power production such as coal, oil, natural gas, and hydroelectric.  One must consider not only the extent of a possible tragedy but also the chance of such a tragedy occurring to determine the full impact of using any particular source.  One must also consider the full life cycle of the power production rocess for each fuel type, fuel extraction, plant building, fuel transport and processing, power plant productive life time, decommissioning, and environmental impact of all of these phases.

According to a study in China, based on experiences in their study, coal-fired energy production has caused twelve times the number of deaths per annual GW as the nuclear energy chain.  By implementing known improvements that are needed, this ratio could be reduced to a 4X disadvantage for coal.  However, this does not include any nuclear accidents, since none have occurred in China to date.  This report also found that the radiation from coal, totaled over the entire product cycle, is nearly twice as high for coal as for nuclear power; again, this does not include estimates for nuclear accidents such as occurred at Three Mile Island, Chernobyl, or Fukushima.  As another means of comparison, a study by the Clean Air Task Force, attributes 13,200 deaths to coal plant emissions annually in the USA, this does not include deaths from any other portion of the energy production chain.

In 1975 China, a one-hundred-year flood event caused 30 dams built for hydroelectric power to fail, drowning at least 230,000 people.

The point of this information is to highlight the fact that no means of energy production is without risks.  Nuclear power generation seems riskier for two reasons: when there is an accident, it tends to be a disaster, killing and injuring a large number of people at one time and causing environmental damage over a large area; plus it is relatively new and there are many unknowns about it.  Nuclear disasters are blatantly obvious and the news media takes advantage of people's fear to raise their ratings and make the consequences seem even more dire than they already are.  Predictably people react in fear and want to stay with power sources they know.  Coal-powered plants are "silent" killers.  Except for the occasional mining disaster, one doesn't hear about the 13,000 annual deaths from breathing particulates emitted by coal-fired plants because they don't happen in bunches nor is it always obvious that coal is the culprit except to a doctor who performs an autopsy and sees the person's black lungs or measures radioactive isotopes charactistic of coal in a person's body.

Whether nuclear power should remain an option should be based on fact-based reasoning and decision making.  One must compare all costs associated with each means of power production from acquisition of the fuel until the plant is decomissioned and torn down or buried.  Not only the direct human impact, some of which has been highlighted here, but also the environmental impact to teh Earth as a whole and the other organisms that share this p[lanet with us.  Currently, fossil-fuel powered plants do not pay for the effect they have on the environment, there is little social cost assessed on carbon emission or the emission of other pollutants.  To make economically sound decisions for the long term, these costs must be included.  Making such an assessment is beyond the scope of today's post but will be the subject of one in the future.


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Some Radiation Information

Radiation Primer

Some revelant definitions and information:
A Sievert is a measure of radiation exposure, a millisievert is one-thousandth of a sievert, and a microsievert is one-millionth of a sievert.

Isotope: An atom with a particular neutron count in its nucleus. Whereas all the atoms of a given element will have the same number of protons present in the nucleus, the number of neutrons may vary. For example, hydrogen has one proton in its nucleus. It has three different isotopes: protium (most common form containing no neutrons), deuterium (one neutron), and tritium (two neutrons).

Iodine-131 and Cesium-137 are the most dangerous contaminants of concern so far. Iodine-131 is readily absorbed by the thyroid gland and can lead to thyroid cancer. By taking potassium iodide, one can saturate one's thyroid and effectively prevent the uptake. Since the half-life (the time it takes for half of the material to decay) of I-131 is only 8 days, this is an effective and practical method of reducing one's risk. In the case of Ce-137, however, the half-life is about 30 years and there is no effective means of reduction of risk other than physically preventing exposure. Cesium is incorporated in one's bones, primarily and can lead to bone cancer and leukemia.
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