Extreme Environments III: radiation resistance

Question 1

natural radiation and sources

Answer 1

• Electromagnetic radiation – from the sun and other stars,
  bioluminescence, fluorescence of minerals, aurora borealis, aurora australialis, anthropogenic light-sources:
• visible light [blue/violet end can be damaging to Life – Ward (1982) and Frankland (1887) given quite elegant demonstrations showing blue light is more toxic to Bacillus spp. than UV light is]
• ultraviolet light (lower wavelength photons than violet light)
• infrared light (higher wavelength photons than red light)
• microwaves (fairly high wavelength photons)
• radiowaves (very high wavelength photons)
• gamma rays (very low wavelength photons)
• Particulate radiation – from unstable atoms:
• alpha particles (helium nuclei, 2 protons, 2 neutrons, really heavy, stopped by paper, range is <10 cm in air) – the most damaging, but least penetrative.
• beta particles (fast-moving electrons, stopped by thin metals or some organs, range is 20 cm to 20 m in air)
  Keep in mind that x-rays are from Bremsstrahlung and x-ray tubes and share same wavelength range as gamma rays nowadays and differ only in source

Question 2

radiation damage

Answer 2

• most damage is to DNA and RNA.
• proteins are thus impacted by mutagenesis – low/medium doses of radiation cause this and thus long-term impacts.
• high doses lyse DNA – look up radiation sickness in humans.
  Radiation resistant Bacteria and Archaea are simply very good at DNA repair!
• UV-B (lower wavelength UV) causes pyrimidine dimers (TT,
  UU, AA) in adjacent (not opposite!) pyrimidine bases in NAs. The formed ‘double base’ can’t be reverted and won’t pair properly –kinks the DNA backbone and replication etc don’t work. If proteins ARE made, they are mistranslated and may not function.
• UV-A (longer wavelength UV) makes oxygen free radicals that oxidise DNA bases – these no longer pair/function.

Question 3

More on radiation damage

Answer 3

• ionising radiation (alpha, beta, gamma, x) can destroy the DNA backbone, lysing DNA.
• think about how an electron or helium nucleus might interact with the DNA strand.
• DNA lyses into nucleotides
• oxygen free-radicals can also lyse proteins and are formed by ionising radiation – forms individual amino acids.
• amino acid side-chains can also be oxidised in this way, as can lipid fatty acyl tails.
• alpha and beta particles can destroy ester linkages and peptide bonds.

Question 4

The dose alone makes the poison

Answer 4

• dose is often measured in the Sievert (Sv):
• 0.4 µSv/h is received by everyone on Earth from radioactive elements on Earth
• 0.034 µSv/h ditto from cosmic rays
• 1.1 Sv/h from a routine chest x-ray (but 0.2 s exposure – so what is the actual dose…?)
• 0.4 µSv total dose from a London-New York flight
• 20 mSv annual dose-limit for Certified Radiation Worker in the UK[based on a working-year, if you were exposed every hour you’re at work, that’s 12 µSv/h]
• 1.0 Sv/h received in immediate area of Fukushima when it happened.
• typical dose per year in the UK is 3.8 mSv – higher in the South West (7 mSv) owing to uranium in the bedrock.
• average UK lifetime dose from natural radiation is 0.29 Sv.
• 5,000 Sv will kill a human on the spot.
• 60,000 Sv needed to kill Escherichia coli instantly.

Question 5

example bugs

Answer 5

Deinococcus radiodurans from the Deinococci of the Bacteria.
Boring generalist heterotroph. Aerobic. Extremely resistant to desiccation, radiation and molecular halogens (Br2
and Cl2).
Fatal dose: 5,000,000 Sv (1,000 times more than Homo sapiens L.!)
Thermococcus gammatolerans from the Thermococci of the
Archaea. Anaerobic fermenter. Extremely resistant to radiation and desiccation.
Fatal dose: 30,000,000 Sv (6,000 times more than Homo sapiens L.!)

Question 6

adaptations to radiation

Answer 6

• highly efficient DNA-repair systems, which in turn have to be radiation-resistant proteins. DNA is continually proof-read.
• D. radiodurans lives as tetrads to allow DNA exchange – each cell has 4 genome copies and can copy DNA from 3 other cells to repair its damaged DNA.
• spreading out of gDNA into chromids and multiple
  chromosomes – redundancy in case of damage.
• DNA is packed into toroids to maintain structure in event of dimer formation.
• high intracellular Mn(II) concentration – acts as an antioxidant, removing oxygen radicals, helps DNA repair enzymes stay functional, can be oxidised into MnO2 which deposits in and around cell – could act as ‘shielding’?

Question 7

but why?

Answer 7

• natural radiation never approaches the doses
  these organisms survive.
• working hypothesis is that desiccation adaptations also work for radiation:
  Battista et al. (2001) Cryobiol. 43: 133.

Question 8

what about radiometals?

Answer 8

• some metals are naturally radioactive owing to large, unstable nuclei e.g. uranium, plutonium, americium etc – mainly lower row of the f-block.
• generally, if metals get into a cell, they are just pumped back out but there are some adaptations that can chelate them or reduce them on the outside of the cell to ensure they don’t get in at all.
• look at the work of Prof Lynne E Macaskie’s group over the last 35+ years (U. Birmingham, before that, U. Oxford) particularly:
• Macaskie et al. (2006) Hydrometallurgy 104: 483.
• any Macaskie and Yong papers.
• all of it done in Serratia sp. N14 which they called ‘Citrobacter sp. N14’ in the earlier papers.