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Thread: What causes photorespiration - ref APD V6 #81

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    What causes photorespiration - ref APD V6 #81

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    I hope Tom Barr does not take issue with me discussing the topic he brought up in APD.

    In APD V6 #81, there was an interesting discussion by Tom on photo-respiration in plants and algae and why it can possibility help control algae problem.

    I am under the impression that green plants take in O2 and give out CO2 only when there is no light (respiration). With lights, green plants do the reverse, that is, take in CO2 and give out O2.

    If so, how and why will photo-respiration happen in plant and algae?

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    I don't profess to understand the mechanics of PR very much (esp with regards to how higher plants do it differently from algae), but I believe all living organisms respirate 24 hours a day, plants being no exception. It's just that during the day, much of the CO2 produced by plants is used up in photosynthesis. Correct me if i'm wrong, but PR is essentially "breathing" and the burning up of stored carbohydrates in the presence of O2. As with animals, PR releases or uses up energy (whereas photosynthesis gathers and stores energy), so apparently, algae can't take the "exertion" and are forced into inert forms (spores) to survive or burn themselves up.

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    Converting of carbohydrates into energy and releasing CO2 is called respiration.

    Photorespiration is different. Photorespiration happens when Rubisco act as a oxygenase in the presence of O2 instead of as a carboxylase in photosynthesis.

    BC

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    OK I've been reading up on photorespiration and its all pretty "cheem" stuff... So I may have got it wrong.

    Plants have an enzyme called RuBisCo... This enzyme has an affinity for both CO2 and O2. Also, this enzyme is activated by light. Whether Rubisco fixes O2 or CO2 depends on the level of CO2:O2 in the tank. Under High CO2 : Low O2, then the affinity is for CO2. Under High O2: Low CO2 then it is for O2.

    So Photorespiration occurs when O2 is fixed by rubisco and CO2 is given off. This is not the same as Dark Respiration... which I gather happens when the lights are out (which was what I was confused with).

    Btw, pls correct me if I got anything wrong... don't wanna mislead people
    Allen

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    Wah, Allen, Sui Sui! Nicely explained.[]

    Just to add, RuBisCo is the abbreviation for Ribulose 1,5 bisphosphate Carboxylase/Oxygenase. So from the name itself, you know it is a multifunctional enzyme. Ribulose bisphosphate is the substrate for Rubisco

    link with more details

    Some plants deal with this by evolving means of concentrating the CO2 in certain areas of the leaf thereby evading much of photorespiration. Algae deal with this by concentrating HCO3 (though not all algae can do this).

    ck

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    so does that translate to:
    in the present of high o2, food production is more energy costly and plants compensate better for it than most algae. so algae dies.

    (and if so, does ozone help? heh.)

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    I'm not entirely sure, but I think thats could be the premise for why healthy plants = less algae.

    Now from the rest of the stuff I've read, some plants have also developed other mechanisms to deal with the problems with Rubisco. Namely C4 and CAM plants. These plants use PEP carboxylase to fix carbon. PEP carboxylase has no affinity to O2, and as such won't face the same problems with photo respiration.

    My own question now is this.... from what I've read, some 90% of plants are C3, and the rest are C4,CAM (this was for terrestrial plants not sure if it holds for aquatic/marsh plants)... so they don't have PEP carboylase... so how do non C4/CAM plants deal with photorespiration? Tom mentioned something to do with peroxisomes which I can't seem to make head or tails about... it seems to be something which also helps fix carbon, but I can't find specific details. Also lacking is information on whether it exist in all plants, or just specific plants.
    Allen

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    I think some times back I read a post in AQ whereby someone mentioned that amano aerate his tanks at night. And someone went to suggest that the possible reason is that by so doing, pearling and bubbling can happen shortly after light is turned on..

    I thought that may not be the actual reasons (you think amano is as crazy as us to love the see plants bubble?) So when I read the Tom's posting, I thought I got the answer. But can someone translate what Allen mentioned into some puffer-brain-proof lingo so that I can understand?

    What exactly is 'O2 fixing' and 'CO2 fixing'?

    Allen, if I understood you (see below) correctly, wouldn't photorespiration happens at full force after 8-10hrs of continuous lightings when O2 (assuming that's O2) is produced at a flat-out rate? If so, why is the plant still giving out O2? Or are we saying sometime in the mid-day, the plant starts to give out CO2 instead of O2?


    ========================== quoted from allen's post ==================

    Plants have an enzyme called RuBisCo... This enzyme has an affinity for both CO2 and O2. Also, this enzyme is activated by light. Whether Rubisco fixes O2 or CO2 depends on the level of CO2:O2 in the tank. Under High CO2 : Low O2, then the affinity is for CO2. Under High O2: Low CO2 then it is for O2.

    So Photorespiration occurs when O2 is fixed by rubisco and CO2 is given off. This is not the same as Dark Respiration... which I gather happens when the lights are out (which was what I was confused with).

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    Some C4 plants: Hydrillia, Orcutt grass, and many bicarb user act sort of like C4 plants.
    Isoetes is a CAM plant. See John Keeley's research for more info on CAM in aquatic systems.

    Algae in general are much less adapted to PR than the higher plants. They also experience less O2 and a more variable CO2/O2 level than the land evolved plants ever do.
    Some algae will PR when exposed to CO2 enrichment. This is counter to what one would expect but does happen with some algae, likely due to a back up of excess energy normally used to concentrate CO2 on the cell's surface from HCO3 combined with faster growth rates than naturally occuring generally.

    But O2 itself has a more negative effect on PR for algae than the plants. Reaeching in the 100-150% above O2 saturation level would likely favor plants over algae I believe. Aeration alone would only reach to 100%max.

    I am adding O2 with a DO meter to measure the amount of algae growth at different O2 levels without any substrate/plants or other factors that might interefer with the results.

    Regards,
    Tom Barr

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    Excellent Tom,

    please do keep us posted on your findings...
    Allen

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    Very interesting. Could that be how Siesta has help in curbing certain algae?

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    not sure what you mean by that FC? why do you think a 'siesta' period helps? elaborate pls.
    Allen

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