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Evolution of Phototrophs · Acedra
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Evolution of Phototrophs
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0:00
hi guys today we're talking about the evolution of phototrophs or photosynthesizing organisms so like we talked about on the previous video over the earliest life on earth this first life might have been a simple chemotroph or a simple phototroph phototrophs get their energy from light and chemotrophs get their energy from chemicals and in that video we assumed that that first organism would have been a chemotroph and then talked about the different types of organisms that
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that could be but in this video we're going to be talking about the evolution of phototrophy which may have predated chemotrophic evolution but in this video as well as the previous one we're going to still assume that chemotrophs were first and now talk about the evolution of phototrophs after the chemotrophs and how they might have expanded from potentially hydrothermal vent origin environments but first i must point out that the forest phototropes were likely not
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these sunflowers or any sort of plant actually they were microbes i talked more in my life fortunes video about endosymbiosis and how plant cells came from endosymbiosis or basically merging of two cell types that eventually worked as one organism and before plants was microbial phototrophs so what was the first kind of phototroph was it the around 3.5 billion year old cyanobacteria that likely made this stromatolite or was it whatever caused
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this 3.9 billion year old formation to have light carbon isotope signatures if you want to know how light carbon isotropic signatures indicate life you can watch my carbon isotope video i'll link it up here for you if you want to check it out but actually the first phototroph was probably neither of these well it might have been whatever caused the light carbonic signatures in this rock here
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but there's a distinction i need to make between oxygenic photos and an oxygenic phototrophs here so oxygenic photos are those that you're probably most familiar with things like plants things even like cyanobacteria that take up carbon dioxide and use it and light energy to split water and make oxygen and then they release the oxygen and then animals like ourselves breathe in oxygen and release carbon dioxide it's a cycle however these types of phototrophs that produce oxygen
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were likely not first and oxygenic phototrophs those that produce compounds that aren't oxygen for example instead of producing oxygen from water or o2 from h2o there are an oxygenic phototrophs that take h2s or hydrogen sulfide and produce sulfate or so4 so they don't actually produce o2 or oxygen that's why they're called an oxygenic and these likely evolved first but i'll show you why that is later in the video first let's just back up a second and talk a little bit about why
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phototrophy might have been beneficial for early organisms so assuming a hydrothermal vent origin of life chemotrophs would have started first at that dark hydrothermal vent and used chemicals to gain energy but then eventually life would have wanted to venture out into new environments but it was restrained by needing the chemicals that were coming out of the hydrothermal vent so how did
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they eventually move away from the hydrothermal vent well one possible way is using a different energy source instead of the chemicals coming out of the vent using light for example one primitive type of pigment in microbial cells was likely bacterial rhodopsin which basically captures light energy to transport protons out of the cell this created a proton gradient more protons out of
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the cell than in the cell creating a proton pump which basically pumps protons through a protein in the membrane of the cell called atp synthase which basically pumps the protons through the protein creating atp as it does so and atp is chemical energy for all of life we our cells and our body create atp through a proton pump and we use that atp to do our daily things and they could have
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used this rhodopsin to make this proton gradient to create atp however early on it might have been present in chemotros as an alternative or extra source of energy for when light was present but maybe the chemicals they wanted weren't super abundant and so in low nutrient environments and conditions they were like well we need some energy so let's use this rhodopsin as backup to absorb light energy and make atp that way since we don't have the chemicals we like right now but
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all other phototrophs use chlorophyll or bacterial chlorophyll as their light absorbing pigment rather than rhodopsin so it's possible that phototrophy evolved twice once with rhodopsin and once with chlorophyll but it's more likely or i've seen more references that say that the iron core of rhodopsin proteins was eventually replaced by magnesium allowing more efficient light-sensitive pigments called bacterial chlorophylls we can see in the figure down here that chlorophyll
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which isn't the same as bacterial chlorophyll but i'll explain that later absorbs different wavelengths of energy both lower and higher than bacterial rhodopsin does and these more efficient absorbing pigments would have increased micro survival on early earth because they became thermotactic this means that they could detect hydrothermal sources and that's because many of these bacterial chlorophyll pigments were capable of absorbing infrared radiation which anything
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that gives off heat gives off ir radiation so that's how they could detect these sources however you might be thinking earlier i mentioned that if they were able to use light for energy why do they need to be near hydrothermal sources still well even though they could use light for energy they still needed an electron donor like molecular hydrogen or hydrogen sulfide and other things and all of these electron donor compounds came out of hydrothermal vents so they had to still
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be near the hydrothermal vent fluid to get their electron donors that they use the light to convert to something else to gain energy however later i will discuss when certain phototrophs evolved they no longer needed to be near hydrothermal vents because their electron donors that they needed were not coming from the hydrothermal vents but more on that later eventually an early version of
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what's called the reaction center complex used in phototrophs evolved from the adaptation of mobile ubiquinones ubiquinones are basically compounds in all living cells that act as electron carrying coenzymes for the electron transport chain if you've ever taken a bio 101 class you've probably heard the term electron transport chain it's exactly what it sounds like it's a chain of reactions and electron carrying compounds that basically move electrons down this chain
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of compounds until it creates something that can help make atp for the cell the end goal is always atp and in phototrophs this reaction center complex allowed electron flow from the excited pigment so the pigment like chlorophyll for example gets excited by light and electrons from this excited pigment were carried by electron carrying compounds and enzymes to kick off the electron transport chain but what type of organism was all of this occurring in what
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was the first phototroph we already said it wasn't a eukaryote so we could cross that off the list but was it part of bacteria or archaea well the lack of photosynthetic archaea basically crosses off archaea from that list and we know from that that photosynthesis likely evolved from the bacterial domain however that's pretty much the only thing it's agreed upon when it comes to what the first phototrophs actually were some genetic evidence suggests that green non-silver bacteria
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were the first phototrophs however some suggest heliobacteria and other evidence suggest purple bacteria so which one was first well if we look at this figure we can see some bubbles listing off these bacteria and all of these little abbreviations with numbers next to it are certain genes that some of those bacteria had and where they might have come from and it's trying to like make a flow chart of which phototrophic genes belong to what and where do they come from and
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we can see heliobacterium here that's the heliobacteria we've got the purple bacteria over here and chlorofluxis is a type of green non-sulfur bacteria so they're all listed here but the flow chart isn't very unidirectional it's kind of all over the place and the reason for this is lateral gene transfer you guys know i talked about lateral gene transfer lgt in my previous video over the earliest organisms same thing happened with chemotrophs and it just makes
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everything a big mess when we look at genes from primitive organisms and try to map phylogeny it's never a straightforward map of who came first and whatnot it's always very confusing because lateral genes transfer or the transfer of genes across species boundaries makes it hard to know if those genes were being passed down or just transferred laterally across species so we may never know exactly which of these groups of bacteria are responsible for the beginning of
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photosynthesis and making lateral gene transfer even more detrimental was microbial mats basically microbial mats are layered matte like structures like i showed earlier with the stromatolite that basically has microbial communities literally right next to each other stacked on top and underneath one another because they benefit from each other's metabolites and they end up stratifying themselves in this you know layered structure i talk about microbial mats in
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a whole video i'll link it up here if you want to check it out uh but that facilitates the lateral gene transfer because one close proximity to other communities of microbes and also eps so eps is like an extracellular mucous type substance that microbes excrete for a number of reasons and functions uh one of which is the transfer of genetic material which eps facilitates um so that also made lateral gene transfer a real pain in the butt for phylogenetic purposes
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so we don't know but one of these three groups is potentially responsible for the first phototrophs so let's talk about them first purple bacteria basically bacterial chlorophyll that pigment we talked about earlier is used in purple bacteria and absorbs near infrared radiation and purple bacteria can be broken up into both purple non-sulfur and purple sulfur bacteria named basically because one uses sulfur and one doesn't and so a purple nonsulfur bacteria can use
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hydrogen to or molecular hydrogen as an electron donor and co2 as an electron acceptor and make uh basically organic carbon this is glucose and water as their products of their metabolism and the way that they kind of set off this reaction is by the light absorption so they use light as energy to do their metabolic pathway and purple sulfur bacteria basically do something similar but instead of
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molecular hydrogen they use hydrogen sulfide and because purple non-sulfur bacteria do prefer using molecular hydrogen this may indicate that they originated near hydrothermal vents where molecular hydrogen was abundant so if these were the first photosynthesizing bacteria then they likely got their start either at a hydrothermal vent or some sort of hydrothermal system where molecular hydrogen would have been available after the first phototrophs or purple
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bacteria or potentially helio bacteria or potentially green non-sulfur bacteria then green sulfur bacteria might have come next i put bacteria here pretend that has a b and it says bacteria my bad these guys might have come next and we know that they were tad more advanced because they had a new reaction center complex that contained iron sulfur clusters and were more electronegative upon illumination and so more electronegative means the light excited them
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more allowing more electrons to be transported through the chain aka more energy created these also could grow in what some phototrophs and other organisms likely considered toxic environments because green sulfur bacteria liked sulfur and not only could grow but liked to grow and very sulfite and anoxic without oxygen environments like the lower layers of microbial mats and i talked more about that and the kind of stratification and who is where in microbial mats
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in my microbial mats video that i mentioned earlier so if you want to know more about that you can see that video eventually though all of these bacteria green non-sulfur bacteria purple bacteria helio bacteria green sulfur bacteria all of these phototrophs would have grown together lightly grown together in microbial mats which is precisely why there was rampant lateral gene transfer but moving on these mats were potentially still near hydrothermal sources because again they
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needed those electron donors for their metabolism but they also had to be shallow enough for light to penetrate now remember that the photic zone how deep like penetrates in the ocean goes to around 200 meters in depth in the modern ocean and likely was similar in the early ocean maybe even more i don't know but it goes pretty deep so you could have had a geothermal area that also received light pretty easily so this is where they likely would have been again they had to use
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those compounds that came out of the vent fluid like hydrogen sulfide molecular hydrogen iron sulfur etc and i promise you later we will get into how they finally broke away fully from the hydrothermal areas it did not need that vent fluid anymore but before we talk about that let me briefly go over photo protection basically because o2 or molecular oxygen was not abundant enough on early earth to make ozone i talked more about when earth was oxygenated in my great oxidation event
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video and my neoprotozoic oxidation event video but basically there weren't oxygen phototrophs to make oxygen with their photosynthesis yet and so because that's the major source of oxygen on earth oxygen hadn't really become that abundant yet so because it wasn't abundant enough to make ozone uv light could get through the atmosphere instead of being stopped by ozone like it is today and it reached our surface and also penetrated into the ocean and it could have been harmful to
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those early phototropes and they would have had to evolve protection mechanisms because as we see here uv light is harmful to dna and everybody on earth has dna so like we need to not have it you know damage our dna so mets might have been not only beneficial because they got the metabolites from their you know sister community and they were like oh thanks for the food and also thanks for some jeans too i like that as well but they also would have been beneficial
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because it protected at least the lower air communities below the surface layer from uv light but they also could have used sediment as a sunscreen rather than their bacterial friends ferric minerals or oxidized iron minerals would have been perfect sunscreen for this uv light because they absorb light in uv light wavelengths around 220 to 270 nanometers and amore basilica could have also been used as a sunscreen for uv light because they
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would have both absorbed uv wavelengths while simultaneously allowing essential wavelengths that the phototrophs wanted and needed through to get to the phototrophs and so they could use it and we actually have plenty of examples in modern organisms as well as ancient ones of the secretion of a more basilica for shells and skeletons and whatnot and because the secretion of silica is so common yet sometimes a little enigmatic because it doesn't make sense to try and use silica for
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shells or skeletons when calcium carbonate is so much easier to secrete because of its saturation state in the ocean however this you know protection this sunscreen type mechanism or function might have been one reason that that evolved and last for photo protection i just want to mention carotenoids so carotenoids are just great for photo protection they are not like the primary pigment for absorption of light and phototrophs just like i talked about the primary
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pigment is bacterial chlorophyll or chlorophyll and so carotenoids are accessory pigments in all phototrophs that are important for just basically protecting from excess or harmful light they also protect against the harmful products of photosynthesis like oxygen radicals and so they kind of act as antioxidants and then not only are these carotenoids great for photo protection but also they attract pollinators and seed dispersers in plants and they also assist in absorbing light
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in low light conditions now finally getting to the switch from bacterial chlorophyll to chlorophyll bacterial chlorophyll and chlorophyll are both pigments used in phototrophs but they absorb different wavelengths of light bacterial chlorophyll can absorb longer wavelengths of light and chlorophyll absorbs anywhere from around 430 to around 680 nanometer wavelengths of light and shorter wavelengths of light are actually higher in energy and the higher the energy the
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more stable compounds the organism will be able to break down through their metabolism that is charged by light energy so with all this energy chlorophyll using organisms were able to oxidize and split these super stable molecules of h2o or water so cyanobacteria were these new kids on the block that were able to use chlorophyll to split water why was this such an incredible feat this splitting of water produced oxygen molecular oxygen and this was the beginning of oxygenic
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photosynthesis and not only was this monumental because oxygen was produced and eventually that led to us being able to evolve and live because we breathe oxygen but this was also monumental because since water is unlimited instead of being limited to hydrothermal vent fluid it was literally everywhere in the ocean it's made of water this allowed cyanobacteria to expand from their hydrothermal origins or potential hydrothermal origins and diversify throughout
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the entire product zone of the ocean including continental shelves and shallow water environments near the ocean and it was just an amazing expansion of life because it wasn't limited to this source of hydrothermal vent fluid or the chemicals that came from it cyanobacteria evolved around 3.5 to 2.7 billion years ago 2.7 is the most like undeniable evidence for cyanobacteria 3.5 is more of a probably these microbial mats represent cyanobacteria but we're not 100 sure
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but in either case those two numbers are both way before what we call the great oxidation of it i mentioned this earlier it's basically when earth became largely oxygenated because of oxygenic photosynthesis which was done by cyanobacteria so why didn't the oxidation event happen right after or like during the evolution of cyanobacteria because cyanobacteria evolved and immediately expanded because of their ability to use water rather than chemicals from hydrothermal
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vents for their metabolism well the reason why the goe or great oxidation event didn't happen until way later or around 2.4 billion years ago is probably because of chemical reservoirs i talked more about this in my goe video but basically these cyanobacteria produced o2 right molecular oxygen but then it's releasing this oxygen into a really reduced earth and i mean chemically reduced
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that just means that there are a lot of compounds out there that could have been oxidized by this oxygen aka they could have reacted with it to transform into something else and because of this need for oxygen to go and react with and oxidize all these reduced compounds out there it had to fill these reservoirs of chemically reduced species before it could build up on its own i don't know why i'm talking like this but i am it's just that this is so cool to
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me anyway yes i'm a nerd i know but basically you have to fill these chemical reservoirs uh to be able to eventually then build up in the atmosphere and oceans so this build up here basically marks the point where boom all those chemical reservoirs are basically filled to the point that oxygen could then build up on its own without reacting with a bunch of things it was like oh i've already reacted with you i can just be my own thing now and although molecular oxygen might seem
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precious to us and all animals on modern earth today on early earth during the archaean it was a toxic byproduct i mean it was just the waste of these photosynthesizers and their expansion was so quick and widespread and they created this huge oxidation event that it actually caused a lot of extinctions because many of these early and oxygenic phototrophs and anaerobic chemotrophs not
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only didn't like or use oxygen but they also many of them couldn't tolerate it it was toxic to them so a lot of them died and went extinct or at least were extremely restricted to the corners of the earth and ocean that weren't oxygenated at that point but eventually organisms did evolve to use molecular oxygen in their metabolism obviously so we will talk more about the many aerobic respiration metabolic pathways which there are more than one more than just the way we do it
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there are plenty of aerobic respiration pathways that use oxygen and metabolism and we'll talk about those in the next or an upcoming geo bio video i'm not sure if it'll be the next one anyway if you want to check out the references i'm using to make this video and many other videos in my geo bio playlist you can check out geomicrobiology by kurt konhauser this is a great book i'm using this for a lot of the videos in this playlist as well as the brock biology and
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microorganisms so those are two great references they're listed and linked in my description below and lastly in the upcoming videos other than the aerobic respiration evolution video we'll talk about extremophiles and all the variations of extremophiles those organisms that love extreme environments or at least what counts as extreme to us humans and then we'll also talk about when life moved to land i know i did a poll on my channel and you guys really liked that idea so
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i will talk about that in a future video as well and until then you can check out the playlist below the geobiology playlist which is shown up for at least will show up on the screen here in a second so check that out if you want to see any other geobiology videos or see if these videos are out by the time you're watching this and i will see you guys next time thanks for watching bye
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