We don’t yet know the reason why light-harvesting molecules evolved for this spectrum. Without knowing the reason, it’s hard to say if a black plant would have any evolutionary advantages at all. It’s possible that this is the most efficient way of producing sugar from CO2 and water. It’s also possible that it isn’t.
Maybe you don’t want a whole plant, just a kind of artificial chloroplasts fixed on a membrane through which air is pumped? (I don’t mean it as a solution, just that we might be thinking in a wrong direction.)
Is it possible that the atmosphere absorbed more green light early on? Perhaps it was optimal at the time, but now they are stuck in a suboptimal configuration.
I think it’s more likely that chlorophyll and its related molecules were just the first photosynthetic pigments that came about, and new pigments couldn’t compete with it because they’d have to start from square one, whereas existing life is already built around the specifics of chlorophyll. And as Romashka said, perhaps chlorophyll is already as efficient as possible so a new pigment wouldn’t be beneficial anyway.
I’ve seen interesting work noting that the absorption spectrum of chlorophyll is very approximately the inverse of that of purple sulfur bacteria, which were most likely much more common in the early earth due to differences in geochemistry and atmospheric composition. Something taking the scraps that were left behind?
Photosynthesis has evolved many many times, but what has only evolved once in the cyanobacteria (and then been appropriated by blue green algae and countless other endosymbiosis events) is oxygen-producing photysynthesis. This allows the fixation of carbon at much higher efficiency since it can use water as the required electron donor instead of sulfur or metal salts or organic molecules and thus had a big advantage.
The choice of electron source is ultimately independent of the pigments used, but they are linked by evolutionary history.
What we see now might be as optimal as it gets, constrained by the electron transport systems that cannot deal with too much energy coming in. It could mean free radicals destroying the cell from inside.
ETA and even if this particular problem is dealt with, accelerated sugar synthesis requires more efficient water transport (= changes in whole body development → tradeoffs with reproduction and ability to withstand cold seasons) and CO2 intake (= thinner cuticle, larger stomata etc. → increased susceptibility to disease).
And when you optimize for those things, a life form threatening existing ecosystems is entirely possible.
One of the big constraints on photosynthetic land plants today is the sheer difficulty of fixing CO2 into organic molecules in the presence of oxygen, plus the resultant water stress.
Each of the double bonds of CO2 has an electron configuration remarkably similar to the electron configuration of the O2 double bond. Even though the RuBisCo enzyme that cracks CO2 and sticks it on an organic molecule to enlarge it is optimized to grab CO2, the ambient oxygen level is >400 times the ambient CO2 level. At that concentration difference, even a very selective enzyme is going to misincorporate oxygen a reasonable fraction of the time, producing a toxic peroxide that requires energy to square away and deal with. The faster the enzyme runs the less selective it can be. End result is that the RuBisCo of land plants today fixes perhaps one CO2 per enzyme per second, fantastically slow, so as to only misincorporate O2 something like 1⁄4 of the time. Slower and it would be a drag on growth, faster and it would poison the plant.
This also produces water stress on plants because of their need to keep their stomata pores open while photosynthesizing because they can’t deal with internal depletion of CO2, they need lots of airflow into their tissues to bring in the very thin gas. This airflow carries away lots of water from their tissue into the air via transpiration in all but the most humid of climates, increasing their water requirements. This is why C4 and CAM photosynthesis plants have evolved. These plants via various mechanisms concentrate CO2 from the air into their photosynthetic tissues. CAM plants like pineapples leave their stomata open only at night, sucking up CO2 in the cool air and sequestering it as organic acids using very little energy, then close up their pores during the day decompose the acids back to a high local CO2 concentration and then run their RuBisCo faster. C4 plants use various mechanisms to do something similar in surface tissues during the day and then transport the acids to specialized photosynthesizing cells where the CO2 is released at high concentration and a faster RuBisCo can run. These plants appear to have originated over 100 million years ago in marginal dry environments, saving water, and become much more common something like 30-40 million years ago as atmospheric carbon levels dropped precipitously. Modern C4 plants we all know and love include corn.
Both of those described systems involve very complicated anatomical structures and regulatory mechanisms. There are people who have tried to put faster RuBisCo into crop plants by itself and it’s totally pointless. Marine RuBisCo runs something like 8 times as fast because there’s oodles of carbonate and bicarbonate ions in the water which marine algae can enzymatically convert to CO2 in the cell for basically no cost. Someone in a tour de force of genetic engineering recently managed to get a marine RuBisCo working in tobacco plants as a demonstration with the justification that increasing carbon fixation rates in plants was good. And their tobacco plants did indeed fix carbon much faster per unit of RuBisCo protein. But it was pointless since they died horribly if exposed to light in air due to all the peroxide damage. They had to be kept in 10x enriched CO2 growth chambers and they still grew slower than normal.
The RuBisCo of land plants is pretty much at the optimum. C4 and CAM plants have a faster one since their anatomical structures and regulatory mechanisms allow it. Anything you do to mess with carbon fixation in open air runs into hard physical limits and requires much more complicated changes than messing with a few enzymes and stomata.
Now messing with NITROGEN fixation on the other hand… just look at Kudzu for an example of what good nitrogen fixation can do for a species.
(EDIT: just a note, plants can get a lot more energy from light than they can ultimately fix into a long-lasting form, so the further above mentioned idea of increasing light capture via new pigments is not hitting the process at the bottleneck)
Marine RuBisCo runs something like 8 times as fast because there’s oodles of carbonate and bicarbonate ions in the water which marine algae can enzymatically convert to CO2 in the cell for basically no cost
So if we could grow crops underwater we could get a lot more energy?
In a manner of speaking yes. That’s part of how kelp and seaweed (and to a lesser extent coral) manage to grow so fricking fast and part of how free-floating phytoplankton replicates fast enough to feed a biomass of zooplankton larger than itself at any given moment.
Only a fraction of the wattage of the biological energy available to plants that is converted from light can actually be captured by the carbon-fixation system as carbs and biomass-production, a lot more just can’t get stored long-term. In marine algae with all the extra carbon floating around, it’s rather a larger fraction that can be stored.
There’s other issues with marine agriculture, having to do with nutrient concentration and hervibory and the fact that the light only goes down through the water so far...
‘M.pyrifera is one of the fastest-growing organisms on Earth. They can grow at a rate of 0.6 meters a day to reach over 45 metres (148 ft) long in one growing season.’
Honestly not sure how these stack up in terms of energy capture per square meter.
EDIT 2: It should also be noted that despite the fact that the ocean coveres 70+% of earth’s surface and is full of carbon, it only represents something between 50 and 85% of the total photosynthesis that occurs on Earth depending on whose figures you listen to. Between low levels of many mineral nutrients, lack of a solid substrate near most of its surface, temperature variations, and dimming of sunlight with depth, it’s not as naturally productive compared to land as its carbon levels would indicate. The aforementioned superkelp grows in shallow water near nutrient-rich upwelling cold water.
EDIT 3: A little more research on my part shows that given the pigments and the chemical processes involved, the maximum theoretical energy yield of photosynthesis in sunlight is ~25% and maximum theoretical carb yield of a plant is ~10-11 % which will go down with light as bright a sunlight because the machinery has a maximum rate per square centimeter at which it can work which is actually partially why most plants arent simple planes but have lots of leaves pointing every whichaway, so the light intensity on any given leaf is less and it can be more efficient. Typical wild land plants are doing a lot of non-carbon-fixing energy-using activities and are hamstrung by low CO2 and manage less than 1% carb production, a bunch of crop plants that have been optimized for energy storage rather than other energy-using processes manage something like 2%, and sugarcane (a domesticated C4 plant) in tropical high light high humidity conditions can manage 7 or 8%. Typical wild algae apparently easily manages ~4-5% and in artificial conditions can be boosted much higher.
I was thinking more like, you need to re-evolve the whole lamina to re-juggle water conductivities in mesophyll, cuticle, vascular bundles and what else is there, to maintain water flow throughout the leaf. And all that newly made sugar which would have to be dealt with (don’t remember how it influences osmosis.) Which means different auxin fluxes, which would rebuild the rest of the body.
This is the absorbance spectrum of the various types of chlorophyll used in plants and bacteria: https://www.assignmentexpert.com/blog/wp-content/uploads/2014/08/absorption-of-light-by-chloroplast-pigments.jpg
We don’t yet know the reason why light-harvesting molecules evolved for this spectrum. Without knowing the reason, it’s hard to say if a black plant would have any evolutionary advantages at all. It’s possible that this is the most efficient way of producing sugar from CO2 and water. It’s also possible that it isn’t.
Maybe you don’t want a whole plant, just a kind of artificial chloroplasts fixed on a membrane through which air is pumped? (I don’t mean it as a solution, just that we might be thinking in a wrong direction.)
Is it possible that the atmosphere absorbed more green light early on? Perhaps it was optimal at the time, but now they are stuck in a suboptimal configuration.
I think it’s more likely that chlorophyll and its related molecules were just the first photosynthetic pigments that came about, and new pigments couldn’t compete with it because they’d have to start from square one, whereas existing life is already built around the specifics of chlorophyll. And as Romashka said, perhaps chlorophyll is already as efficient as possible so a new pigment wouldn’t be beneficial anyway.
I’ve seen interesting work noting that the absorption spectrum of chlorophyll is very approximately the inverse of that of purple sulfur bacteria, which were most likely much more common in the early earth due to differences in geochemistry and atmospheric composition. Something taking the scraps that were left behind?
Photosynthesis has evolved many many times, but what has only evolved once in the cyanobacteria (and then been appropriated by blue green algae and countless other endosymbiosis events) is oxygen-producing photysynthesis. This allows the fixation of carbon at much higher efficiency since it can use water as the required electron donor instead of sulfur or metal salts or organic molecules and thus had a big advantage.
The choice of electron source is ultimately independent of the pigments used, but they are linked by evolutionary history.
What we see now might be as optimal as it gets, constrained by the electron transport systems that cannot deal with too much energy coming in. It could mean free radicals destroying the cell from inside. ETA and even if this particular problem is dealt with, accelerated sugar synthesis requires more efficient water transport (= changes in whole body development → tradeoffs with reproduction and ability to withstand cold seasons) and CO2 intake (= thinner cuticle, larger stomata etc. → increased susceptibility to disease).
And when you optimize for those things, a life form threatening existing ecosystems is entirely possible.
One of the big constraints on photosynthetic land plants today is the sheer difficulty of fixing CO2 into organic molecules in the presence of oxygen, plus the resultant water stress.
Each of the double bonds of CO2 has an electron configuration remarkably similar to the electron configuration of the O2 double bond. Even though the RuBisCo enzyme that cracks CO2 and sticks it on an organic molecule to enlarge it is optimized to grab CO2, the ambient oxygen level is >400 times the ambient CO2 level. At that concentration difference, even a very selective enzyme is going to misincorporate oxygen a reasonable fraction of the time, producing a toxic peroxide that requires energy to square away and deal with. The faster the enzyme runs the less selective it can be. End result is that the RuBisCo of land plants today fixes perhaps one CO2 per enzyme per second, fantastically slow, so as to only misincorporate O2 something like 1⁄4 of the time. Slower and it would be a drag on growth, faster and it would poison the plant.
This also produces water stress on plants because of their need to keep their stomata pores open while photosynthesizing because they can’t deal with internal depletion of CO2, they need lots of airflow into their tissues to bring in the very thin gas. This airflow carries away lots of water from their tissue into the air via transpiration in all but the most humid of climates, increasing their water requirements. This is why C4 and CAM photosynthesis plants have evolved. These plants via various mechanisms concentrate CO2 from the air into their photosynthetic tissues. CAM plants like pineapples leave their stomata open only at night, sucking up CO2 in the cool air and sequestering it as organic acids using very little energy, then close up their pores during the day decompose the acids back to a high local CO2 concentration and then run their RuBisCo faster. C4 plants use various mechanisms to do something similar in surface tissues during the day and then transport the acids to specialized photosynthesizing cells where the CO2 is released at high concentration and a faster RuBisCo can run. These plants appear to have originated over 100 million years ago in marginal dry environments, saving water, and become much more common something like 30-40 million years ago as atmospheric carbon levels dropped precipitously. Modern C4 plants we all know and love include corn.
Both of those described systems involve very complicated anatomical structures and regulatory mechanisms. There are people who have tried to put faster RuBisCo into crop plants by itself and it’s totally pointless. Marine RuBisCo runs something like 8 times as fast because there’s oodles of carbonate and bicarbonate ions in the water which marine algae can enzymatically convert to CO2 in the cell for basically no cost. Someone in a tour de force of genetic engineering recently managed to get a marine RuBisCo working in tobacco plants as a demonstration with the justification that increasing carbon fixation rates in plants was good. And their tobacco plants did indeed fix carbon much faster per unit of RuBisCo protein. But it was pointless since they died horribly if exposed to light in air due to all the peroxide damage. They had to be kept in 10x enriched CO2 growth chambers and they still grew slower than normal.
The RuBisCo of land plants is pretty much at the optimum. C4 and CAM plants have a faster one since their anatomical structures and regulatory mechanisms allow it. Anything you do to mess with carbon fixation in open air runs into hard physical limits and requires much more complicated changes than messing with a few enzymes and stomata.
Now messing with NITROGEN fixation on the other hand… just look at Kudzu for an example of what good nitrogen fixation can do for a species.
(EDIT: just a note, plants can get a lot more energy from light than they can ultimately fix into a long-lasting form, so the further above mentioned idea of increasing light capture via new pigments is not hitting the process at the bottleneck)
So if we could grow crops underwater we could get a lot more energy?
In a manner of speaking yes. That’s part of how kelp and seaweed (and to a lesser extent coral) manage to grow so fricking fast and part of how free-floating phytoplankton replicates fast enough to feed a biomass of zooplankton larger than itself at any given moment.
Only a fraction of the wattage of the biological energy available to plants that is converted from light can actually be captured by the carbon-fixation system as carbs and biomass-production, a lot more just can’t get stored long-term. In marine algae with all the extra carbon floating around, it’s rather a larger fraction that can be stored.
There’s other issues with marine agriculture, having to do with nutrient concentration and hervibory and the fact that the light only goes down through the water so far...
EDIT:
http://en.wikipedia.org/wiki/Aquaculture_of_giant_kelp
‘M.pyrifera is one of the fastest-growing organisms on Earth. They can grow at a rate of 0.6 meters a day to reach over 45 metres (148 ft) long in one growing season.’
http://en.wikipedia.org/wiki/Seaweed_farming
Honestly not sure how these stack up in terms of energy capture per square meter.
EDIT 2: It should also be noted that despite the fact that the ocean coveres 70+% of earth’s surface and is full of carbon, it only represents something between 50 and 85% of the total photosynthesis that occurs on Earth depending on whose figures you listen to. Between low levels of many mineral nutrients, lack of a solid substrate near most of its surface, temperature variations, and dimming of sunlight with depth, it’s not as naturally productive compared to land as its carbon levels would indicate. The aforementioned superkelp grows in shallow water near nutrient-rich upwelling cold water.
EDIT 3: A little more research on my part shows that given the pigments and the chemical processes involved, the maximum theoretical energy yield of photosynthesis in sunlight is ~25% and maximum theoretical carb yield of a plant is ~10-11 % which will go down with light as bright a sunlight because the machinery has a maximum rate per square centimeter at which it can work which is actually partially why most plants arent simple planes but have lots of leaves pointing every whichaway, so the light intensity on any given leaf is less and it can be more efficient. Typical wild land plants are doing a lot of non-carbon-fixing energy-using activities and are hamstrung by low CO2 and manage less than 1% carb production, a bunch of crop plants that have been optimized for energy storage rather than other energy-using processes manage something like 2%, and sugarcane (a domesticated C4 plant) in tropical high light high humidity conditions can manage 7 or 8%. Typical wild algae apparently easily manages ~4-5% and in artificial conditions can be boosted much higher.
I was thinking more like, you need to re-evolve the whole lamina to re-juggle water conductivities in mesophyll, cuticle, vascular bundles and what else is there, to maintain water flow throughout the leaf. And all that newly made sugar which would have to be dealt with (don’t remember how it influences osmosis.) Which means different auxin fluxes, which would rebuild the rest of the body.
But yes, of course it is at least that difficult.