3 - Microbes and the Carbon Cycle Flashcards
Biogeochemical cycles
- Movement and transformation of chemical elements and compounds between living organisms, the atmosphere, and the Earth’s crust
- Catalysed by biological or chemical means (or both)
Nutrient cycles
- E.g. Carbon, Nitrogen, Oxygen, Water
- Are tightly coupled (transformations in one cycle may impact other cycles)
Two important types of metabolism
Assimilative and dissimilative
Assimilative
Compound usually incorporated into cell material (it is a nutrient)
Dissimilative
Compound used as an electron acceptor, generates “waste”
Example of coupled cycle
C and N which make up the bulk of living organisms
Rate of primary production (CO2 fixation)
- Controlled by the availability of nitrogen
- N usually low –> process is decreased, if high then increased
Carbon cycle
- Microbes require carbon, energy and electrons (autotrophs get carbon from co2, heterotrophs get carbon from organic matter)
- 3 important chemical forms of carbon and 3 important components of the carbon cycle
- All other nutrient cycles are linked in some way to carbon cycle
3 important chemical forms of carbon
- CO2 (highly mobile, rapidly exchanged)
- Methane (important in climate change)
- Organic matter (important sink, also primary production)
3 important components of the carbon cycle
- Sources
- Sinks
- Reservoirs
Sources
- Generate/release C
- E.g. Heterotrophic microbes releasing CO2 during respiration
Sinks
- Absorb, accumulate and store C for an indefinite period
- E.g. Biomass (plants) that take up CO2 from atmosphere, dead plant material (humus), oceans
Flux
Movement between sources and sinks
Respiration equation
C6H12O6 + 6O2 –cellular respiration–> 6CO2 + 6H2O + ATP
Reservoirs
Store C for long time periods
Examples of reservoirs
- Rocks and sediments make up majority
- Also found in land plants (mainly dead)
- Soil carbon
Major reservoirs that carbon cycles through
- Rocks
- Oceans
- Methane hydrates
- Fossil fuels
- Biosphere
Biological pump
Process by which inorganic carbon is removed from the atmosphere and sequestered in the ocean
Biological pump process
- CO2 converted to organic matter (photosynthesis & primary production, microbial and non microbial)
- Some organic matter sinks (into deep sediments)
- some is recycled back into atmosphere (flux)
Forms carbon is present in
- Most oxidised form is CO2, CO
- Most reduced form is methane and complex organ matter
Primary production
The synthesis of organic compounds from atmospheric or aqueous carbon dioxide
Carbon fixation
- Starts with conversion of CO2 into organic matter or methane
- CO2 is the most rapid means of transfer of carbon (removed from the atmosphere via photosynthesis and returned via respiration)
3 ways CO2 is converted into organic matter
- Oxygenic photosynthesis
- Chemolithoautotrophy
- Anoxygenic photosynthesis
Oxygenic photosynthesis
- By photoplankton
- CO2 +H2O –> CH2O (organic matter) + O2
Chemolithoautotrophy
In both oxic and anoxic environments
How much of Earths’ oxygen is created by phytoplankton through photosynthesis
50-85%
How is CO2 reduced to methane
- Methanogenesis (biological formation of methane)
- Methane is end product (waste)
Methanogenesis
- A major process in anoxic environments (freshwater sediments, intestines of animals)
- Performed by methanogens (archaea) - strict anaerobes
- Methane formed from CO2 + H2 or H2 + acetate or organic matter decomposition
Major mechanism of methane formation
- Reduction of CO2 (TEA) using H2 as an electron donor
- Most methanogens use this process
Minor mechanism of methane formation
- CH4 from organic matter
- Much more complicated and requires help from syntrophs
Methanogenesis equation
CO2 + 4H2 → CH4 + 2H20
Syntrophy
- Cooperation by two or more different microbes to degrade a single substance that neither can degrade alone
- Important for aromatic and aliphatic hydrocarbons
- Also important for fatty acids and alcohols as methanogens cannot
directly catabolise these
What do primary fermenters produce
- H2, CO2, acetate, fatty acids and alcohols
- Methanogens can use acetate and H2, but not fatty acids and alcohols
- Rely on syntroph partner
Partner syntrophs
- Secondary fermenters
- They metabolise products of primary fermenters and produce acetate, H2, CO2
- In doing so, syntrophs supply methanogens with molecules that they can use
- Syntrophs cannot do this alone in pure culture
Why cant syntrophic metabolise products of primary fermenters in a pure culture
- As they rely on methanogens to consume the H2 produced
- H2 consumption makes reactions energetically favourable
- Known as inter-species hydrogen transfer
Example syntrophy
- Butyrate fermentation has positive energy (+ 48.2 kJ)
- Requires energy for reaction to occur
- Will not happen in pure culture (single organism)
- When H2 is consumed by methanotrophs and concentration reduced to extremely low levels,
the reaction is pulled towards the reaction products - Dramatically alters energetics of the reaction and the reaction becomes exergonic G = -18 kJ
Another example of syntrophy
- Ethanol fermentation to acetate coupled with methanogenesis
- Ethanol fermentation (+19.4) is energetically unfavourable so organism will not grow alone
- Methanogenesis (-130.7), energetics Ok. produces H2 that ethanol fermenter consumes
- Coupled reaction has a negative ΔG0’
What is CO2 converted to
Organic molecules or methane
Cycling of organic matter
- Fixed carbon enters a pool of organic matter (can be oxidised back into CO2 via respiration or fermentation)
- Microbial decomposition of organic matter contributes most of the CO2 to the atmosphere
What is bioavailability and degradation of organic matter influenced by
- Type of organic matter
- Oxidation-reduction potential
- Availability of competing nutrients
- Abiotic conditions (pH, temp, o2)
- The microbial community present (metabolic/functional diversity)
Example of complex organic substances
Lignin
Lignin
- Second most abundant organic molecule on earth after cellulose
- Family of complex polymers
- Exceptionally stable plant structural material
- Linked by carbon-carbon and carbon-oxygen (ether) bonds
Lignin decomposition
- Fungi are excellent degraders
- Bacteria have role but are slower
Aerobic lignin decomposition
- Mostly Actinomycetes (filamentous soil microbes)
- Secrete hydrolytic enzymes e.g. lignin peroxidase, laccase
- Degrade lignin by oxidative depolymerisation
- Process requires oxygen
Anaerobic lignin degradation
- Very few microbes capable of this
- Occurs very slowly
- This is why lignified materials accumulate in anoxic environments
Methane produced in sediments
Can be oxidised aerobically or anaerobically to CO2
Aerobic oxidation of methane produced in sediments
By methanotrophic proteobacteria after diffusing to oxic areas
Anaerobic oxidation of methane produced in sediments
By anaerobic methane oxidation or reverse methanogenesis
Human impacts on carbon cycle
- Current era called anthropocene
- Greatest impacts from burning fossil fuels, deforestation and land use changes leading to increase atmospheric CO2
Climate change and global warming
- Atmospheric CO2 levels have increased by ~40% since the industrial revolution
- CO2 is <0.5% of atmosphere but very effective at trapping infrared radiation emitted by earth
- Oceans have absorbed substantial CO, slowing the effect
Ocean acidification
- Dissolution of CO2 in seawater produces carbonic acid (H2CO3)
- Spontaneously dissociates into weak acids bicarbonate and carbonate
- Extra H ions leads to ocean acidification
Issue with excess H ions
- Pushes equation towards bicarbonate which reduces available carbonate ions
- Carbonate ions are important for combining with calcium carbonate
Why is calcium carbonate important
- For shells and skeletons of marine animals
- Coccolithophores (primary producers)
- Corals
- Shellfish
Methane hydrates
- Methane present in atmosphere at lower levels than Co2 but 20 times more effective at trapping heat
- Slow release of CH4 from sea floor hydrates feeds anaerobic methane-oxidising archaea and animal communities containing aerobic, methane-oxidising
symbionts
Human impact on methane hydrates
- Vast quantities of CH4 are trapped underground beneath permafrost or in marine sediments
- Mostly derived from microbial activity (convert organic matter to methane)
- Highly dynamic, release and absorb methane in response to changed conditions
Fate of methane
Could be oxidised to CO2 before reaching atmosphere
