2 - Metabolic & Functional Diversity Flashcards
Metabolic diversity
Range of different metabolic strategies that microbes have to obtain energy
Phylogenetic diversity
- Evolutionary relationships between organisms
- Genetic and genome diversity of evolutionary lineages
- Usually based on rRNA gene phylogeny
Important processes underpinned by microbial metabolism
- Primary production (photosynthesis)
- Carbon capture
- Decomposition
- Nitrogen fixation
Microbial metabolism
The means by which a microbe obtains the energy and nutrients it needs to live and reproduce (cant make energy from nothing, needs to be captured or conserved
Three critical components of metabolism
- Carbon (auto, hetero)
- Energy (photo, chemo)
- Electrons (litho, organo
Autotrophs
CO2 principal carbon source
Heterotrophs
Reduced, preformed, organic molecules from other organisms
Phototrophs
Light energy source
Chemotrophs
Oxidation of organic or inorganic compounds
Lithotrophs
Reduced inorganic molecules electron source
Organotrophs
Organic molecules electron source
Five major nutritional types of microorganisms
- Photolithoautotroph
- Photoorganoheterotroph
- Chemolithoautotroph
- Chemolithoheterotroph
- Chemoorganoheterotroph
Chemical work
Synthesis of complex molecules
Transport work
Uptake of nutrients, elimination of waste
Mechanical work
Motility, movement inside cell (e.g. chromosomes during cell division)
Energy obtained from light, organic or inorganic molecules
Must be converted to useful form (most often ATP)
ATP
- High energy molecule
- Hydrolysis to ADP strongly exergonic
Oxidation-reduction (Redox) reactions
- Electrons move from an electron donor to an electron acceptor
- Molecules that can donate lots of electrons are energy rich
- Pairs with more negative potential will spontaneously donate electrons to pairs with more positive potential
Electron donor
- Loses energy
- Is oxidised
Electron acceptor
- Gains energy (more energy rich)
- Is reduced
Standard reduction potential
Measures the tendency of the donor to lose electrons (one half of reaction)
Free energy
- Energy available to do work
- Change in free energy expressed as ΔG0’
negative ΔG0’
Reaction will process and release free energy (exergonic)
positive ΔG0’
Reaction requires energy to proceed (endergonic)
Two ways Chemoorganoheterotrophs capture energy and electrons
- Respiration
- Fermentation
Chemoorganoheterotrophs
- Chemo: energy from chemicals (not light)
- Organo: electrons from organic molecules
- Hetero: carbon from organic molecules
Respiration
- Electrons released by oxidation of energy source (e.g. NAD and FAD) are accepted by carriers
- These are now reduced (NADH. and FADH2) and donate electrons to the electron transport chain (ETC)
Fermentation
- Does not have electron transport chain
- Electron acceptor is endogenous
- Almost all ATP is synthesised by substrate level phosphorylation (SLP)
- Generates less energy
ETC
- Electrons pass through ETC to the terminal electron acceptor (TEA)
- Generates proton motive force (PMF)
- Used to synthesise ATP from ADP + phosphate (via oxidative phosphorylation)
Aerobic respiration
- TEA is oxygen
- good energy yield
Anaerobic respiration
- TEA varies (but is not O2)
- Need to process a lot of substrate (NO3) to get good energy
- e.g. organic molecules fumarate and humic acids
Bacteria living in nutrient and oxygen poor environment
- Catabolise molecules and use products as building blocks for essential cell components (uses lots of energy)
- Use aerobic respiration until O2 is consumed, then switch to anaerobic with alternative TEA
- If no alternative TEA then fermentation
- Slow growth
Three examples of metabolic diversity
- Phototrophy
- Chemolithotrophy
- Fermentations
Phototrophy
- Use of light energy
- Usually also autotrophs (carbon from co2)
Photosynthesis
- Conversion of light to chemical energy
- Requires chlorophylls or bacteriochlorophylls
Photoautotrophy reactions that run in parallel
- Light reactions (ATP generation)
- Dark reactions (co2 reduction)
- May be oxygenic or anoxygenic
Light reactions
Energy from light captured and converted to chemical energy
Dark reactions
Use ATP and reducing power to fix Co2 and synthesise cell components
Oxygenic
- O2 produced
- Cyanobacteria
Anoxygenic
- No O2
- Purple and green bacteria
Great oxygenation event (GOE)
Point when Cyanobacteria made Earth’s atmosphere oxygenic
Main type of chlorophyll of oxygenic phototrophs e.g. cyanobacteria
- Chlorophyll a
- Absorbs red and blue light and transmits green
Bacteriochlorophylls
- Purple and green phototrophic bacteria produce one or more
- Bacteriochlorophyll a is present in most purple
bacteria
Structure of Chlorophyll and bacteriochlorophyll in oxygenic phototrophs and purple anoxygenic phototrophs
- Both Chl and Bchl are attached to proteins, housed within membranes to form photocomplexes (not free within cell)
- Contain 50 – 300 Chl /Bchl molecules
- A few of these photocomplexes are named reaction centres (where ATP generation occurs)
Antenna pigments
- Light harvesting Chl/Bchl molecules surround the RC
- Absorb light and funnel some towards the reaction centre
Site of photosynthesis in eukaryotes
Within chloroplasts
Site of photosynthesis in prokaryotes
Chromatophores, lamellae, thylakoids, chlorosomes
Chlorophyll absorption
- Only absorb narrow range of light (other light is “wasted”)
- Additional accessory pigments assist in absorbing wasted light energy
Accessory pigments
- Carotenoids and phycobilins
- Absorb light in blue-green to yellow range
- light energy then transferred to chlorophyll
- Enable photosynthesis to occur over a broader range of light wavelengths
- Also quench toxic oxygen species produced by bright light
Carotenoids
- Most widespread accessory pigments
- Tend to mask colour of Bchls, thus responsible for the colours seen in anoxygenic phototrophs
Phycobiliproteins
- Present in cyanobacteria
- Main light-harvesting systems
- Assembled into phyobilisomes
Chemolithotrophy
- Derive energy from oxidation of inorganic compounds
- Most are also autotrophs (some are mixotrophs)
- Can utilise wide range of inorganic compounds as electron donors
Mixotrophs
- Require organic compound for carbon
- Chemolithoheterotrophs
Which generates more energy
Glucose oxidised completely to Co2 compared to energy derived from inorganic compounds
Ecological impacts of oxidation of large quantities of substrate
Contributes to global nitrogen, sulphur and iron biogeochemical cycles
Why does energy yield from oxidation vary widely
Depends on redox pair
Autotrophs also need reducing power (NADPH) to fix co2
- Some substrates have more positive reduction
potentials than NAD(P)+ /NAD(P)H pair (cannot donate electrons directly to NAD(P)* - Instead use reverse electron flow to make NADH
When does fermentation occur
- Organisms are incapable of respiration (lack ETCs)
- Organisms unable to respire due to conditions (TEAs for AnO2 respiration are absent)
- Choose not to respire (synthesis of ETC components repressed)
During fermentation
- ATP is synthesised by substrate level phosphorylation
- NAHD must be oxidised back to NAD+ despite lack of ETC
Fermentations
- Many kinds (lactic acid most common)
- Pathways named after acid or alcohol produced
Lactic acid
- Pyruvate reduced to lactate
- Two groups (homolactic and heterolactic)
Homolactic
Use Embden–Meyerhof pathway and reduce almost all pyruvate to lactate
Heterolactic
- Use pentose phosphate pathway and make lactic acid, CO2, ethanol
Three possible explanations for distantly related bacteria sharing same traits
- Gene loss (trait is present in ancestor but is lost by some descendants)
- Convergent evolution (trait evolves independently
- Horizontal gene transfer
Further divisions of functional diversity
- Physiological diversity
- Ecological diversity
- Morphological diversity
Physiological diversity
- Relates to functions and activities
- Usually described in terms of microbial metabolism and cellular biochemistry
Ecological diversity
relationship between organisms and their environments
Morphological diversity
Apperance of cells
Where did photosynthesis first arise
Anoxygenic phototrophs
Differences between the six bacterial phyla anoxygenic photosynthesis is present in
- Extensive metabolic diversity present amongst phyla
- Found in a wide range of habitats
- Horizontal gene transfer thought to play an important role in their evolution
Only bacteria capable of oxygenic photosynthesis
Cyanobacteria
Examples of Anoxygenic phototrophs
- Purple sulfur bacteria
- Purple non sulfur bacteria
- Green sulfur bacteria
- Green non sulfur bacteria
Purple sulfur bacteria
- Found in illuminated, anoxic areas where H2
S is present (e.g. lakes) - Colour comes from carotenoids
- Use H2S as electron donor (oxidise H2S to S0, deposited as sulphur granules in cell)
Purple non sulfur bacteria
- Not always purple (may be red or orange)
- Very metabolically diverse
- Usually photoheterotrophs
Green sulfur bacteria
- Low metabolic diversity
- Phylogenetically related
- Non-motile, strict anaerobes
- Oxidise H2S to S0 then to SO42-
- S0 deposited outside the cell
S0
Elemental sulphur
Green non sulfur bacteria
- Many are filamentous, capable of gliding motility
- Some form thick microbial mats
- Grow best as photoheterotrophs, but capable of photoautotrophy
