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[Music]

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[Applause]

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what I'm gonna be talking about today is

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some work that we've been trying to do

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to look at how we might be able to

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distinguish the identity of different

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carbon fixation pathways in deep time so

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I mean we care about this right because

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not only are we taking carbon dioxide

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and converting it to biomass but it has

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to do with energetics material transfer

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and perhaps more importantly the

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Lindemann style trophic dynamics of the

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global biosphere at the time and one of

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the things that you can do to identify

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which autotrophic pathway may actually

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be fixing carbon is by looking at the

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ratios of carbon-13 to carbon-12 it

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turns out that due to the tension

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between equilibrium isotope effects and

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enzymatically controlled kinetic isotope

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effects you will end up with at the end

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of this process biomass that's enriched

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in the light isotope of carbon and

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leaving behind carbon dioxide that is

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enriched in the heavy isotope of carbon

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okay so how do we actually get at this

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effect and how do we use it to identify

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different carbon fixation pathways well

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one of the things that gave me a great

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pleasure to here is that I'm gonna be

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talking before Jody who has one of what

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I think the most crystal-clear

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presentations on how you actually can go

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out into the environment and figure out

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from the carbon isotopic composition of

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dissolve co2 or its counterpart

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dissolved bicarbonate and then the

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carbon isotopic composition of primary

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biomass in here a particulate organic

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carbon and figure out from the isotopic

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difference there the isotope

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fractionation associated with primary

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production and if you go and read this

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paper one of the amazing things about

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this data set is that if you correct for

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nitrate and you correct for temperature

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you're left with the temporal signal

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that may have something to do with

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enhanced co2 access as time has

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increased okay

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so one of the things though that we have

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to worry about is that rocks are not

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biomass and dissolved co2 so how do we

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go from the dissolve co2 carbon isotopic

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composition the composition of biomass

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and figure out something from the

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geologic record well the way we do we do

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is we approach it in this sort of haze

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diagram since we've got the carbon

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isotopic composition of these different

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compounds on the Left axis here is what

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we want this difference between dissolve

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co2 and biomass but we have to be

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concerned with the temperature dependent

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isotopic fractionation associated with

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carbonate speciation we have to be

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concerned with the mineral logic

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associated fractionation associated with

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the different precipitation of carbonate

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minerals and then of course there's

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going to be some small but important

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fractionation associated with the

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reworking of primary biomass into

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sedimentary total organic carbon okay so

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one of the things is that what does this

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record of mineral carbon isotopic

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compositions and total organic carbon

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isotopic compositions look like in deep

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time one of the things that were very

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fortunate for is that there was recently

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an incredible statistical reanalysis of

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the carbon isotope record in deep time

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and the carbon isotope record in deep

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time it turns out has two distinct types

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of States there's going to be the

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intervals that we're going to ignore

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here so called transitional intervals

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associated with climatic upheavals and

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non steady-state behavior in the carbon

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cycle but we're going to be focusing

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instead on these non transitional

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intervals largely the phanerozoic middle

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Proterozoic and back in time so what we

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want to do is we want to invert this

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haze diagram then and go from the carbon

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isotopic composition of sedimentary

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carbonate minerals the carbon isotopic

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composition of sedimentary organic

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carbon and go through a procedure that

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allows us to extract to the best of our

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ability the isotopic difference between

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dissolve co2 in primary biomass in deep

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time

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so it turns out that this technique was

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developed by Sarah Hurley a postdoc I'm

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fortunate enough to be working with and

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what it happens as we go like this let's

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look at a histogram for a certain time

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period of the isotopic composition of

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carbon eight carbon and what Sara

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realized is that if you do an agnostic

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resampling of the isotopic effects

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associated with carbonate mineral

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precipitation the temperature derived

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isotopic effects associated with

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carbonate speciation what you can end up

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with is you can end up with a frequency

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distribution of the Delta 13c carbon

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isotopic composition of dissolved co2

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for the time period that you're

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interested similarly if you take the

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isotopic distribution associated with

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sedimentary organic carbon and run it

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through a random resampling looking at a

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distribution that might have something

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to do with how or primary biomass is

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fractionated during microbial reworking

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you can end up with a frequency

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distribution of the carbon isotopic

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composition of biomass in deep time

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combining these two effectively just

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taking their difference gives you a

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frequency distribution of the so called

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epsilon P for primary productivity

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epsilon for a difference in Delta values

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that then may tell you something about

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what carbon fixation pathways were

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operating in deep time okay so when

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Sarah did this with the non transitional

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intervals in the carbon isotope record

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what she returned was the following type

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of distributions if we start over here

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something centered at about 50 million

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years ago going back to something about

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250 million years ago what you can see

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is a slight increase in the most

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frequent value of carbon isotope

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fractionation by the dominant primary

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producers at that time though there are

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a number of evolutionary events that may

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be associated with this it's also

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possible that what you're looking at is

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CIN

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we the effect of the gross long-term

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increase in co2 as we go back through

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the phanerozoic and as co2 increases

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what you end up with is you end up with

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less of a more efficient operation of

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the carbon co2 concentrating mechanisms

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that are associated with these organisms

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leading to a larger fractionation we're

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going to focus next on two intervals for

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a case study the case study one is this

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mid Proterozoic interval and what we

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want to do is we want to ask the

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following question was the middle

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Proterozoic actually the age of

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cyanobacteria what I'm showing here is a

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diagram that has a bunch of different

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compilations of oxygen levels from today

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back to the early archaea and shown here

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is of course the great oxidation

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oxidation interval but also shown down

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here in green are some indications both

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from geochemistry and also from micro

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fossils of cyanobacterial oxygenic

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photosynthesis and then on this side of

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the diagram some information from

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molecular clocks from micro fossil

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evidence and from biomarker evidence for

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a primary production global primary

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biospheric primary production dominated

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by algae and what we're asking is in

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this mid Proterozoic interval wasn't

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really an age of cyanobacteria can we

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use this technique to say okay the

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primary producer at this time which it

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seems like it was was a sign of

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bacterial population or was it something

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else so one of the things that is a

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complicating factor in this is that all

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extant cyanobacteria have a co2

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concentrating mechanism probably the

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most famous bacterial organelle is known

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as the carboxy zone it's where cyanosis

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today have their Rubisco have all their

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carbonic anhydrase they bring in bicarb

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it gets converted through the activity

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of carbonic anhydrase to carbon dioxide

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right at the site of carbon fixation

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and what you can do is you can overcome

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some of the catalytic inefficiency of

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Rubisco in that way and there have been

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a whole bunch of different estimates for

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when the cyanobacterial co2

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concentrating mechanism may have come

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into play and what it is usually

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associated with is drops in carbon

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dioxide levels it makes sense if carbon

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dioxide levels fall you would expect

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that a co2 concentrating mechanism would

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be developed so we decided to test this

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and Sarah in the lab was able to create

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a carboxy zone this mutant of the

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wild-type cyanobacterium Senate caucus

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7002 and these green fluorescence images

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show the distribution of Rubisco in

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these mutant cyanobacteria she grew them

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under different co2 levels and it turns

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out that they're a high carbon requiring

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mutant which means that growing them at

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very low co2 levels is very difficult

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fortunately though it's a relatively

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simple system to model where you just

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have co2 diffusing into the site of

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carbon fixation so we were able to model

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what might happen at lower co2 levels

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even at relatively low co2 levels what

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you can see is that the range of carbon

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isotopes that are produced by these

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potentially physiologically ancestral

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cyanobacteria does not cover more than

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about 30% of the epsilon primary

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productivity range that we see in the

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middle Proterozoic so what happens when

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you just take a wild-type cyanobacterium

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and grow it again under different co2

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levels well what happens is you produce

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a range of carbon isotope values that

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covers 95% of the middle protozoic

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epsilon primary productivity

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distribution suggesting then that one of

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the simplest interpretations of the

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middle Proterozoic

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primary productivity record is that

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cyanobacteria with a carbon can't see

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you too concentrating mechanism for the

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dominant primary producer at this time

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great so what I'm gonna do now is I'm

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gonna speed into the second case study

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which is looking in very deep time and

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saying okay what might this carbon

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isotope fractionation distribution mean

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there's some similarity here between

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what you see in the middle Proterozoic

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and what you see in the early Archaean

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and in fact that may be used to say okay

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the same carbon fixation pathways were

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an operation at this time what we

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decided to do is a thought experiment

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and working with some colleagues at BU

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Daniel se gray and Josh Gould furred we

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decided to look at what might

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potentially happen if you created a

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endergonic and biomass capable

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metabolism from a simple set of

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potential prebiotic seed compounds shown

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down here acetate carbon dioxide 4 made

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hydrogen sulfide and some others and so

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essentially what Daniel and Josh did is

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they went ahead and they led to a

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network expansion associated with this

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metabolism and they were able to show

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that not only was it endergonic it was

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capable of producing biomass and what we

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wanted to know is what's the carbon

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isotopic consequences of this expansion

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so it turns out the carbon isotope

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fractionation is both incredibly

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confusing but at some level relatively

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straightforward so it depends on two

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things primarily one the nearest

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neighbors that the carbon atom is bonded

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to and then the nearest neighbors to

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those nearest neighbors and so what a

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colleague Chris butch was able to do was

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take this full set hundreds of different

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compounds and actually calculate what

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the isotopic fractionation would be as

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this explained

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happens okay and here shown on the left

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is the iterations here are two images of

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that epsilon primary productivity on the

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top the number of compounds in the

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network is shown linearly on the bottom

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we've got it plotted versus the log

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number of networks allowed number of

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compounds in the network this one is

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easier to see the differences the early

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networks are shown here in blue and in

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red but as the network increases in

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diversity and potentially in complexity

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isotope fractionation actually goes the

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other way it is narrowing up and

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approaching a single mode at about 18

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per mil for the difference between

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dissolve co2 and biomass so what's the

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reason behind this it turns out that

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when you count the bonds and the bonding

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partners that are involved in this as

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Chris has done here what you see is that

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as the network expands your overall

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bonding environment is getting more

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reduced you're seeing a lot more

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carbon-carbon bonds and carbon hydrogen

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bonds and here what I've shown is the

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distribution of Chris's chemo

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informatics prediction along with the

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early Archaean distribution and though

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they're not exactly the same there may

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be some kinetic effects here and less

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equilibrium effects here one thing

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that's very clear is that an epsilon

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primary productivity of around 18 per

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mil or 20 per mil a'somethin that would

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have been in the past associated with

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say the carbon fixation by the Calvin

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cycle may simply reflect the development

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in these networks of aliphatic

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carbon-carbon bond okay I'll leave you

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now with what we know what we think we

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know and what we want to know and thanks

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you