L11 : Life in High Toxicity Flashcards
Name earth, space and anthropogenic sources of ionising radiation?
Earth:
- Radioactive decay of mantle/crust, minerals
- high altitude
Space:
- Solar radiation
- Cosmic radiation
Anthropogenic:
- Healthcare applications
- Nuclear power and waste, accidents
Explain why radioresistant lineages are unclear?
Scattered distribution of radioresistant species
- Ability lost gradually from lineages through natural selection/genetic drift
- Ability evolved independently through horizontal gene/convergent evolution
What is the link between radiation and origin or life?
Much higher background radiation when life emerged
Early life must have had considerable radiation resistance and used energy from ionising radiation
How does direct dionising radiation damage the cell (DNA, protein, lipid)?
DNA:
- Mutations
- Direct breaks
- Loss of nitrogenous bases
- Cell cycle arrest
Proteins:
- Denaturation
- Misfolding
- Carbonylation
Lipids:
- Oxidation
- Membrane disruption
- Ion leakage
How does indirect ionising radiation damage the cell?
- Organelle damage
- Mitochondrial dysfunction (ATP depletion)
- Dehydration (H2O becomes ROS)
- Inflammation (cytokines, chemokines released
What is the consequences of ionising radiation on cells?
Apoptis or necrosis
What are 3 examples of reactive oxygen species (ROS)?
Superoxide (O2-)
Hydrogen peroxide (H2O2)
Hydroxyl free radicals (OH-)
What are 6 DNA adaptations for high radiation?
- Robust DNA repair mechanisms
- Gene redundancy
- Formation of highly compact, stable genome
- Use of pigmentation
- DNA dimer prevention
- Robust cell cycle checkpoint
What are proteins and components involved in the cellular stress response to high radiation?
- Extracellular polymeric substances (EPSs)
- Retain H2O, form biofilms - Outer membrane/LPSs
- Extra protection, retain H2O - Aquaporins
- Allow H2O into cell - Thick membrane, modified lipids (unsaturated)
- High levels antioxidants (carotenoids, CAT, SOD)
- Osmolytes
- Protect from osmotic stress, dehydration - High levels of dark pigments
- Protect cell, antioxidants - Mn(II)-phosphate complexes
- Protect from ROS, enhance recovery - Hydrolases, ATP-dependent proteases]
- Removal of damaged DNA, proteins, organelles - Chaperones (HSPs)
- Protect from denaturation/aggregation
Example of radioresistant archaea?
Euryarchaeota
(Hyperthermophilic - thermococcus gammatolerans)
Example of radioresistant bacteria?
Deinococcus
(Deinococcus radiodurans)
Bacteroidetes
(Hymenobacter xinjiangensis)
Example of radioresistant eukaryotes?
Fungi
(Cryptococcus neoformans, cladosporium)
Algae
(Hematococcus)
Tardigrades
Deinococcus radiodurans
C
Chernobyl fungi variety
Cross contamination concerns
What are xenobiotics?
Substances that are foreign to an organisms normal chemistry
How can xenobiotics cause damage?
Dose dependent
Heavy metals compete with normal components
- Bind to and damage lipids, proteins, DNA
What are metalophiles?
Organisms that have an affinity for or thrive in the presence of xenobiotics and metals
Examples of toxic environments for metalophiles?
- Mining sites
- Metal processing plants
- Industrial sewage
- Earth crust
- Hydrothermal vent systems
- Volcanic areas
- Healthcare and cosmetics
Why do cell membrane adaptations help with high toxicity?
- Increased permeability
- Maintain osmotic gradient
- Expel heavy metals
What are cell membrane adaptations to high toxicity?
- More unsaturated phospholipids - increase permeability
- FAs with either bonds and isopropenoid chains - aid nutrient transport
- More phosphate lipid head groups - bind +ve HM ions
- More free -COOH groups - bind +ve HM ions
- Metal specific ion channels (eg. ArsB/C)
- Non specific ion channels (porins, voltage, pH etc.)
- Efflux pumps (eg. ABC transporters)
- Proton pumps - power transporters
Why do cell environment stress responses help with high toxicity?
- Remove excess metals
- Neutralise toxicity
- Prevent cellular damage
What are cell environment stress responses to high toxicity?
- Extracellular sequestration/ chelation in periplasm
- Precipitation/ mineralisation - transform HM into solid mineral forms and neutralise toxicity
- Antioxidant production (eg. Glutathione, SOD, thioredoxins)
- Intracellular chelation (citrate, histidine)
- Metal redistribution through compartmentalisation - minimise HM interference
- Chaperones (HSPs) - regulate adaptation to HM stress
- Sequestration - HM binding with metallothioneins, phytochelatins etc.
- Vacuolar sequestration - HM storage
What are adaptations against protein misfolding in high toxicity?
- Cys and His - strong affinity for HMs
- Asp and Glu acid - COOH bind +ve metal ions
- Zinc finger motifs - stabilised by Zn+, allow cellular interaction under metal stress conditions
- Alpha helices to tolerate mild unfolding
- Chaperones (HSPs) to mediate folding
- Cysteine - contain thiol (-SH) that binds HMs
- Helix-turn-helix motifs - sense and bind HMs
What are adaptations against protein denaturation and aggregation in high toxicity?
- Salt bridges
- Metal cofactors (eg. Rubredoxins bind Fe+)
What are adaptations against loss of enzyme activity in high toxicity?
- Enzyme redundancy - compensation
- Evolution of specific binding motifs for metal ions
What are DNA adaptations against high toxicity?
Must withstand oxidative stress from high HM presence
- Robust DNA repair mechanisms
- Metal resistance gene expression
- Dps (DNA protection during starvation) proteins - bind and shield DNA
What are energy utilisation adaptations in high toxicity?
- Dissimilatory metal reduction (DMR) - use of HMs as terminal electron acceptors in respiratory chain
- Assimilation - uptake and integration of HMs into cellular components for metabolic use
- Metabolism adjustment - prioritise ATP production for active transport
- Use alternative energy pathways (glycolysis even in presence of o2)
- Enhanced redox recycling of HMs - to convert into useable energy
- Use anaerobic pathways to cope with o2 lack
- Use phosphatase hydrolysis - generates energy for HM sequestration, chelation, transformation
- Some use symbiotic relationships
Examples of archaea with resistance to heavy metals?
Sulfolobus acidocalarius - sulfur-rich geothermal springs
Ferroplasma acidarmanus - acidic mine drainage
Examples of bacteria with resistance to heavy metals?
Cupriavidus metallidurans - metal-contaminated soils and mines
Acidithiobacillus ferrooxidans - thrives in high Fe2+ and sulfur (mines)
Examples of eukaryotes with resistance to heavy metals?
Fungi (Aspergillus niger) - contaminated soils
Common sunflower (Helianthus annuus) - metal contaminated soils
Earths toxic environments as astrobiological models
