Lecture 6
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Lecture 6 - Leaderboard
Lecture 6 - Details
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| 🇬🇧 | 🇬🇧 |
| What is an isotope | Same element, same amount of protons, but different amount of neutrons and tus different masses |
| Stable isotopes do not | Decay over time |
| What is the oxygen (d18O) reference standard called? | VSMOW = Vienna Standard Mean Ocean Water |
| Why do we need isotope reference standards? | Absolute isotope quantity is difficult to measure accurately, however, relative differences can be measured accurately. Common reference point/baseline to ensure accuracy, reliability and reproducibility of isotopes data from different laboratories and studies. Calibration, interlaboratory comparison, and quality control of procedures. |
| ... is the partial separation of isotopes during physical or chemical processes | Isotopic fractionation |
| The two most relevant types of isotopic fractionation are: | Kinetic (results when rates or reactions or physical processes differ, e.g., biosynthetic pathways) and equilibrium (occurs because the thermodynamic properies of isotopically substituted species differ e.g., fractionation betwee inorganic carbon species in water) |
| The product is often isotopically ..... why the unused part is often isotopically .. | Depleted/more negative & enriched/more positive |
| During kinetic isotopic fractionation, eutotrophic organisms fractionate ... 13C durig inorganic carbon uptake (CO2) in different degrees | Against (epsilon value works in the negative direction, larger number is larger fractionation) |
| What is rubisco ? | Earth's most abundant enzyme, used by autotrophic organisms to convert CO2 into organic compounds via the Calvin-Benson pathway |
| What factors influence fractionation in autotrophs | Growth rate (fractionation is reduced at faster growth rates, more product), concentration of CO2 (more CO2 = pickier, more fractionation) & cell geometry (smaller, pickier, more fractoination) |
| Fractionation is inconsistent due to limitations by for example .... Growth conditions are important! | Light & nutrients |
| 13C-contents depends on ......... and environmental factors (e.g., pCO2 and growth rate for Rubisco) | Carbon acquisition pathway |
| Fractionation for rubisco: | (straight chain) lipids (alkyl) most depleted, then isoprenoids and sugars are most positive (unused substrate) |
| Biomass is always depleted compared to | Atmosphereic CO2 |
| Reversed TCA - occurs in some bacteria and some archaea. Fractionation: | Reverses tge way they produce compound. So sugars most depleted, then isoprenoid lipids then straight chain lipids/alkyl |
| Reductive acetyl CoA pathway, not cyclic pathway - used by some bacteria (eg sulphate reducers ) and archaea (methanogens). Fractionation: | Very large fractionation compared to CO2, straigh chain lipids/alkyl & isoprenoids is super depleted vs biomass. Methanogens super depleted in d13C and dD(deuterium/hydrogen) as methane is aswell |
| Anammox fractionation by bacteria | Relatively large fractionation, isoprenoid and alkyl lipids (straight chain) strongly depleted in 13C versus biomass (NH3 & NO2- into N2) |
| 3-Hydroxy propionate pathway, used e.g. by phototrophic green non-sulphur bacteria. Fractionation | Uses bicarbonate instead of CO2, it is more positive by about 7-8 permille than CO2 – so bacteria using this will already be more positive from the start -> biomass lil. depleted vs CO2, alkyl enriched vs CO2, isoprenoids more depleted compared to alkyl and biomass |
| 3-Hydroxypropionate/4-Hydroxybutyrate pathway -> - Occurs in some hyperthermophilic Crenarchaeota and also in Thaumarchaeota (producers of crenarchaeol biomarker lipid) | Isoprenoids similar to biomass, DIC and sugars are enriched |
| We can use the known fractionations to determine the carbon acquisition pathway of a microbe | Analyse and compare CO2, biomass and lipids |
| C3 plants live in ... regions and are generally ... compared to C4 plants | Humid & depleted (they depend on pCO2) |
| C4 plants live in ... regions and are generally ... compared to C3 plants | Dry (predominantly grasses on savannahs) & enriched -> C4 plants have concentrating mechanism (limited exchange with atmosphere) -> fractionation does not depend on CO2 |
| We can measure our biomarker lipid and then reconstruct C3/C4 plant n-alkanes can be used to reconstruct higher vegetation and differentiate between C3/C4, but | Different contributions of different plant types with different 13C contents may bias the overall n-alkane isotope patterns -> no quantification possible, but at least reconstruct some changes in vegetation |
| What determines the isotopic composition of heterotrophs ? you are what you eat +- a few permille | D13C of food source & biosynthetic pathways |
| Methanotrophs | Very depleted in d13C and dD |
| Nitrogen isotopes are used for ... studies while carbon isotopes are used for ... studies | Food web studies (position)(because nitrogen increases by 3.5 permillle per trophic level and 13C much less) & food source |
| When amino acids are changed more strongly with the trophic level the N-isotopic composition is changed ... | More strongly aswell |
| N2 -fixation | Hardly any isotopic fractionation, slightly negative N isotopes |
| Denitrification (mainly perfeormed by anaerobes) | Generates 15N-depleted N2 resulting in 15N-enrichment of remaining nitrate |
| Negative n-isotopes of bulk sediment may point to | N2-fixing cyanobacteria |
| Sedimentary 15N can give paleoenvironmental information on relative importance of N2-fixation (......15N) denitrification (......15N) | Negative & positive |
| What is this and what is it a biomarker of? | Porphyrins - nitrogen isotopes |
| What are the two stable isotopes of hydrogen making of the dD? | Protium (1H) and deuterium (2H, D). -->deuterium has one proton, one neutron and one electron |
| DD is measured against? | SMOW (Standard Mean Ocean Water) and SLAP (Standard Light Antarctic Precipitation) |
| DD has a much larer range than d13C because? | Because of the larger mass difference |
| Clouds are ... in deuterium | Depleted (due to the evaporatio-precipitation balance and fractoination) |
| Rain is ... compared to the cloud | Enriched (heavier will rain out first, leaving the cloud more depleted) |
| Largest step in hydrogen isotopic fractionation in organisms is by formation of .... and estimated to be ca. 171 per mill | NADPH - but internally there is more fractionation taking place, but it is hard for us to know which enxymes/pathways are responsible |
| Hydrogen isotopic fractionation of lipids is depending on carbon/energy metabolism and can help us determine ..? | If it is a chemautotroph or photo(auto/hetero)troph |
| Hydrogen isotopic fractionation in plants: Lipid classes have different hydrogen isotopic compositions (but differently from 13C), name the pattern | N-alkanoic acids least depleted, then sterol, then lastly phytols (compared to water!) -> compared to bulk: phytols least depleted and then n-alkanoic&sterols |
| Straight chain compounds are more ... than isoprenoid compounds | Enriched (n-alkanoic acids vs phytols) |
| Differences in hydrogen isotopic composition between isoprenoid lipids due to different ... | Biosynthethic pathways |
| Where it is synthesised in the cell determines .... | The starting pool of hydrogen isotopes |
| Hydrogenation of doublebonds during biosynhtesis leads to ... in dD | Increasing depletion |
| DD of n-alkanes (from plants) mostly reflects ... | Precipitation/evaporation balance because this affects the dD of the source water |
| Influences on dD of terrestrial n-alkanes | Source water dD(precip/evap), fractionation due to biosynthesis of lipids and minorly by C3/C4 metabolism |
| There is no large impact of ... on hydrogen isotopic fractonation | Temperatures -> but there is of salinity |
| Lower dD = ? | More rain out(wet climate)/fresh water |
| Hydrogen fractionation is impacted by light | More fractionation at light optimum |
| The hydrogen isotopic composition of lipids from aquatic organisms is influenced by the following major factors: | - Metabolism of organisms - Biosynthesis lipids - dD of the source water - Salinity – in marine environment - Light – we like to ignore this - Only minor effect of growth rate/nutrients |
| Radiocarbon is produced from | Cosmic rays |
| 14C of atmospheric CO2 has varied over time (cosmic rays). This why we need to.. | Need to calibrate 14C measured against age |
| The bomb spike (spike in 14C) is usefull | Because the spike is represented in all material of that age and acts as a great tracer |
| Suess effect ? | Depletion in d14C due to input of large amounts of fossil fuel derived CO2 (contains not 14C) (occurred before bomb spike due to burning of fossil fuels, 1950s) |
| Difference in radiocarbon of atmosphere and ocean --> Marine reservoir effect: between 400-800 yrs – have to compensate for this, were probably different in the past. What is the marine reservoir effect? | Carbon taken up by a marine organism is older than a terrestrial organism as deeper waters are not in contact with the atmosphere |
| Compare radiocarbon of biomarker lipids from surface sediments with surface waters... | If they have same composition, they are from surface waters, if lipids are more depleted they originate from bottom waters and made them there |
| Age difference between biomarker lipid (organic) and foram proxies (inorganic) due to ?? | Lateral transport -> causes age offsets |
| Biomarker lipids have different offsets due to ... | Degradation & transport |
| We like to avoid sight where ... is a big factor | Lateral transport |
| Age of terrestrial lipids exported by rivers to the marine environment reveals the turnover and export of the soil organic carbon pool because | They are much younger and have higher d14C (fatty acids even more so that lignin phenols) |
| Compound specific radiocarbon is laborious and not sensitive. Need large amounts of material and long work up procedure. | Need atleast 50ug, but improvement is being made |
| Compound specific radiocarbon can be used to elucidate the age of a biomarker and particularly its origine e.g. ..... ? | Local vs laterally transported? depth in watercolumn (Was it in contact with atmosphere? Surface ocean? Deep ocean?)? Influx from land? |