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Some of the effects of CO2 on micro algae

Several studies on 5 species of microalgae reveal that under "low CO2"
conditions, the cells had a well developed pyrenoid with much more starch
than those grown under "high CO2" conditions.

These algae have less "storage" of food reserves(stored as starch, like most
plants do) when grown in high CO2 conditions(like our tanks).

Another indication is that the affinity for CO2 in Photosynthesis is much
higher in "low CO2" cells than in "high CO2" cells.

Under low CO2 conditions, these algae are much better able to use CO2 than
the same species of algae grown in high CO2 conditions(Aizawa and Miyachi

The rate of Photosynthesis is much smaller in "high CO2" cells but their
maximum PS rates are still similar. So unless you are growing the algae
under optimum conditions, as a rule they are harmed by adding CO2 and have
less reserves allowing a more effective use of blackout and other methods of

Some similar effects have been found in Cyanophyta (I'll call its botanical
name since 99% of all the work done on the group was done by Botanist) but
the greatest difference was found in the Green algae, Chlorophyta. These
effects are on the enzyme responsible for converting HCO3 into CO2, called
carbonic anhydrase. Don't fear this enzyme, we have in in our body also,
regulates pH in our blood(we have a CO2/HCO3 buffer system too) but our
enzyme is a little different but not too much.

I think the take home message for folks is that adding CO2 makes the algae
become "lazy" and inefficient using CO2 and not store as much food reserves,
making them more vulnerable to being wiped out.

Then there's the photo respiration issues. Most algae don't have the same
enzymes in their peroxisomes as higher plants do. Chara, Green spot
algae(Coleochaete orbicularis) do. These enzymes prevent loss of organic
products due to photorespiration. Many algae photorespire when CO2 is added.

Tom Barr