Test III Flashcards
1.b Describe the three stages of biofilm development and at least four benefits that biofilms provide for the inhabitants. .
- Biofilms are assemblages of bacterial cells attached to a surface and enclosed in an adhesive matrix secreted by the cells.
- A matrix is a mixture of polysaccharides, proteins and nucleic acids.
- They typically contain porous layers.
3 Stages of Biofilm development:
Attachment
-Adhesion of a few motile cells to a suitable solid surface
-Starts with random collision of cells with a surface, resulting in attachment.
-Attachment achieved through appendages (e.g. pili, flagella) or cell surface proteins.
-Attachment triggers the expression of biofilm specific genes.
→c-di-GMP signals switch to biofilm growth
→Production of extracellular polysaccharides for matrix formation
→Loss of motility
Colonization
- Intercellular communication, growth and polysaccharide formation
- Intercellular communication also aid in biofilm formation, via concentrations of signaling molecules
Development
- More growth and polysaccharide
- Environmental cues (e.g. oxygen depletion) may trigger active dispersal. (E.g. secretion of proteases that cleaves surface adhesion proteins).
1b. Four benefits that biofilms provide for the inhabitants.
- Self defense against phagocytosis (e.g. protozoa or macrophages), physical forces and antibiotics.
- Inner layers of biofilm cells have more time to initiate stress response
2Allows cells to remain in favourable niches
- Nutrient rich surfaces
- Flowing areas where nutrients are replenished
- Nutrient depletion creates zones of altered activity. These anoxic environments may favour certain microbes. Creates new niches.
- Allowing cells to remain in close proximity to one another
- Intercellular communications
- Exchange of nutrient and genetic material - Maybe the default mode of growth in nature
- Doesn’t require much energy for the organisms, quick way to protect themselves to some extent.
1b. Give one example of quorum sensing involvement in the development or functioning of biofilms
Quorum sensing= Mechanism to assess the presence and density of other microbial cells, usually of the same species.
Aliivibrio fischeri
- A marine bioluminescent bacterium
- Luciferase expression (e.g. lux operon) is activated by LuxR
- LuxR is induced by a sufficient level of N-3-oxohexanoul homoserine lactone (an AHL)
- The luxI gene encodes an enzyme that makes the A.fischeri AKL
- Leads to the discovery of quorum sensing.
- These organisms can coordinate when they become bioluminescent.
- Other organisms start making AHL, it diffuses into a cell. This activates laxR, which activates the expression of luciferases. AHL is also produces.
2b. Why are freshwater bodies in temperature climates often stratified, and why is such stratification less common in colder climates?
- Most freshwater bodies are stratified in temperate climates. Stratification is influenced by temperature.
- The surface is close to the atmospheric conditions.
- As you go deeper, the penetration of sunlight decreases and so temperatures drop. This is not in a linear fashion though.
- Over time the colder water stays at the bottom since its denser relative to warmer water.
- Separated by temperature and density. As you go further and further down, the temperature drops.
There are three layers:
-Epilimnion (surface layer)
-Thermocline (in the middle)= steep change in temperature
-Hypolimnion (deeper layer).
The layers are separate so transfer of material between layers normally facilitated by diffusion rather than mixing (to move between boundaries).
- Epilimnion is fairly well mixed since the wind blows on the surface of the lake, well oxygenated.
- But when it hits the thermocline there is a transition.
- Not all the oxygen can diffused across the thermocline.
- Results in a deep drop in the oxygen concentration. There is even less oxygen in the hypolimnion. So the bottom waters can experience extended anoxia.
Cold temperatures
- Stratification is not always maintained.
- The surface before it ices, cools very fast, making it way denser than the water in a thermocline.
- The cold oxygenated water sinks displacing the hypolimnion. This causes an inversion of the strata.
Temperate environment
- The water is not well mixed and the water is very stratified, the bottom water is often anoxic and fairly rich in nutrients.
- This results in water that are not clear but cloudy.
In e.g. southland, you see lakes that are clear and can see to the bottom. These lakes being in a colder area are well mixed throughout the year. They are nutrient deprived. The entire water may stay oxygenated. A pristine lake is not as productive as a habitat.
- H2S increases slowly with increasing depth within the hypolimnion.
- Across the strata, as oxygen decreases, temperature decreases, but concentration of H2S increases and is present in the hypolimion layer.
2b. Describe how concentrations of O2 and H2S, as well as temperature, change across the strata. How does this stratification phenomenon affect the distribution of microorganisms?
- In freshwater microbial ecology they contain both oxygenic phototrophs (e.g. algae and cyanobacteria) and oxygen consuming heterotrophs).
- Primary producers largely determine the activity and diversity of heterotrophs.
- Heterotrophs are relying on the primary producers to generate the carbon they consume.
- So the amount and timing of the carbon produced determines the heterotrophs.
- Paradoxically, primary production boom can lead to anoxia in bottom water due to respiration of excessive organic material.
- Lots of algal and cyanobacteria mass being generated, they will eventually die.
- When they die, it’s often because the nutrient input has been used up.
- This means that boom in primary production is not being sustained.
- The dead primary produced will then be respired by the heterotrophs.
- Have a sudden crash in photogenetic phototrophs and a sudden boom in heterotrophs.
- Heterotrophs required oxygen as they respire the primary production biomass.
- These heterotrophic bacteria will use up all the oxygen, which leads to anoxia.
- This kills all the metazoans that can live in oxic conditions.
-The freshwater is dominated by Proteobacteria, Actinobacteria, Bacteroidetes and Cyanobacteria.
3b. Describe pelagic waters’ characteristics as a microbial habitat. How do those characteristics affect the physiology of pelagic microorganisms?
Pelagic waters typically contain very little key inorganic nutrients for phototrophic organisms. E.g. N, O, Fe.
- Mostly are well oxygenated.
- Microbial cell levels an order of magnitude lower than freshwaters, but overall productivity still higher.
- Most organisms in ocean waters are very small. This is an adaptive feature of an oligotrophic lifestyle.
- They have a hard time obtaining nutrients due to low concentrations in the environment.
- Want to minimize the surface area to volume. A smaller volume means organisms require less energy for cellular maintenance. But since they can rely on nutrients to diffuse across the membrane they requires more effective nutrient transport system.
- These organisms have nutrient transport systems to obtain the limiting nutrients in the environment.
- These transporters aren’t necessarily found in freshwater organisms.
3b. How do coastal waters differ from pelagic waters, and why do those differences lead to oxygen depletion?
Ocean is divided into pelagic and coastal waters.
-Coastal waters are more nutrient rich and variable.
-They contain variable salinity and have and input of terrestrial nutrients.
-Have a much higher primary productivity that sometimes leads to anoxia.
-Regions of oxygen-depleted waters are between depths of 100 and 1000 m.
• Over wide expanses of the open ocean and coastal ocean
• Associated with nutrient rich regions of high surface productivity and limited mixing
• Warming of surface waters increases stratification and reduces oxygen transfer.
-Cell abundance decreases with depth. Surface water contains 10-1000 times more cells than water at 1000m. Bacteria predominate above 1000 m.
4b. Compare the geologic origins of and environmental conditions at Cold Seeps and Hydrothermal Vents and describe how their differences and similarities shape ecosystems found at these habitats.
Cold seeps
Differences and similarities.
- Cold seeps occur mostly along continental margins where gases (CH4, H2S) seep through the sediments and provide energy to sustain large endemic biomass.
- Metazoans e.g. tubeworms, soft coral, crabs mussels rely on symbiotic chemoautotrophic bacteria to harvest energy and nutrients from the environment.
- This is because they have no way of getting nutrients on their own. Some feed on other species but the ultimate energy source come from chemoautotrophy.
- The ecosystem is driven by chemoautotrophy.
The differences and similarities between them shape the ecosystems found at these habitats.
- They determine what type of organisms are found, what type of relationships there are between the organism and what type of environment it is, especially in terms of temperature.
- Organisms living in or around hydrothermal vents are adapted to the high temperatures whereas the organism in and around cold seeps are adapted to much lower temperature.
- There are different nutrients available as well which in turn determines the organisms and symbiotic relationships.
4b. Compare the geologic origins of and environmental conditions at Cold Seeps and Hydrothermal Vents and describe how their differences and similarities shape ecosystems found at these habitats.
Hydrothermal vents
-Deep-sea hydrothermal vents are volcanic springs occurring at or near mid ocean ridges.
-Depths from <1000 to >4000 m.
-Seawater enters the Earths crust through openings in the sea floor (rifts) and becomes hydrothermal fluid (very hot) rich in reduced inorganic compounds (H2S, CO, H2 and metal ions).
-When water comes out, it come out in 2 forms; hot vents, or out a series of channels.
-The warm diffuse vents are 5-50 C while the hot vents (black smokers) are 270 to >400 C.
-In deep-sea hydrothermal vents, they contain rich and diverse metazoan populations that rely on chemoautotrophic bacteria to fix carbon and harvest energy.
-Some reside on the inside walls of chimneys. Some vents microbes are endosymbiotic but many are free living.
Free-living hydrothermal vent microbes are dominated by chemolithotrophs especially Epsilonproteobacteria which oxidizes sulphide and sulfur using O2 or nitrate as TEA. Most are presumed to be thermophilic or hyperthermophilic and lives in the gradients between hot vents and surrounding water. A lot are free-living but some are both symbiotic and free-living.
5b. What is Rhizobia and what benefits does it provide to/gain from its host?
Rhizobia
- Collection of microbes that is found near the roots of legumes which is composed of nitrogen fixing alpha and beta proteobacteria
- Inhabit root nodules and provide nitrogen nutrients to the host
- Can also be free-living
- Highly specific symbiosis
- They are acquired horizontally. The plants initially grow with the microbes but then they acquired them from the environment. The plant has to be free living at some point of its life.
- They provide huge benefits to the host.
Benefits to Rhizobia
- Leghemoglobin provides protection from O2. Microbes gain protection from oxygen. Nitrogenase is oxygen sensitive. Leghemoglobin moves oxygen away from it.
- Only produced by the nodule when it’s inoculated with the correct rhizobial species.
5b. Describe the biochemical processes that occur in root nodules and the sources of nutrients involved.
- Rhibozia gets carbon substrates from the plant.
- Plant makes electron donors through photosynthesis, (e.g. succinate, malate and fumarate). These organic acids then feed into the TCA cycle.
- TCA cycle generates a PMF and eventually ATP.
- ATP is used to facilitate nitrogen fixation, also gaining reductive power from pyruvate.
- Nitrogen fixation gives you ammonia
- N2 → NH3 (ammonia). NH3 is assimilated by glutamine synthetase in the plant cytoplasm.
- Glutamine synthase turns ammonia into glutamine in the plant cytoplasm. This can be transported around as an organic nitrogen compound.
6b. What is the goal of wastewater treatment? Describe the main components of a modern wastewater treatment system and three potential environmental issues created by untreated wastewater.
- Wastewater includes sewage, gray water (non consumable water) and industrial wastewater.
- The primary goal of wastewater treatment is to reduce nutrient and toxic material loads. This is measured in terms of reduction in BOD. Typical domestic wastewater is about 200 BOD units and industrial wastewater can reach 1500 BOD units. The goal is less than 5 BOD units.
Wastewater goes first through primary treatment and then secondary treatment. Primary treatment consists of screening then sedimentation. Secondary treatment consists of anaerobic digestion and aerobic oxidation. The wastewater is then disinfected.
Primary treatment
- Physical separation steps to remove solid and particular materials
- Outflow can still have very high BOD
Secondary treatment-Anaerobic
-Anaerobic degradative and fermentative reactions in sludge digesters
• Breakdown of suspended macromolecules (polysaccharidases, proteases and lipases)
• Soluble nutrients are removed through fermentation. First generate fatty acids, H2 and CO2. Fatty acids are then fermented by syntrophs to produce acetate. Products are consumed by archaeal methanogens to produce CH4.
-Used for wastewater with very high BOD
Secondary treatment- Aerobic
-Aerobic degradation of organic materials in wastewater with low BOD (e.g. household wastewater)
-Activated sludge
• Continuous aeration. Slime forming bacteria grows and forms aggregated masses (i.e. flocs).
• Flocs are settled and removed. A small amount is retained as inoculant
• BOD reduction (95%) mostly through aggregation
• Addition of oxygen is an energy intensive process.
-Tricking filter methods
• Spread wastewater on crushed rocks (about 2m thick)
• Biofilms develop on rock surfaces and help oxidise organic material.
Disinfection with UV, chlorination or ozone
-Treated effluent to discharge.
6b. Three potential environmental issues created by untreated wastewater.
- Pharmaceuticals
- Most medicine taken is not metabolized so ends up in the water.
- Synthetic estrogen creates hermaphrodite fish
- Wastewater treatment aren’t necessarily designed to deal with estrogen
- Oxazepam changes fish feeding behavior. - Cosmetics
- Face scrubs micro beads (0.004-1.25 mm) absorb and concentrate chemical pollutants.
- Fish often mistake these beads for food and so they ingest them thus increasing chemical pollutants in the environment and may kill the fish - Algal Blooms
- Directly attributable to nitrogen and phosphorus nutrient pollution.
- Nitrogen: animal waster and excessive fertilizer use
- Phosphorus: household cleaning products
- Directly linked to climate change. Increased storm frequency and intensity. Elevated temperature promotes algal growth. Most commonly Cyanobacteria create anoxia and decimate aquatic life.
- caused by high nutrient content in water.
- Many algae produce natural toxins.
- Results in shellfish poisoning
- Many toxins are concentrated through filter feeding. - Introduction of inorganic nutrients.
- Kill organisms and disrupt the ecological niche.
- Untreated wastewater would also basically kill many organisms and habitats wherever it is discharge.
- Not only does it affect the environment, but also humans; diseases and deaths
7b. Describe enrichment as a technique in microbial ecology and the three requirements for a successful enrichment attempt.
Enrichment is a culture dependent method.
- Design a medium and a set of incubation conditions that are selective for the desired organism and counter selective for undesired ones.
- Replicate (as much as possible) the conditions of the organism’s ecological niche
- Must start with the right and viable inoculum (e.g. containing the organism of interest)
- Hundreds of enrichment strategies exist.
- Need to have some knowledge of the ecology and physiology of the culture you are trying to grow.
- Cultures must be duplicated as closely as possible to the resources and conditions of the organism’s niche. It must also be replicated as much as possible.
- Must start off with the right and viable inoculum (containing the organism of interest), thus the making of an enrichment culture may begin with collecting a sample from the appropriate habitat to serve as the inoculum.
- Enrichment cultures are established by placing the inoculum into selective media and incubating under specific conditions. Conditions are set to be selective for the desired organism and counter-selective for the undesired ones. e.g. when growing cyanobacteria you want a medium without a lot of carbon, otherwise you will get a lot of other organisms growing instead.
- Need to take the ecology of the microbe into account. When taking a sample, need to take it in a certain way e.g. taking a soil sample, letting it dry will kill some of the microbes present.
7b. List and explain three significant shortcomings for enrichment as a technique for studying microbial ecology.
Negatives
- Impossible to determine true negatives
- Positive outcomes do not imply ecological significance. Sometimes the dominant microbe grown is not necessarily the most important microbe in the ecology, could just be a fast grow culture or may not replicated the exact conditions form where is was taken from.
- The organism of interest maybe in the sample but the resources and conditions of the laboratory culture maybe insufficient for growth. This may result in positive outcomes, which may not be a true reflection of the microbes in that environment.
- The most dominant microbe growth may not be the true microbe that dominants in the environment.
-Isolation of the desired organisms form an enrichment culture says nothing about the ecological importance or abundance of the organism in its habitat. A positive enrichment proves only that the organism was present in the sample/inoculum.
8b. Describe the most-probable-number (MPN) technique and its use in microbiology. List three types of isolation procedure—which one of these is MPN most common used with?
MPN is based on the assumption that the last tube with growth in the dilution series has 10 of fewer cells. It is used to estimate the concentration of viable microbes in a sample by means of replicate liquid broth in ten fold dilutions.
Carry out 10 fold dilutions; each tube concentration is decreased by 10-fold dilution. You keep doing dilutions until you assume you have only one colony.
Isolation procedure is when a pure culture is yield (containing only one species) and begins with an enriched culture. Three types of isolation procedures are:
- Streaking agar: doesn’t work for all microbes
- Agar shake: useful for anaerobic microorganisms.
- Liquid dilutions: a serial 10-fold dilution until no growth is observed. Liquid dilution is the procedure that MPN is most commonly used with.
8b. List three ways in which the purity of a microbial isolate can be verified and why these attributes individually cannot verify the purity of a potential isolate.
Verifying purity
- Cell morphology
- Colony characteristics
- Growth in other media (esp. media that favours potential contaminants)
- Molecular genetic techniques (as supplemental information)
These attributes individually cannot verify the purity of a potential isolate because microbes can be similar in one attribute but diff with another, therefore all ways of verification are needed.
E.g. one microorganism may have similar morphology to another but their colony characteristic may be different, if only morphology was used for verification. I would be possible that the pure culture may not by the culture you wanted.
10b. What are phylogenetic markers? List four examples of protein-coding genes that can be used as phylogenetic markers.
Phylogenetic marker= is a fragment (locus) of either coding or non-coding DNA which is used in phylogenetic reconstructions, i.e. which is known to have no or predictable variation with a given species and which sequences are available for most or all species of a genus.
The two ribosomal genes are organized in an operon. These RNA molecules function as RNA molecules and so are well conserved at the nucleotide level. Protein coding genes are well conserved at the nucleotide level due to degeneracy and wobbly codons. If you don’t have good conservation at the nucleotide level, wont be able to design good PCR primers.
Examples of protein coding genes:
Can take a gene and use it to tell what metabolic process is present. May not be a true representation of all the microbes using that process as PCR is very sensitive.
narG
- Metabolic process: Denitrification
- Encoded enzyme: Nitrate reductase.
nifH
- Metabolic process: Nitrogen fixation
- Encoded enzyme: Nitrogenase
amoA
- Metabolic process: Nitrification
- Encoded enzyme: Ammonia monooxygenase
apsA
- Metabolic process: Sulfate reduction
- Encoded enzyme: Adenosine phosphosulfate reductase.
10b. Describe the differences between rRNA genes and the ITS region in terms of their use as phylogenetic markers.
16S and 23S ribosomal RNA is for bacteria,
- They are joined together with the internal transcribed spacer (ITS). It basically links up the two genes.
- The entire operon is transcribed as one thing and then the ITS gets chopped out.
ITS does not serve any function.
- Since it is not used, there is no evolutionary pressure to conserve the sequence. So the sequence and its length can be variable because there is no function and no pressure to keep it stable.
- ITS can vary between strands of the same species. E.g., E. coli there is lots of strands, some pathogenic, some harmless. Can use ITS to distinguish between them which will have identical 16S ribosomal RNA.
- Use 16 S to get a high level over view.
- To get fine resolution, use ITS.
- Since ITS is not functional, its not conserve in any way. So can’t use it to predict what organism it is.
ITS (internal transcribed spacer)
- Non-functional, variable length
- Useful phylogenetic marker for subspecies level resolution of bacterial diversity.
11b. Describe how DNA is sequenced using the Sanger technique. How are the four nucleotides (A, T, C, and G) distinguished, and what specific attribute of the technique allows only one nucleotide to be read at a time?
Sanger Sequencing
- Fluorescently labeled dye terminators
- Analyzed using capillary electrophoresis and bases are read by fluorescence detector automatically
- One sequence at a time.
- DNA is first separated into two strands. The strand that is to be sequenced is copies used chemically altered bases.
- These altered bases cause the copying process to stop when a particular letter is added into the growing chain.
- This process is carried out for all 4 bases and then the fragments are put together like a jigsaw to reveal the sequence of the original DNA.
- Regular dNTPs are mixed with fluorescently terminal labeled dye terminators.
- For a DNA strand to keep growing need to have a 3’OH group.
- With the dye terminators, we attach a dye to that position instead so you can’t add another nucleotide instead.
- The ddNTP is specific to one nucleotide and will stop elongation when that nucleotide is incorporated in the growing chain, thus forming different sized fragments of DNA.
- Each base has a different colour. As this travels through a capillary in a DNA sequencer this is read by a laser and fluorescent detector
This method limits you to sequencing one sequence at a time.
- Wouldn’t work if you have two sequences as you will have a point where there are 2 peaks from two bases. Won’t be able to tell what base you have.
- This can only be done one nucleotide at a time because if more than one nucleotide was used, then more than one ddNTP would be used, thus wrong fragments that doesn’t match the sequence of the original DNA.
Next generation sequencing
- High throughput (3 to 3000 million reads per run)
- Read length approaching or exceeding Sanger sequencing (as long as 30 Kb per read)
- Simple sample preparation (<24 hours by a single technician)
- Low cost (as low as $5 per GB)
- Requires small amounts of DNA (<1ug)
11b. Contrast these with one of the next-generation sequencing platforms (i.e., Illumina, Ion Torrent, or PacBio).
Ion Torrent Sequencing
Ion Torrent sequencing technology
- Semiconductor-based, non-optical detection of nucleotide incorporation
- Detect chemical signals associated with nucleotide incorporation
- Low cost instrument and reagents.
- Ion chips have many wells covering those pixels. The wells captures chemical information from DNA sequence signals and translate it into digital information.
- The sequencing process starts when DNA is cut up into millions of fragments each fragment then attaches to its own bead and it copied until it covers the bead.
- This automated process covers the different beads with the different sized fragments. These beads flow across the chip each depositing in a well.
- The wells are flooded with one of the 4 DNA nucleotides.
- Whenever a nucleotide is incorporated into the single stranded DNA, a hydrogen ion is released.
- This is how the ion torrent sequencing sequences DNA by reading this chemical change directly on the chip.
- The hydrogen ion changes the pH of the solution in the cell.
- An ion sensitive layer beneath the well measures that change in pH and converts it to voltage. This voltage change is recorded indicating which nucleotide is incorporated. Each well works as the smallest pH meters.
- If a nucleotide is washed over and its not complementary to the strand, its not incorporated, no change in pH or voltage.
- If there are two identical bases next to each other, two nucleotides are incorporated and the voltage doubles, recording two bases added.
- This process happens simultaneously in million of wells. Doesn’t matter if you have chips with a million wells or a billion, the sequencing process occurs in a few hours.
12b. Describe in detail two ways in which analyses of stable isotopes can be used to study microbial ecology.
Stable isotope probing
Not all isotopes are radioactive. E.g. 95% of carbon in nature is 12C, 5% is 13C and very little is 14C. Only 14C is radioactive.
Isotopes are metabolized different by microorganisms.
- Enzymes typically prefer lighter isotopes
- Therefore, biologically fixed carbon is depleted in 13C (compared with inorganic carbon) and shows isotopic fractionation.
Stable Isotope Probing
- Add substrates containing less common isotopes (i.e. 13C, 15N, 18O) and identify organisms that have incorporated these isotopes into cellular material.
- Use substrate to specify the pathway of interest.
- Typically used to reveal the diversity behind specific metabolic transformations in the environment
- Coupled with molecular genetic analyses.
- Particularly useful for studying a specific pathway in a metabolically diverse community (e.g. methane consumption in forest soils).
- Stable isotopes are typically heavier. If it’s the only thing available, organisms will use it.
- If you add stable isotopes in the form on carbon substrates (e.g. CO2, ammonia), even though biological systems don’t like it, if it’s the only thing available, they will use it.
- SIP reveals microbial diversity by yielding isotope-labelled DNA that can be used to analyse specific genes or the entire genome of the organism(s) that consumed that labelled substrate.
- SIP uses substrates to specify the pathways of interest. When coupled with molecular genetic analysis SIP is particularly useful for studying a specific pathway in a metabolically diverse community (e.g. methane consumption in forest soils).
Example
- We have an environmental sample, we take out all the air/substrate and replace it with 13C labeled glucose. Only the cells involve for example in methyltrophy.
- If you only want to find the methylotrophs, you feed then 13C labeled methane. Only the methylotrophs will take up the 13C methane. You incubate it for a bit. You then extract the final DNA sample and put into a fast centrifuge.
- Spinning for a few days, will eventually be able to separate the DNA bands on the basis of stabilized content. 12C DNA gets separated from the 13C DNA due to different weights.
- The only things that can be present in the 13C DNA fraction are the things that can use the substrate added. This is called stabilize isotope probing. By looking at the fraction, you know which organisms used the substrates added, and the other fraction contains things that are dead or are not involved in the pathway you are looking at.
- Can then remove and analyze (PCR 16S rRNA or metabolic genes or do genomics)
12b. Describe in detail two ways in which analyses of stable isotopes can be used to study microbial ecology.
Isotopic Fractionation
- The two elements most useful for stable isotope studies in microbial ecology are carbon and sulfur. Carbon exists as 12C, 95% abundance, and 13C, 5% abundance.
- Sulfur has four stable isotopes, the dominant isotope is 32S, some sulfur is found as 34S and very small amounts of 33S and 36S.
- The relative intensities of these isotopes change when C or S is metabolised by microorganisms because they typically favour lighter isotopes.
- The isotopic composition of a material can reveal its past biological or geological origins.
- Methane produced by methanogenic archaea is isotopically extremely light, indicating that methanogens discriminate strongly against 13CO2 when they are reducing CO2 to CH4.
- By contrast, carbon in isotopically heavier marine carbonates are clearly of geological origins.
- Therefore, biologically fixed carbon is depleted in 13C (compared with inorganic carbon) showing isotopic fractionation.
