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I've been reading some papers and research on PR in algae and higher plants. Now while some have postulated that variations in CO2 have led to strategies to deal with PR, from mopst of what I've read seem more to indicate that variations in O2 levels play a much larger role in PR issue for algae.
Although I've stated this in the past I've found numerous research publications to back up my claims regarding why algae declines in a plant tank when the plants thrive.
Low or high CO2 levels do not influence the ability of algae to handle PR since many have excellent CCM(Carbon concentrating mechanisms).
But, O2 levels really influence PR in algae much more so.
When algae PR, they often leak the by product of PR, P gylcolate in a process that consumes O2 and gives off CO2(A reverse of sorts to that of photosynthesis) and it loses much of carbon it has fixed.
P glycolate is a strong inhibitor of photosynthesis. So higher concentrations will cause a decline in photosynthesis. Plants recycle this PR by product with peroxisomes. Many algae don't have peroxisomes but many have peroxisomal like pathways to deal with PR from higher CO2 levels but these pathways are not nearly as efficent when high levels of O2 are added(say by high plant or algae production).
P Glycolate is an inhibitor in regulation of RUBISCO and activation of RUBISCO.
Adding O2, CO2 and a combinations of each at 200% saturation, amibent, 50% and 5% ambient to algal cultures should show that O2 is a stronger influence in controlling growth of algae.
This type of experiment would remove the potential plant influences and focus in on O2 and CO2 levels that are often typically found in planted aquariums.
One idea I've thought that is a likely mechansim for algae not growing well is a "back up" when the algae are PR-ing due to CO2 being added. This drives the algae "too fast" and they are typically using HCO3 as their DIC carbon source. This takes a fair amount of energy to drive this, and if the algae no longer use this energy, it is likely that it backs up and lower the efficiency a great deal.
Some research indicates that the CCM in algae prevents the formation and excreation of large amounts of P glycolate(Tolbert 1992). So if you alter this, it will harm the algae's abilty to grow. CO2 can help but O2 will likely do much more harm.
Some paleo O2/CO2 levels around the time of higher plants show some interesting patterns.
Interestingly, aquatic plants also leak out P glycolate as well.
But they and Chara have better methods of regeneration of P glycolate than most green algae(Mitochondrial pathways vs peroxisomal).